Easy Laser Printer Maintenance & Repair - Bigelow - ocr.pdf

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Easy Laser PrinterMaintenance

and RepairStephen J. Bigelow

W i n d c r e s t / M c G r a w H i l lN e w Yo r k S a n F ra n c i s c o Wa s h i n g t o n , D C . A u c k l a n d B o g o t a

C a ra c a s L i s b o n L o n d o n M a d r i d M e x i c o C i ty M i l a nM o n t r e a l N e w D e l h i S a n J u a n S i n g a p o r e

S y d n e y To k y o To r o n t o



©1995 by W indcrest .Published by Windcrest, an imprint of McGraw-Hill, Inc.The name "W indcrest" is a registered trademark of McGraw-Hill, Inc.

Printed in the U nited States of America. All rights reserved . The publisher takesno responsibility for the use of any materials or methods descr ibed in this book, norfor the products thereof.

Product or brand names used in this book may be trade names or trademarks. Where we believe that theremay be proprieta ry claims to such trade names or tradem arks, the name has been used with an initialcapital or it has been capitalized in the style used by the name claimant. Regardless of the capitalizationused, all such names have been used in an editorial manner without any intent to conv ey e ndorse men t of orother affiliation with the name claimant. Neither the author nor the publisher intends to expr ess anyjudgme nt as to the validity or lega l status of any such proprietary claims.

Library of Congress Cataloglng-in-Publication DataBigelow, Stephen J.

Easy laser printer maintenance and repair / by Stephen J . Bigelow.p. cm.

Includes index.

pbk 9 10 DOC/DOC 9 9 8h e 1 2 3 4 5 6 7 8 9 DOC/DOC 9 9 8 7 6 5 4

ISBN 0-07-035975-X ISBN 0-07-035976-8 (pb k.)1. Laser printers— Mainte nanc e and repair. I. Title.

TK7887.7.B538 1995681'.62—dc20 94-33620

CIP

Acquisitions editor: Roland PhelpsEditorial team: B.J. Peterson, Editor

Susan W. Kage y, Managing E ditorJoanne Slike, Execu tive EditorJoann Woy, Ind exer

Production team : Katherine G. Brown, DirectorRhonda E. Baker, CodingSusan E. Hansford, CodingBrenda S. Wilhide, Compu ter A rtistLisa M. Mellott, Desk top Op erator 0359768

ELIesigner: Jaclyn J. Boone



D i s c l a i m e r a n d c a u t i o n s

It is important tha t you read and unders tand the fo l lowing in fo rmat ion . P leas e read

it carefullyThe repair of laser printers involves some amount of r isk to yourself and other people in your work area. Use extreme cau t ion wh en work ing wi th laser beam s, ac (a lternat ing curren t ) , and h igh-vo l tage power sources . Every reasonab le effo r t hasbeen made to identify and reduce areas of personal r isk . Read this book carefully be

fore a t tem pt ing the p roce dur es d iscussed . I f you are uncom for tab le fo l lowing theprocedures that are outl ined in th is book, do no t a t t em p t th e m — refe r y o u r se rv iceto qualif ied service personnel.

Neither the author, the publisher, nor anyone directly or indirectly connectedwith the pub l ica tion o f th is book shall mak e any warran ty E ITH ER EX PR ES SE D OR

IMPLIED, with regard to th is material , including, but not l imited to , the implied warranties of QUALITY, merchantabil i ty, and fi tness for ANY particular purpose. Further,neither the author, publisher, nor anyone directly or indirectly connected with thepublication of this book shall be liable for errors or omissions contained herein, or forincidental or consequential damages, injuries, or financial or material losses resultingfrom the use, or inabil i ty to use, the material contained herein . This material is provided AS-IS, and the reader bears al l responsibil i t ies connected with i ts use.



Dedica t ionThis book i s ded ica ted to my wonder fu l wi fe , Kath leen .

Without her lov ing suppor t and encouragement ,this book would st i l l have been possible , but not nearly worth the effort .



Contents

A c k n o w l e d g m e n t s x i i i

Int roduct ion x v

The electrophotographic printer 1A n o v e r v i e w 2Wh at is e l ec t ro p h o to g rap h ic? 2Featu r es and speci f ica t ions 2

P o w e r r e q u i r em e n t s 3In ter face compat ib i l i ty 3

Prin t capaci ty 4Prin t character is t ics 4Reliability/Life information 5Environmental in fo rmat ion 6Physical in fo rmat ion 6

Typical assembl ies 6T h e ac p o w er su p p ly 6Th e dc po we r supp ly 7High-vo l tage po we r supp ly 7Fusing assembly 7

Erase lamp assembly 8Main m o to r 8Writ ing mechan ism 8Scanner-motor assembly 8Paper-con tro l assembly 8Main logic assembly 9EP car t r idge assembly 9Contro l -panel assembly 9



2 Typica l components 1

Mechan ical par ts 10G ears 10P u l l ey s 12Rollers 13

Reducing f r ic t ion 13E lec t ro m ech an ica l co m p o n en t s 14

E l e c t r o m a g n e t i s m 14R elay s 15S o len o id s 16Mo to rs 17

P ass iv e co m p o n en t s 18Resis to rs 18Reading res is to rs 20Capaci to rs 22

Reading capaci to rs 24Inducto rs 25

A c t i v e c o m p o n e n t s 27D io d es 27D io d e m ark in g s 32Transis to rs 33Transis to r mark ings 37In tegrated c i rcu i ts 38

3 Soldering and test instrum entsSmall tools and materials 43

Hand too ls 43Mater ia ls 44

Solder ing 45Solder ing background 45Irons and t ips 45Solder ing 47Surface-mount so lder ing 48D eso ld e r in g 50

Mu l t im e te r s 51Measu r in g v o l t ag e 52

Measur ing curren t 52Measu r in g f r eq u en cy 54Measur ing res is tance 54Check ing a capaci to r 55S em ico n d u c to r ch eck s 56

L o g ic p ro b es 58Osci l loscopes 59

Contro ls 60Osci l loscope speci f ica t ions 60



Osci l loscope s tar tup p rocedures 61Vo l tag e m easu rem en t s 62Tim e an d f r eq u en cy m easu rem en t s 64

4 Service guide lines 65The t roub leshoo t ing cycle 65

D ef in e y o u r sy m p to m s 66Identify and isolate 66Repair o r rep lace 67R e- tes t 68

Gather ing techn ical data 68Elect r ic i ty hazards 69

Static electrici ty 71Static formation 72D ev ice d am ag e 72Controll ing stat ic electrici ty 73

Reassembly and d isassembly h in ts 74Housing d isassembly 74Elect romechan ical d isassembly 74R eassem b ly 75

5 Electrophotog raphic techn ology 77T h e e l ec t ro p h o to g rap h ic ap p ro ach 78

Cleaning 79Charg ing 80Writ ing 81

D e v e l o p i n g 82Transfer 83Fusing 84

Writ ing mechan isms 84Lasers 85L E D s 88L C S s 89

T h e e l ec t ro p h o to g rap h ic ca r t r id g e 90Pro tect in g an EP car t r idge 91

6 Po we r suppl ies 93

Power supp l ies—ac and l inear dc 94Tran s fo rm ers 94T h e ac p o w er su p p l ie s 96Rect i f iers 96Fi l ters 98Regulato rs 99

Troub leshoo t ing ac and linear dc supp l ies 102Const ruct ion and operat ion o f dc swi tch ing supp l ies



Troub leshoo t ing dc swi tch ing supp l ies 110High-vo l tage supp ly t roub leshoo t ing 114

7 Image-fo rmat ion sys tem 116

System s tar t -up p rob lems 117

L ase r-d e l iv e ry p ro b lem s 119Fusing-assembly p rob lems 122Im ag e- fo rm at io n p ro b lem s 124

8 M ech an ica l sys tems 137

P ap er p ro b lem s 138Sensor and in ter lock p rob lems 146

Resis t ive sensors 146Mechan ical sensors 147Optical sensors 148

Troub leshoo t ing sensors and in ter locks 148Scanner-motor /main-motor p rob lems 150E P ca r t r id g e p ro b lem s 151

9 T he electronic control package 154

C o m m u n ica t io n 155P r in te r co m m u n ica t io n b ack g ro u n d 156

A S C II ex p la in ed 156Contro l codes 157N u m b er sy s t em s 157Binary digits 158

Communicat ion l inks 159Communicat ion s tandards 160Paral le l communicat ion 160Ser ia l communicat ion 162Iso la t ing the comm unicat ion in ter face 164

Troub leshoo t ing a para l le l in ter face 164Troub leshoo t ing a seria l in ter face 167M e m o r y 169

P e r m a n e n t m e m o r y 169Te m p o r a r y m e m o r y 170

Tro u b lesh o o t in g m em o ry 171Contro l panel 171Sealed swi tches 172Troub leshoo t ing a con tro l panel 173Main logic 173

Micro p ro cesso r o p e ra t io n s 174T h e sy s t em c lock 175A S IC o p era t io n s 175

Troub leshoo t ing main log ic 176



A P P E N D I C E S

A Typical c leaning and ma intenance 178

B Com prehens ive troubleshoot ing guides 184

C Vendors 192

Glossary 196

Index 2

Ab ou t the au thor 2 6



A c k n o w l e d g m e n t s

In today ' s fas t -moving wor ld o f personal com pute rs , it is v i r tual ly impossib le to p repare a thorough , com plete book on comp ute r serv ice wi thou t the coop erat ion and

support of o ther individuals and corporations. I offer my heartfel t thanks to the following fo r mater ia l and techn ical sup por t that helped t o make th is book :

Mr. J im Bro ich , Hew let t -Pa cka rd Co .Mr. Gregg E lmore , B+K Precis ionMr. Reza Ghaleb i , ROHM Corp .Ms. Jan Marciano , Eps on Am erica Corp .Mr. Ron Trum bla , Tan dy Corp .M s. K i r s t en Wisd o m , H ew le t t -P ack ard C o .

Of course , I a lso wish to thank the ed i to rs and the deve lopm en t s taff at M cGr aw-Hill for their endless patience and consideration in making this book a reali ty.

xii i



Introduction

EP (e lect rop ho t ogra ph ic) p r in ters have bec om e an invaluab le too l in the h igh- techrevo lu t ion . It seems that abou t every c om put er ow ner has an EP p r in te r on hand o rhas access to one at work. You can find EP printers in offices, laboratories, stores,c lassrooms, fac to r ies , and homes th roughou t much o f the wor ld . S ince EP p r in tersbecame widely available in the early 1980s, they have rapidly evolved into flexible,rel iable, and surprisingly cost-effective devices using any one of several maturepr in t ing techno log ies . Th is book d iscusses those techno log ies in deta i l and showsyou how m any o f today ' s EP p r in ters w ork .

No mat ter how re l iab le a laser o r LED pr in ter migh t be , i t wi l l even tual ly needsome sor t o f main tenance and repai r. Th is book is in tended to p rov ide the neededbackground in fo rmat ion on p r in ter mechan ics and e lect ron ics , a long wi th the techn iques that gu ide you th rough the t roub leshoo t ing and repai r o f abou t any com mercial EP printer. The book is writ ten for the hobbyist or repair technician who has anin termediate knowledge o f e lec t ron ics and some work ing knowledge o f mechan ics .A kn ow ledge o f e lementary t roub leshoo t ing is help fu l , bu t i t i s no t a p rere qu is i te—th is book descr ibes the bas ic operat ion and use o f several com m on tes t ins t ruments .Troub leshoo t ing p rocedures are p resen ted in a d iscuss ion fo rmat that a ids youroveral l unders tand ing o f the EP p r in ter and how i t work s .

Easy Laser Printer Maintenance and Repair beg ins s lowly wi th a deta i led explanation of the electrophotographic conc ep t . Chap ter 1 shows you how to in terpre t the sometim es-confu sing fe atures and spec ifications of E P printers. A br ief

discussion of typical assemblies explains the purpose and relat ionship of each majorpr in ter subsect ion . Chap ter 1 is a handy in t roduct ion fo r readers wh o are ne w to E Ppr in ters . Chap ter 2 o ffers a rev iew o f the many d iverse com pon en ts that you wi l l f indin a typ ical EP p r in ter. Mechan ical , e lec t romech an ical , pass ive , and act ive com ponen ts are covered in deta i l . Even exper ienced readers wi l l f ind chap ter 2 to be ahelpful review. Chapter 3 presents detailed information on the practical tools andtes t equ ipment needed to perfo rm many deta i led repai rs .

Hand too ls , so lder ing and deso lder ing , mul t imeters , log ic p robes , and osci l loscopes are discussed carefully. Even if you 've never held a soldering iron or test in-

x v



x v i Introduction

s t rument befo re , th is sect ion shows you how to use each dev ice . Serv ice gu idel inesare covered next in chapter 4 . You will learn the universal troubleshooting cycle, andunderstand the hazards of ac and stat ic electrici ty.

Once you have the prelirninary chapters out of the way, use chapter 5 to dig into the theory beh ind e lect ropho tograph ic concep ts . You can learn each s tep in theEP process and how each con tr ibu tes to creat ing the f inal paper image. You canmake a comparison of laser, LE D (L igh t-emitt ing d i od e) , and LS C ( l iquid crystalshu t ter ) mec han isms. Th e real t roub leshoo t ing beg ins in chap ter 6 wi th a thoroug hevaluat ion o f l inear power supp l ies , fo l lowed by an equal ly complete rev iew ofswi tch ing power supp l ies . Troub leshoo t ing p rocedures are p resen ted fo r each sect ion . High -vo l tage supp l ies and t roub leshoo t ing are a lso cov ered .

Chap ter 7 deta i l s each e lemen t o f EP imag e fo rmat ion . Each s tep a long the w ayoffers com preh ensive t roub leshoo t ing p roced ures . Chap ter 8 cover s the op erat ionsand troubleshooting of the mechanical systems of a printer, including paper handling, sensors and interlocks, laser scanners, and EP cartr idges. Chapter 9 providesa comprehensive d iscuss ion o f the p r in ter ECP ( e l ec t ro n ic co n t ro l p ac k ag e) . Yo u

can learn to unders tand and t roub leshoo t communicat ion , memory, con tro ls , andmain logic.

Append ix A prov ides you wi th a re l iab le c lean ing and main tenance p rocedure .Each o f the t roub leshoo t ing symptoms and p rocedures in the book are condensedinto a quick-reference form in appendix B. Look to appendix C for a detailed l ist ofpar ts , mater ia ls , and serv ice vend ors .

Fo l lowing the append ices is a com preh ensive EP p r in ter g lossary.I am intereste d in you r success I have taken ever y possible measu re to ensure

that th is book is thorough and comprehensive. Your comments, questions, and suggestions about th is book , as we ll as any of your pers onal troublesh ooting exp erie nc es

that you m ight Like to share, are wel co m e at any t ime. Fee l free to conta ct me directly :S tephen J . B igelowDynamic Learn ing SystemsP.O. Bo x 805Marlboro , Massachuset ts 01752U.S.A.

CIS:7 3 6 5 2 ,3 2 0 5



C H A P T E R

The electrophotographicprinter

Laser printers (Fig . 1-1) have co m e a long way in the last few y ears . No t too long ago,laser p r in ters were expensive commodi t ies res t r ic ted to the few ser ious businessesthat cou ld affo rd them . How ever, the p rec is ion , speed , p r in t ing consis tency, and image qual ity o ffered by laser p r in ters ma de them ex t rem ely a t t rac t ive per ipherals . Ascom pu ter design s continued to adv anc e into the 1990s, laser printe rs also advan cedwhile printer prices plummeted. Today, laser printers are available for under $600(U.S . ) and have become commonplace in homes and smal l businesses a l l over the

1-1 A Hew lett-Pack ard LaserJet III printer.

1



Features and specifications 3

p r in ter can do , which m igh t help you to tes t i t m ore th orough ly dur ing and af ter you rrepair. A l ist ing of specifications is usually contained in an in troductory section ofthe printe r instruc tion manual or at the en d in an app end ix. If yo u do not have a co pyof the p r in ter doc um entat ion on hand , the manu factu rer can o f ten fax a copy o f thespecifications directly to you. Remember that there is no standard format for l ist ingpr in ter speci f ica t ions . The fo rm at i s up to the p re feren ces o f each manufactu rer. R e

gardless of how the specifications are l isted, you will most often find the followingsub jects : power requ irements , in ter face compat ib i l i ty, p r in t capaci ty, p r in t characterist ics, rel iabil i ty /l ife information, environmental information, and physical informat ion . Each o f these sp eci f ica t ions has some im por ta nce, so you shou ld be familiarwith them in detail .

Power requi rementsAs wi th any e lect r ica l dev ice , a p r in ter requ ires p ow er to funct ion . Vol tage, f re

quency, and po we r con sum pt ion are the th ree typ ical speci f ica t ions that you wi ll f indhere . Dom est ic U.S . vo l tage can vary f rom 105 to 130 Vac (a l ternat ing-curren t vo l t s )

a t a f requ ency o f 60 Hz (h er tz ) . Eur opea n vo l tage can range f rom 210 to 240 Vac a t50 Hz. Many curren t laser p r in ters have a pow er s e lect ion sw i tch that togg les thepr in ter betw ee n 120 and 240 V operat ion . Po w er co nsum pt ion is ra ted in watts ( W ) .Dep end ing on the par t icu lar mod el , laser p r in ters can use up to 900 W dur ing p r in ting . However, most models use an au tomat ic power-down mode that shu ts down themajor power-consuming componen ts af ter the p r in ter i s id le fo r several minu tes .Chap ter 6 d iscusses the ope rat ion and repai r o f laser p r in ter pow er supp l ies .

Interface compatibil i tyA p rinter is a peripheral de vic e. Th at is , i t serv es no pu rpo se at al l unless i t can

co m m u n ica te (o r interface) wi th a com puter. A com mu nicat ion l ink be tw eenpr in ter and com pu ter can be es tab l ished in ma ny d i fferen t w ays , bu t th re e in ter facetechn iques have become s tandard : RS-232 , Cen tron ics , and IEEE 488 . Only a p roper ly wired and terminated cab le i s needed to connect the p r in ter and computer.Pr in ter communicat ion and t roub leshoo t ing are d iscussed in chap ter 9 .

RS-232 is a serial in terfac e use d to pass binary digits ( or bits) one at a t ime betw ee n the c om pute r and p r in ter. Ser ial links o f th is typ e are ver y co mm on, no t jus tfo r p r in ters , bu t fo r o ther ser ia l communicat ion app l ica t ions such as modems andsimple digital networks. RS-232 is popular due to i ts h igh speed, physical simplici ty,and i ts abil i ty to handle data over long distances.

Centronics is the standard for parallel com mu nication. Alth oug h one used exclus ively by p r in ters , para lle l commu nicat ion has be com e popu lar fo r o ther per ipherals such as para l le l -por t tape d r ives and por tab le CD -RO M (com pac t d isc read-on lymemory) drives. Centronics is a de facto standard, so i t is not official ly endorsed bystandards organizations such as the IEE E (Insti tu te of Electrical and Elec tron ic Eng i n e e r s ) , EIA (E lect ron ic Indust ries As socia t ion) , o r CC ITT (In ternat ional Consu ltat ive Commit tee fo r Telephone and Telegraph) . Ins tead o f pass ing one b i t a t a t ime,Centronics in terfaces pass entire characters from the computer to the printer as setsof bits . Centronics is popular because of i ts functional simplici ty. Although parallel



4 The electrophotographic printer

connections require more in terconnecting signal wires than an RS-232 cable, thehard ware requ ired to handle parallel information is simpler.

IEEE 488, a l so known as GP IB (g enera l -purp ose in ter face b us ) , i s an o ffic ia lIEEE standard for parallel communication. I t is not as widely used as Centronics orRS-232 , bu t GPIB suppor ts network and b id i rect ional communicat ion between ins t ru m en t s . T h e G P IB t ech n iq u e w as o r ig in al ly d ev e lo p ed b y H ew le t t -P ack ard C o m

pany, w he re i t is sti ll wid ely used in their line of printers a nd p lotter s.

Print capacityPrint capacity is a gen eric term including several d ifferent laser printer specifi

cations that outl ine what a printer can do. One of the most common print capacityspecifications is print speed, wh ich is measu red in pages per minu te (p p m ). Inexpensive EP printers work at 4 ppm, but 8 to 10 ppm printers are available. Next, youmust be concerned wi th resolution, wh ich is the num ber of individual dots that canbe placed per l inear inch. Typical EP printers offer 300 x 300 dpi (dots per inch) resolution (300 l ines pe r inch at 300 dots p er l inear inch, or 90,000 dots pe r square i n c h ) .

A resolution this h igh is adequate for most business and personal graphics. The current gene ration of laser and LE D p rinters is capable of 600 x 600 dpi resolution.

You might f ind a section on paper specifications. Al thou gh do t matr ix and inkje t p r in ters are very f lex ib le in accep t ing a wid e var ie ty o f pap er th icknesses and f inishes, the p ap er u sed in laser printers mu st fal l within ce rtain we ight s and fin ishes ifthe EP p roces s i s to work correct ly. In most cases , s tandard le t ter-s ize , x erogr aph y-grade p aper (16 -24 po und bo nd ) wi l l work . The p aper a lso shou ld have a p la in f inish . Shiny or gloss-fin ished papers will cause problems with the EP process. Mostlaser printers will handle envelopes, transparencies, and labels. Before choosingsuch materials, be certain that they are labeled as safe or tested with laser printers.Poor-qu ali ty materials can jam and dam age you r printer.

Memory is anoth er imp ortan t specification for E P printe rs. Bec aus e laser printers assemble images as full pages of individual b its , the more memory that is availab le , the larger and mo re c om plex an imag e can be p r in ted . Typ ical laser p r in terso ffer 512 K (k i loby tes ) to 1 Mb ( m eg ab y t es ) , bu t 2 -3 M b is requ ired fo r fu l l-pagegraph ic images that migh t be p rodu ced wi th so f tw are such as CorelDraw. M ost laserp r in ters o ffer memory upgrade op t ions .

Print characteristicsPrint characterist ics specify just how printer images will appear, how they will

be p rod uce d , o r how ch aracters f rom the p r in ter wi l l be in terp r e ted . Fon ts , so f twareemulation, and character sets are the three specifications that you should be mostfamiliar with .

kfont is a sty le of typ e with certain visual charac terist ics that d ist inguish i t fromother typ e s ty les . Th ese character is tics migh t include d i fferences in bas ic characterfo rmat ion , accen ts , and decorat ive add i t ions ( that i s , Cour ier versus Helve t ica t y p e ) .Figure 1-2 is an example of several basic printer fonts. Early laser printers rel ied onfon t car t r idges that con ta ined ROMs (read-on ly memories) that held the image datafo r each fon t . To change a fon t , you cha nged the car t r idge. How eve r, wi th the r i se o fMicrosof t Windows and improved memory sys tems, most cu rren t laser p r in ters use



Features and specifications 5

10 point Anal

1 0 P O IN T A L G E R IA N

1 2 p o i n t B r i t a n n i c B o l d

12 p o i n t C e n t u r y G o t h i c

1 4 p o i n t I m p a c t

1 6 P O I N T S T E N C I L

1 p o i n t T i m e s N e w R o m a n

18 point Q ivaldi

1 8 p o i n t W i d e L a t i n1-2 A sample of software-driven EP printer fonts.

soft fonts, where the data fo r des i red fon ts is d o w n l o a d e d to the printe r du ring the

actual printing process. Soft fonts also al low easy enhancements such as und er l ining, bold , i tal ic, supe rscript , su bscript , and so o n.

All p r in ters use the i r own bu i lt - in so f tw are " lang uag e" that is in th e p r in ter perm an en t m em o ry. Th e langu age s pecifies such things as fon t fo rmat ion (dot p l acem e n t ) and s ize , how to r eco g n ize and r e sp o n d to con t ro l codes or con trol p anelinpu t , and m ore . This softw are lan gua ge also tel ls th e p r in t e r ho w to o p e ra t e , co mmunicate , and r e sp o n d to p ro b lem s .

Mo s t of these languages wer e o r ig inal ly dev elop ed by lead ing p r in ter manufacturers such as H ew le t t -P ack ard , IB M, and Epson . Other m anufactu rers that wish t o

mak e thei r p r in ters funct ional ly com pat ib le m ust use a so f tware language that emu

lates one or m o r e of the exist ing langu age standa rds. For exam ple , most laser p r in ters will emulate th e opera t ion of a H ew le t t -P ack ard L ase r Je t III. Em ulation in thisuse means that even though a laser printer is physically and electro nically differentf rom a LaserJe t III, it wi l l respond as if it w e r e a LaserJe t III w h e n c o n n e c t e d to a

host computer. Another typ ical p r in ter language is Postscr ip t .Ord inar i ly wh en a character cod e is sent t o a printer, it is p r o c e s s e d and p r in ted

as a fu lly fo rmed a lphanumeric ch aracter or o ther specia l symbol . Ho we ver, becausea character code is not la rge enou gh to car ry every p oss ib le type of t e x t or specialsy m b o l (for exam ple , fo re ign- language characters or b lock g ra ph ics) , characters are

grouped in to character sets that the p r in ter can sw i t ch b e tw een . S w i tch in g a charac te r set is o f ten accom pl ished th rough a series of c o m p u t e r c o d e s or control panelc o m m a n d s . A s tandard character set consists o f 96 A S C II (A m er ican S tan d ard C o d efor In fo rmat ion In terchange) characters . The 96 characters include 26 u p p ercasele t ters , 26 lo w ercase l e t t e r s , 10 d ig i t s , punctuat ion , sym bols , and som e con tro lcodes . Other character se ts can inc lude 96 i talic ASC II cha racter s, in tern ationalcharacters (German , French , Span ish , e t c . ) , and unique b lock g raph ics.

Reliability/life informationReliabil i ty and life expectancy in fo rmat ion expres ses th e ex pe cte d work ing l i fe

of th e laser printer or its c o m p o n e n t s in p a g e s or t i m e . Fo r e x a m p l e , a typ ical EP




toner ca r t r idge is ra ted fo r 200-250 page s , and the image -fo rmat ion "en g in e" is ra tedfor up to 300 ,000 pages . Many p r in ters are ra ted a t 5,000 p ag es p e r m o n th ( ab o u t200 pages per workday) . You migh t see th is same in fo rmat ion expressed as MTBF(m ean t im e b e tw een fa i lu res ) .

Environmental informat ionE n v i ro n m en ta l sp ec i f i ca t io n s in d ica t e th e p h y s ica l o p e ra t in g r an g es o f y o u r

pr in ter. Storage temperatu re an d operating tempera ture a r e th e tw o m o s tco m m o n en v i ro n m en ta l co n d i t io n s . A ty p ica l l a se r p r in t e r can b e s to red in t emp era tu res b e tw ee n -1 0 an d 5 0 °C , b u t can o n ly b e u sed f ro m 1 0 to 3 2 °C (o n av e ra g e ) . I t i s a go od idea to le t you r p r in ter s tab i l ize at the am bien t t em pe ra tu re andh u m id i ty fo r sev e ra l h o u r s b e fo re o p e ra t in g i t . Relative humid ity can o f te n be a llow ed to rang e f rom 10 to 90% du r ing s to ra ge, bu t mu st be l imi te d to a ran ge o f40 to 70% dur ing operat ion . Keep in mind that humid i ty l imi ts are g iven as non-co n d en s in g v a lu es . Noncondensing m ea n s th a t y o u can n o t a l lo w w a te r v ap o r toco n d en se in to l i q u id fo rm . L iq u id w a te r i n th e l a se r p r in t e r w o u ld ce r t a in ly d amag e it s im ag e- fo rm at io n sy s t em .

Your printer also might specify physical shock o r vibration l imits to indica tethe amount of abuse the printer can sustain before damage can occur. Shock or vibration is usually rated in units of g-force. Ke ep in mind that laser p r in ters are remarkably delicate devices; any substantial shock or vibration might disturb theop t ics that d i rect the laser beam . LED pr in ters are a b i t more rugg ed , bu t a lso useop t ics that can be d am aged o r misal igned by rough hand l ing .

Physical informat ionPhysical information about a printer includes such routine data as the printer

heigh t , wid th , dep th , and we igh t . In some cases , an operat ing no ise level speci f icat ion i s included to ind icate jus t how loud the p r in ter wi l l be dur ing operat ion andstandby. Noise speci f ica t ions are usual ly g iven in dBA (A-weigh ted decibels) .

Typical assembliesNo mat ter how d iverse o r un ique EP p r in ters migh t appear f rom one model to

another, their d ifferences are primarily cosmetic. I t is true that each printer mightuse d i fferen t ind iv idual componen ts , bu t every laser p r in ter must perfo rm a verysimilar set of actions. As a result , most laser printers can be broken down into a se

ries of typical sections, or functional areas as shown in Fig . 1-3 . Before you trou-b leshoo t a p r in ter, you m ust unders tand the p urpos e o f each area .

The ac power supplyA n ac p o w er su p p ly i s u su a l ly a s im p le e l ec t ro n ic m o d u le th a t p ro v id es en

ergy fo r the fus ing assembly heaters and erase lamp assembly. There i s typ ical lyl i t t le that go es w ron g wi t h the ac supp ly un less a ser ious fau l t in the fuser o r er aseassem b l i e s d am ag es th e su p p ly. Yo u can l ea rn ab o u t p o w er su p p ly o p era t io n an drepai r in chap ter 6 .



Typical assemblies 7

Mainmotor

Writingmechanism

Scannermotor(optional)

Fusingassembly

* Paper ex i t sensor

* Fusing bulb

* Thermisto r

Eraselampassembly

* Top-cover in ter lock

* Main -motor d r iver

Main logic(dc control ler)

Fusing interlock

Interface/formatter

Control-panelassembly

High-voltagepowersupply

Paper controlcircuit

* EP car t r idge

sensor swi tches

* Registration

solenoid clutch

* Paper-out sensor

* Manual feed

EPcartridgeassembly

1-3 Diagram of a typical laser printer.

Th e dc pow er supp lyTh e dc (d i rec t cu rr en t ) p ow er supp ly conver ts main ac en ter ing the p r in ter in to

one o r more levels o f dc that are used to power the p r in ter e lec t ron ics and e lect romechan ical dev ices (such as motors and c lu tches) . L ike the ac supp ly, a dc powersupply is a fair ly ru gge d and reliable d evi ce un less a fault in som e part of the printercircuitry damages the supply. A cover in terlock in the dc supply shuts down printeroperat ion i f a p ro tect ive cover i s opened . You can learn abou t power supp ly operation and repair in chapt er 6 .

High-vo l tage power supp lyThe EP process re l ies on h igh-vo l tage (1 ,000 V or more) that i s used to create

and dissipate the powerful stat ic charges that move toner within an EP printer. Even

though specia l ized componen ts are used in h igh-vo l tage supp l ies , h igh vo l tagesplace a great deal of stress on electronic parts, so high-voltage supplies tend to fai lmore commonly than o rd inary ac o r dc supp ly modules . See chap ter 6 fo r more info rmat ion on power supp l ies .

Fusing assemblyImage s are de velo pe d on paper us ing a f ine pow de r ca l led toner. Th e tone r must

be f ixed to the paper (o therwise , the toner would smudge o r s imply b low off thep a g e ) . Heat and p ressure are used to fuse ton er to the pape r. Th e fusing asse mb ly




uses a se t o f two ro l lers in compress ion , where the top ro l ler i s heated to mel t thetoner. A p aper ex i t sensor detect s the passage o f paper th roug h the p r in ter, and thetherm is to r sensor is used to regu la te tem pera tu re in the hea ted fus ing ro l ler. Chapters 5 and 7 p resen t m ore d eta i led in fo rmat ion abou t the fus ing assembly.

Erase lamp assemblyTh e imag e that appears on a p r in ted p age has be en t rans ferred th ere as a la ten t

image wri t ten to a specia l pho tosensi t ive d rum. Each t ime the d rum ro ta tes , the laten t image mu st be erase d befo re a ne w image is wr i t ten . The erase lamps c lear thedrum thorough ly and a l low the pho tosen si t ive su rface to acce p t a ne w image . Eraselamp failure is usually easy to spot as you can see in cha pter 7 .

Main moto rEP printers rely on substantial mechanical activ ity. Paper must be drawn from a

supply tray, fed to the image form ation s ystem , fixed, the n fed to the output tray. Th emechanical force needed to support al l of these activ it ies is provided by a single motor and mechanical drive assembly. Chapter 8 describes mechanical systems in detail .

Wri t ing mechanismTh e data that mak es up an imag e must be t ransferred (o r "w ri t t en ") to the pho

tosensit ive drum. As you can see in chapter 5 , th is transfer is achieved by directinglight across the drum surface. For a laser printer, writ ing is accomplished by scann ing a laser bea m across a d rum. For an LE D pr in ter, the ligh t gene rate d b y ind iv idual micro scop ic L ED s (on e LE D for eve ry do t ) t ransfers image data to the d rum .Wri t ing is con tro l led by the main log ic assembly (o r E C P ) .

Scanner-motor assemblyWhen a laser i s used as a wri t ing mechan ism, the beam must be scanned back

and fo r th across the d rum surface . Th is scann ing p rocess uses a hexa gona l m irro rthat is ro ta te d wi th a motor. No te that scanners are no t nee de d fo r L ED pr in ters because there i s no beam to scan across the d rum . You can find m ore in fo rmat ion onscanner assemblies in chapter 8 .

Paper-control assembly

Paper must be g rabbed f rom the paper t ray, reg is tered wi th the la ten t image,passed th rough the im age fo rmat ion sys tem , fused , and passed ou t o f the p r in ter. Although the main motor turns constantly, not al l port ions of the paper handling system can be in mo t ion a t a ll t imes . Th e pap er con tro l assembly p rov ides the sensorsthat detect the p resence o f paper in the paper t ray, the p resence o f paper in themanual feed slot , and the sensit iv ity of each EP cartr idge for optimum printing. Inadd i t ion to sensors , the paper-con tro l assembly p rov ides the paper p ickup and registrat ion roller clutches that grab and register the page during printing. Chapter 8discusses the paper-handling assembly in detail .



Typical assemblies 9

Main log ic assemblyTh e main log ic assembly (o r e lec t ron ic cont ro l pa ck ag e— E C P) i s the hear t and

soul of your EP p rinter. Th e main logic assem bly has m ost of the circui try that op era tes the pr in te r, inc lud ing e lec t ron ics tha t communica tes wi th the computer andcontrol panel . Main logic is a lso responsible for checking and responding to inputprovid ed by a var ie ty of sensors . Problem s tha t occur in main log ic can range f romsubtle problems to major malfunct ions. Chapter 9 covers the main logic circui tsfound in EP printers .

EP cartridge assemblyTh e Elec t ropho tograp hic ( E P ) car t r idge of a p r in te r is a remarkab le p iece of en

gineer ing tha t combines the toner supply and much of the pr in te r image- format ionsys tem in to a s ing le , rep laceable car t r idge . By rep lac ing the EP car t r idge , you a l sorep lace de l ica te , wear-prone par t s such as the pr imary co rona , EP drum , and d eve loper ro l le r. The modular i ty of the car t r idge s impl i f ies the main tenance of yourprinter and improves i ts overal l rel iabi l i ty. The image-formation system is discussedin chapter 7.

Control-panel assemblyUsers must be able to interface with the printer to select various opt ions or op

era t ing modes . Not on ly do cur ren t cont ro l pane ls p rovide mul t i func t ion but tons ,bu t mos t a lso provide an LCD disp lay for p r in te r s ta tus and men u prom pts . The control panel is covered with main logic in chapter 9.



Mechanical parts 11

Drive motor



12 Typical components

Not only can gears transfer force, they also can alter speed and amount of forcethat is app lied at the seco nda ry shaft . Figu re 2-4 show s the effects of simple gear rat ios . Gear ratios are usually exp res sed as the rat io of the size of the primar y gear tothat of the secondary gear (or of the number of gear teeth on the primary to that ofth e seco n d a ry ) . F o r a high rat io , the primary gea r is larger than the secon dary gear.

As a result , the secondary gear will turn faster, but with less force. (For mechanicalparts that rotate, the force in the rotating parts is known as torque). Th e effect isjust the opposite for a low rat io . A sm all prima ry gea r will turn a larger seco nda ryslower, but with more force. Finally, an equal rat io causes a primary and secondarygear to turn at the same speed and force.

High ratio Low ratio Equal ratio

2-4 An exam ple of gear ratios.

PulleysPulley assem blies are com m on in man y laser printers . Like gears, the y are used

to transfer force from one point to another. Instead of direct contact , however, pulleys are jo in ed b y a drive linkage, wh ich is usually a belt , wir e, or chain . Th e ac tionis much the same as the fan belt in your automobile or the drive chain on your bicycle. A b asic pulley s et is show n in Fig . 2-5 . A m otor turns a drive pulley that is con -

2-5 Top view of a basic pulley set.



Mechanical parts 13

nected to a secondary pulley through a drive bel t that is under tension. As the drivepulley turns, force is t ransferred to the secondary through the l inkage, so the secondary pulley also turns. Notice that both pul leys turn in the same direct ion. A pulley/bel t configurat ion is sometimes used as a conveyor bel t to carry paper evenly tothe fusing rollers.

Pulleys and drive l inkages wil l vary depending on their part icular applicat ion.Low -force appl ica t ions can use nar ro w pul leys ( l i t t l e mo re than a wh ee l wi th agro ove in i t ) co nne c ted w i th a wi re l inkage . Wire is no t very rugge d , and it s contac tsurface area with both pul leys is small . Therefore, wire can sl ide when i t s t retchesunder tension or i f load becomes excessive. Belts and their pul leys are wider, sothe re is m uch m ore su rface contac t around ea ch pulley. Belts are usual ly s tron gerthan wire, so there is less tendency to s tretch under tension. Greater s t rength makesbel t-driven pulleys bet ter sui ted for heavier loads.

You can replace pul leys with sprocket wheels and a chain l inkage. Because eachchain l ink meshes with the sprocket wheels , any chance of s l ipping is e l iminated.Chains are almost immune to s tretching under tension, so chain drives are used tohandle the highest loads.

RollersRollers are real ly a focal point of laser pr inter oper at ion . Rollers not only serve

to grab a shee t of paper from the pap er t ray, but rol lers posi t ion (o r register) the paper before print ing. The image formation system uses several rol lers to dis tr ibutetoner and transfer the image to paper. By passing the paper through a set of heatedrol lers , the t ransferred image is fused to paper. Damaged, old, or dir ty rol lers mighthave an adverse effect not only on paper handling, but on overal l image qual i ty.

Reducing frictionAs with al l mechanical systems, parts that are in contact with one another wil l

wea r while the system ope rates due to unavoidable fr ict ion that occurs be tw een p arts .Therefore, reducing fr ict ion wil l extend the working l i fe of your printer. Lubricat ion,bushings, and bearings are three commonly accepted methods of reducing fr ict ion.

Use of oils or grease is on e wa y to reduc e fr ict ion (an d migh t prov e effect ive insmall do ses ) , bu t th is lubrication must be repla ced regular ly for i t to rema in effect ive. Otherwise, i t can wear away, dry out , or harden into thick sludge. Lubricantsalso are notorious for col lect ing dust and debris from the environment , which eventual ly defeats any benefi ts that the lubricant can provide.

Bushings are usual ly "thr ow -aw ay " we ar surfaces as show n in Fig. 2-6. A bushing is made of softer materials than the parts i t is separat ing, so any fr ict ion generated by moving parts wil l wear out the bushing before the other parts touch eachother. W he n a bushing wea rs out , s imply rep lace i t with a new o ne. Bushings aremuch less expensive and easier to replace than major mechanical parts such assl ides or frames. Today, bushing materials are rel iable , and can last throughout theworking l i fe of the printer.

Probably the mos t e ffec t ive devices for reduc ing f r ic t ion be tween par t s a rebearings. Bearing s consist of a hard meta l case with s teel bal ls or rol lers packed inside as shown in Fig. 2-7. Because each steel bal l contacts a load-bearing surface at




Roller

2-6A bushing assembly.

Frame

Shaft

Ball bearings

Case

Roller

2-7A b earing assembly.

only one point , fr ict ion (and wear) is substantial ly lower than for bushings. Unfortunately, bearing assemblies are often much more expensive than bushings, so bearings are used only to handle heavy loads, or in places that would be too difficult tochange bushings. Most laser printers avoid the expense of bearings in favor of inexpensive bushings.

Electromechanical componentsElectromechanical components are a part icular class of dev ices that conve rt

electrical energy into mechanical force or rotation. Relays, solenoids, and motors areth ree common elect romechan ical componen ts that you shou ld unders tand . Each o fthese important devices rel ies on the principles of electromagnetism.

ElectromagnetismW hen eve r e lec t r ica l cu rren t passes th rough a conducto r, a mag net ic f ie ld i s gen

erate d around the circu mfe renc e of that cond ucto r as show n in Fig . 2-8 . Such a ma gnetic f ield can exe rt a physical force on permeable materials (an y materials that can

be m agn et ize d) . The s t reng th o f a magn et ic f ie ld around a condu cto r i s p ropor t ional

Magneticforcelines

Metalconductor

Direction ofcurrent

• /

2-8 Th e magnetic field formed around a conductor.



Electromechanical components 15

to the amount of current f lowing through i t . Higher amounts of current resul t instronger magnetic f ie lds, and vice versa.

Unfortunately, i t is vir tual ly impossible to pass enough current through a typicalwire to pro du ce a ma gnetic f ie ld th at is s t rong en ou gh to do any useful work. T hema gnet ic fie ld mus t som eho w be concen t ra ted , usua lly by coiling the wi re as shownin Fig. 2-9. When arranged in this way, the coi l takes on magnetic poles—just l ike apermanent magnet . Not ice how the d i rec t ion of magnet ic f lux a lways po in ts to thenorth po le of the co i l. I f the d i rec t ion of cur ren t f low wer e reversed , the m agnet icpoles of the coi l a lso would be reversed.

Coil Magnetic force field

2-9 Th e concentration of magnetic force in a coil.

To concen t ra te mag net ic forces even fur ther, a perm eab le core mater ia l can beinserte d into the co i l cent er as in Fig. 2-10. Typical ly, i ron, s teel , and cobal t are considered the classical core materials , but i ron-ceramic blends are used as well . Coilsof wi re such as these form the founda t ion of a l l e lec t rom echan ica l dev ices .

2-10 Concentrating magnetic force with a permeable core.

RelaysA relay i s s imply a mecha nica l swi tch tha t i s ac tua ted wi th the e lec t rom agn et ic

force generated by an energized coi l . A diagram of a typical relay is shown in Fig.2-11. Th e swi tch (or contact set) can be norm al ly open ( N O ) or normal ly c losed(NC) whi le the co i l i s de-energ ized . When ac t iva ted , the magnet ic f ie ld of the co i lcauses normal ly open contac ts to c lose , o r normal ly c losed contac ts to open . Con-




Contacts

2-11A schematic view of relayconfigurations.

Normally NormallyOpen (N O ) Closed (N C )

tacts are held in their actu ated posit ions as long as the coil is ene rgiz ed. If the co il isturned off, con tacts will return to their norm ally ope n or closed states. Ke ep in mindthat a coil might drive m ore than one set of conta cts.

Relays are not always easy to recognize on sight. Most relays used in electroniccircuits are hou sed in small rectangu lar containers of metal or p last ic. Lo w -p ow er relays are available in oversized IC (integrated circuit) sty le packages and solderedright in to a PC (printed circuit) board just l ike any other in tegrated circuit . Unlessthe relay in ternal d iagram is printed on i ts outer case, you will need a printerschematic or manufacturer 's data for the relay to determine the proper input andoutput functions of each relay pin.

SolenoidsSolenoids conver t e lec t romagn et ic fo rce d i rect ly in to mot ion as shown in F ig .

2-12. Unlike ordinary electromagnets whose cores remain fixed within a coil , a solenoid core is al lowed to f loat back and forth without restr ict ion. When energized, the

ma gne tic f ield gen era ted by a coil exe rts a forc e on its cor e (c alle d a plunger)thatpushes it out from its rest position.

Coil body

2-12A typical solenoid.

If left unrestrained, a plunger would simply shoot out if its coil and fall away.Plung ers are usually retain ed by a spring or some o ther sort of mechan ical return assembly. Being retained, a p lunger extends only to some known distance when thecoil is fired; then it automatically returns to its rest position when the coil is off.

Solenoids are com mo nly used as clutches in laser printer paper-handling sys tems .Remember that the motor providing mechanical torque in the printer is constantlyrunning while a page is being p rinted. Ho wev er, the pape r-grab and registrat ion rollersneed only turn briefly during the printing cycle. Solenoid "clutches" are used to engage and d isengage the motor to rque as needed .



Electromechanical components 17

Mo to rsMotors are an essential part of ev ery laser printe r ma nufac tured today. Mo tors

operate the en t i re mechan ical t ranspor t sys tem. Chap ter 8 p resen ts a deta i led d iscuss ion o f mech an ical sys tems and serv ice . For now, concen t ra te on the motor itself.

All motors conver t e lec t r ica l energy in to ro ta t ing mechan ical fo rce ( to rque) . In

turn, that force can be distr ibuted with mechanical parts to turn a roller or move abelt . An induction mo tor p rov ides to rque th rough a ser ies o f powerfu l e lec t rom agnets (co i ls ) around a permanen t magnet co re as shown in F ig . 2 -13 . The core(known as a rotor) is l i t t le m or e than a shaft that is free t o rotat e as i ts pole s encoun ter e lec t rom agne t ic fo rces . Each co i l (a lso cal led a phase or phase winding) isbuil t in to the motor stat ionary frame (or stator).

2-13 Simplified vie w of a typical induction motor.

By powering each phase in i ts proper order, the rotor can be made to turn withsom e amount o f fo rce . Th e amount o f angu lar fo rce g ene rate d by a mo tor i s know nas torque. Induct ion motors general ly requ ire two ac s ignals separated by a 90 degree phase difference. These sinusoidal driving signals vary the strength of eachphase even ly to create smooth ro ta t ion . Induct ion motors are rare ly used in today ' scom merc ia l p r in ters because they do no t lend themse lves to the p recise posi t ion ingrequ irem ents of mo st printers. Instead , a close cousin of the induc tion mo tor is used ,and the motor is called a dc servo motor.

The dc servos are powered by bipolar dc signals rather than by sinusoidal ac.Like the induction motor, a dc servo is turned by bipolar driver signals in rapid succession. Although dc servos are not very precise for posit ioning, they can hold a con

stant speed very accurately. This characterist ic makes dc servos ideal for driving thescanner mirrors in laser printers that must rotate at a precise rate.

A popular variat ion of the dc servo motor is called a stepping motor. Physically,a s tepp ing motor (o r stepper) is ver y similar to a dc serv o, but the rotor/statorarrangem ent i s mu ch mor e in t r ica te . L ike dc servos , s tepp er m otors are d r iven by aseries of square wav e pulses sepa rated b y a phas e differenc e. T he sudd en shift indr ive s ignals coup led wi th the in t r ica te mecha n ical ar rangem ent causes the ro to r tojump (o r s tep ) in cer ta in angu lar increments , no t a smooth , con t inuous ro ta t ion .On ce the r otor has reac hed i ts next step , i t wil l hold its posit ion as long as driver sig-




nals maintain their condit ions. If driver signals hold steady, the motor could remainstationary indefinitely. A typical stepping motor can achieve 1 .8 degrees per step,which mean s a mo tor must mak e 200 (360 deg rees d iv ided by 1.8 de gr ee s) ind iv idual s teps to com plete a s ing le ro ta t ion . Howe ver, gear ra t ios can b reak d own m otormo vem ent in to much f iner d iv is ions .

S tepp ing motors are ideal fo r p recise posi t ion ing . Because the motor moves inknown angu lar s teps , i t can be ro ta ted to any posi t ion s imply by app ly ing the app ropr ia te ser ies o f d r iver pu lses . For e xam ple , suppose you r mot or had to ro ta te 180 degrees. If each step equals 1 .8 degrees, you need only send a series of 100 (180degrees d iv ided by 1 .8 degrees) pu lses to tu rn the ro to r exact ly that amount . Log iccircuits in the printer generate each pulse, then driver circuits amplify those pulsesinto the high-power signals that actually operate the motor. Chapter 9 explains theoperat ion and repai r o f p r in ter e lec t ron ics .

Passive componentsIf you intend to do any subassembly or component-level troubleshooting of laser

printer systems, you must have an understand ing of the various electronic com pon entsthat are available. Most circuits contain both active and passive comp onen ts work ingtogether. Passive components include resistors, capacitors, and inductors. They arecalled passive bec aus e their only pu rpo se is to store or dissipate circuit energy.

Ac t ive com pone n ts mak e up a b road er g roup o f semicond ucto r-based par ts suchas diodes, transistors, and all types of in tegrated circuits. They are referred to as active because each com pon en t uses c i rcu it energy to per fo rm a specif ic se t o f funct ions — the y a l l do someth ing . A com pon en t migh t be as s imple as a rec t i f ier o r ascom plex as a microproc essor, bu t ac t ive par ts are the key e lem ents in m ode rn e lec

t ron ic c ircu i ts . Th is par t o f the chap ter sh ows you each gen eral typ e o f com pon en t ,how t hey w ork , how to read thei r mark ings , and how th ey fa il .

ResistorsAll res is to rs ever made serve a s ing le purpose—to d iss ipate power in a con

trolled fashion. Resistors appear in most circuits, but they are usually used for suchthings as voltage division, current l imiting, etc. Resistors dissipate power by presen t ing a res is tance to the f low of cu rren t . Wasted ene rgy is then shed b y the res istor as heat . In printer logic circuits, so l i t t le energy is wasted by resistors thatv i r tual ly no temperatu re increase i s detectab le . In h igh-energy c i rcu i ts such as

power supplies, resistors can shed substantial amounts of heat . The basic unit of resistance is the ohm. Th e sym bol fo r res is tance i s the Greek symbol omeg a ( Q ) . Youwil l see res is tance a lso p re sen ted as k i lohms ( k f t o r thousands o f oh m s) o r m egoh m s(M£2 or mill ions of ohms).

Carbon-film res is to rs , as shown in F ig . 2 -14 , have largely rep lace d car bon-composit ion resistors in most circuits requiring through-hole resistors. Instead of carbonfil ling, a ve ry pr ecise lay er of carbon fi lm is app lied to a th in ceram ic tube . Th e th ickness o f th is coat ing affects the amount o f res is tance—thicker coat ings y ie ld lowerlevels of resistance, and vice versa. Metal leads are at tached by caps at both ends,



Passive components 19

Leads

2-14Cross-sectional view of acarbon-film resistor.

Metal capEpoxy covering

Resistive layer

Ceramic core

and the entire f in ished assembly is d ipped in epoxy or ceramic. Carbon-fi lm resistorsare general ly more a ccurate than carbon com posi t ion res is to rs becau se a f ilm can b edeposi ted very p recise ly dur ing manufactu re .

A surface-mount resistor is sho wn in Fig . 2-15. A s with carbon-fi lm resistors,surface-m ount resistors are form ed b y depos it ing a layer of carbon film ont o a th inceramic substrate. Metal tabs are at tached at both ends of the wafer. Surface-mountres is to rs are so ldered d i rect ly on the top o r bo t tom s ides o f a p r in ted c i rcu i t boardinstead of using leads to penetrate the PC board. Surface-mount resistors are incredibly small devices (only a few square null imeters in a r e a ) , ye t they o ffer veryt igh t to lerances . Surface-mount res is to rs are used ex tensively in computers andprinters.

Resistive layer

Ceramic substrate

2-15Highly magnified vie w of asurface-mount resistor.




Adjustable resistors, known as potentiometers or rheostats, are usually use d inlaser printers to adjust contrast by varying the level of h igh voltage. A typical potentiometer consists of a movable metal wiper rest ing on a layer of resist ive fi lm. Although the to tal resistance of the fi lm, end-to-end, will remain unchanged, resistancebe tw een e i ther end and the wip er b lade wi l l vary as the wipe r is m ove d . Th ere are tw o

typical types of adjustable resistor: knob-type, where the wiper is turned clockwise orcoun terc lockwise us ing a ro ta t ing metal shaf t , o r s l ide- type, where the wiper i sm ov ed b ack and forth in a straight l ine.

In addit ion to value and tolera nce, resistors also are rated by th eir power-han dlingcapacity. Pow er is norm ally me asur ed in watts (W ) and dep end s on the amount of current (7) and voltage (V ) applied to the resistor as given by Ohm's law (P = IxV]. R esistors are typically manufactured in Ks, , H, t 1, 2 , and 5 W sizes to handle a widevariety of p ow er condit ions. Size is d irectly related to p ow er dissipation abil ity, so largerresistors usually can handle more po w er than a smaller resistor of the sam e value.

As long as power dissipation is below its rat ing, a resistor should hold i ts resistance value and perfo rm indef in i te ly. Ho we ver, wh en a res is to r i s fo rced to exc ee d i t spo we r ra t ing , i t canno t shed heat fas t enough to main ta in a stab le temp eratu re . Ul t imately, the resistor will overheat and burn out. In al l cases, a burned-out resistorforms an open circuit . A faulty resistor might appear sl ightly discolored, or i t mightappear burned and cracked . I t rea l ly depends on the sever i ty and durat ion o f i t soverheat ing . Ex treme overheat ing can burn a p r in ted c i rcu i t board , and poss ib lyd am ag e th e p r in t ed co p p er t r aces .

Fai lu res among po ten t iometers usual ly take the fo rm of in termi t ten t connect ions between the wiper b lade and res is t ive f i lm. Remember that f i lm s lowly wearsaway as the wiper moves back and fo r th across i t . Over t ime, enough f i lm can wearaway that the wiper canno t make good con tact a t cer ta in po in ts . The poor con tact

can cause al l types of erratic or in termittent operation. With EP printers, i t is rarelynecessary to con t inual ly ad just p r in t ing con tras t onc e op t im um leve ls are found , soit is unlikely for adjustable r esistors to w ear out in laser printer s, but dust an d debr ismigh t co l lec t and cause in termi t ten t operat ion when ad jus tment i s needed . Tryclean ing an in termi t ten t po te n t iom eter w i th a h igh-qual ity e lec t ron ic con tactc leaner. Rep lace any in termi t ten t po ten t iometers o r rheosta ts .

Reading resistorsEvery resistor is marked with i ts proper value. Marking allows resistors to be

iden t i fied on s igh t and compa red versus schem at ics o r part layou t d raw ings . Now thatyou k now w hat res is to rs look l ike , you shou ld know h ow to iden t ify thei r value w i thout having to rely on test equ ipm ent. Th er e are three way s to mark a resistor: ex plici tmarking, color coding, and numerical marking. I t is important to decipher al l threetypes of marks because many circuits use resistors with a mix of marking schemes.

Exp lici t marking is just as the nam e imp lies— the actual value of the part is wr itten right on the part . Large, ceramic power resistors often use explici t marking. Theirlong, rectangular bodies are usually large enough to hold clearly printed characters.

Co lor cod ing has long been a popu lar mark ing schem e fo r carbon-f i lm res is to rsthat are s imply too smal l to ho ld exp l ic i t mark ings . The twelve co lo rs used in co lo r




codin g are show n in Tab le 2-1. T he first ten colors (black thr oug h wh ite ) are used asvalue an d multiplier colors. Silver and go ld colors ser ve as tolerance indicators.

Tab le 2-1. Th e standard resistor color code

Colo r 1st Band 2nd Band Mu ltip lier ToleranceBlack 0 0 1Brown 1 1 10Red 2 2 100Oran ge 3 3 1,000Yellow 4 4 10,000Green 5 5 100,000Blue 6 6 1,000,000

Violet 7 7 10,000,000Gray 8 8 100,000,000White 9 9

(Non e) ± 20%Silver ± 10%Gold ± 5%

The color code approach uses a series of colored bands as shown in Fig . 2-16.Band num ber 1 is always loca ted c losest to the end of the resistor. Bands on e andtwo are the value bands, and band three is the multip lier. A forth band (if present)will be si lver or gold to indicate the resistor to lerance. On rare occasions, you mightenc oun ter a f if th band that indicates th e rel iabil ity of a resistor (an d is used on ly formil itary - and aerospace-grad e re s is to rs ) .

/

,Value band 1

/ /Value band 2

^ Multiplier band

1 HIMBI

2-16Resistor color-coding scheme.

As an example of color coding, suppose the resistor of Fig . 2-16 offered a colorsequence o f b rown, b lack , and red . Note f rom Tab le 2 -1 that b rown=l , b lack=0 , andred=100 (because the red band occup ies the multiplier p o s i t i o n ) . T h e seq u en cewo uld be re ad as [band l] [ba nd 2] x [band 3] or 1 0 x 100, or 1 ,000 (1 k f t ) . I f thefirst three color bands of a resistor read red, red, orange, the resistor would be readas 2 2 x 1,000, or 22,000 cl (22 k Q ), and so on .

W hen a fo rth band is found , i t shows the res is to r to leranc e. A go ld ban d re presents an exc ellen t to lera nce of ± 5 % of rated value. A si lver band repre sen ts a fair to lerance o f ±1 0% , and no to lerance band ind icates a poo r to lerance o f ±20 % . W he n afaulty resistor must be replaced, i t should be replaced with a resistor of equal orsmal ler to lerance whenever poss ib le .




Color-coded res is to rs are rap id ly being rep laced by su rface-mount (SM) res isto rs . SM resistors are far too small for clear color coding. Instead, a three-digit numerical cod e is used ( ev en thou gh you migh t nee d a smal l magn ify ing g lass to see i t ) .Each digit corresponds to the first three bands of the color code as shown in Fig .2 -16 . Th e f irs t two nu mb ers are value d igits , and the th ird nu mb er is the multiplier.

The mul t ip l ier d ig i t ind icates how many p laces to the right that the value's dec ima lp lace must be sh i f ted . For the ex am ple o f F ig . 2 -17 , a num erical code o f 102 den o tesa value o f 10 wi th 2 zeros ad ded on to make the num ber 1 ,000 (1 k Q ) . A m ark ing o f331 is rea d as 330 Q, and so on .

Metalleads

2-17 Surface-mount resistor markings.

CapacitorsCapaci to rs are s imply energy-s to rage dev ices . Th ey s to re ener gy in the fo rm of

an e lect r ica l cha rge . By them selves , capaci to rs have l i t t le p ract ica l use , bu t the capaci to r p r incip le has impor tan t app l ica t ions when combined wi th o ther componen tsin f i lters , resonan t o r t iming c i rcu i ts , and po we r supp l ies . Capa ci tance i s m easu redin farads ( F ) . In ac tual p ract ice , a farad i s a ver y large amount o f capaci tan ce , somost norm al capaci to rs measu re in the micro farad (u F o r mi l lion ths o f a farad ) an dpicofarad (p F or mill ionths of a mill ionth of a fara d) ran ge.

In principle, a capacitor is l i t t le more than two conductive plates separated by aninsulator (called a dielectric) as show n in Fig . 2-18. T he amount of capa citance provid ed by this typ e of assembly de pen ds on the area of each plate, their d istance apart ,and the dielectric ma terial that separ ates them . Ev en larger values of capa citance canbe crea ted b y roll ing up a plate/dielectric a ssem bly and housing i t in a cylinder.

W he n vo l ta ge i s app l ied to a capa ci to r, e lec t ron s wi l l f low in to i t un t i l i t i s fu l lych arg ed . A t th a t p o in t , cu r ren t s to p s f lo w in g ( ev en th o u g h v o l t ag e m ig h t s t i l l b eap p l i ed ) , an d v o l t ag e ac ro ss th e cap ac i to r w i l l eq u a l i t s ap p l i ed v o l t ag e . I f ap p l i edv o l t ag e i s r em o v ed , t h e cap ac i to r w i l l t en d to r e t a in th e ch a rg e o f e l ec t ro n s d epos i ted on i t s p la te s . Jus t ho w long i t can do th is depe nd s on the speci f ic ma ter ia ls used to const ruct the capaci to r, as wel l as i t s overal l s ize . In ternal res is tanceth ro u g h th e d i e l ec t r i c m a te r i a l w i l l ev en tu a l ly b l eed o ff an y ch a rg e . F o r th e p u rposes o f th is book , a l l you real ly need to remember i s that capaci to rs are bu i l t tos to re e lec t r ica l charge .




Leads

Top view Side view

2-18 Sectional vie w of a conven tional plate capacitor.

Th ere are gene ral ly two ty pes o f capaci to rs that you shou ld be famil iar wi th . T hety p es can b e ca t eg o r i zed as fixed o r electrolytic. A se lect ion o f capaci to r typ es isshown in F ig . 2 -19. F ix ed cap aci to rs are nonpo lar ized d ev i ces — the y can be inser tedinto a circuit regardless of their lead orientation. Many fixed capacitors are assembled as small wafers or d isks. Each conductive plate is typically aluminum foil . Common d ie lect r ics include paper, mica , and var ious ceramic mater ia ls . The completeassem bly is then co ate d in a hard plast ic, epo xy, or ceram ic housing to kee p out hu-

Aluminumelectrolytic

capacitor(axial)

Aluminumelectrolyticcapacitor(radial)

1 fMylar

capacitor(radial)

Ceramicmonolithiccapacitor(axial)

Tantalumelectrolyticcapacitor

(radial)

Ceramicmonolithiccapacitor

(DIP)

Surface-mountcapacitor

Ceramicdiskcapacitor

2-19 Outlines of various capacitor typ es.




midi ty. Large r capaci to rs can be assem bled in to large , herm et ical ly sealed can is ters .F ixe d capaci to rs a lso are des igned in su rface-mount fo rm.

Elect ro ly t ic capaci to rs are po lar ized componen ts—they must be inser ted in to aci rcu i t in the p roper o r ien ta t ion wi th resp ect to the ap p l ied s ignal vo l tag e . Ta n ta lumcapaci to rs are o f ten found in a d ipped (o r teardrop) shap e, or as small canisters.

Aluminum elect ro ly t ic capaci to rs are usual ly used in general -purpose app l ica t ionswhere po lar ized dev ices are needed . The d i fference between f ixed and e lect ro ly t iccapacitors is primarily in their materials, but the principles and purpose of capacitance remain the sam e.

Capacitors also are designated as axial o r radial dev ices , wh ich ind icates thecapaci to r lead conf igurat ion . When bo th leads emerge f rom the same end o f the capacitor, the device is said to be radial. If the leads eme rg e from eithe r side, the capacito r is kn ow n as axial.

Surface-mount capacitors are usually f ixed ceramic devices using a dielectriccore ca ppe d by e lec t rod es a t bo th ends . I f an e lect ro ly t ic capaci to r i s nee de d , a surface-mount tan ta lum dev ice i s typ ical ly used . Al though the const ruct ion o f a su rface-m ount tantalum capa citor d iffers substantial ly from a cera m ic surface-m ountcapaci to r, they bo th appear v ery s imilar to the unaided e ye. Al l po lar ized capaci to rsare marked with some type of polari ty indicator.

L ike res is to rs , most capaci to rs tend to be rugged and re l iab le dev ices . Becausethe y only store ener gy (no t d issipate i t) , i t is v irtually imposs ible to burn the m ou t.You can dam age o r des t ro y capaci to rs i f you exc ee d thei r work ing vo l tag e ( W V ) ra ting o r rever se the o r ien ta t ion o f a po lar ized d ev ice . Dama ge can occu r if a fai lu reelsewhere in a circuit causes excessive energy to be applied across a capacitor, or ifyou should install a ne w electr olytic c apa citor inc orrectly .

Reading capacitorsLike resistors, al l capacitors carry markings that identify their value. Once you

unders tand the mark ings , you can determine capaci to r values on s igh t . Capaci to rsare typ ically marke d in tw o ways : exp l ic it mark ing and num erical cod es .

Explicit marking is used with capacitors that are phy sically large enoug h tocarry thei r p r in ted value . Larg e, ceramic d isk , mylar, and e lect ro ly t ic capaci to rs haveplenty of surface area to hold readable markings. Note that al l polarized capacitors,regardless of size, must show which of their two leads are posit ive or negative. Becer ta in to pay c lose a t ten t ion to po lar izer mark ings whenever you are tes t ing o r replacing capacitors.

Smal l, nonpo lar ized capaci to rs and many s izes of su rface-mount capaci to rs n owmak e use o f num erical cod ing schem es. The pat tern o f num erical mark ings is easy tofollow because i t is very similar to the marking technique used with numericallycoded res is to rs . A ser ies o f th ree numbers i s used—the f i rs t two numbers are thevalue d ig it s , and the th ird num ber i s the mul t ip l ier d ig i t (h ow many ze ros are a dd edto the value digits) . Capacitor numerical marking is shown in Fig . 2-20.

Most capac i to r numerical mark ings are based on p icofarad me asurem ents . Thus ,the capaci to r mark ed 150 wou ld be read as a value o f 15 wi th no zeros ad ded (15p F ) . A m ark ing o f 151 is 15 wi th one zer o adde d (1 50 p F ) . The m ark ing 152 would




2-20Reading the numerical markingson a capacitor.

1 5 1 150 pF = 0 . 00 01 5 M-F

1 5 2 1 5 0 0 p F = 0 . 0 0 1 5

1 5 3 15 00 0 pF = 0 . 01 5 M-F

be 15 wi th tw o zeros add ed (1 ,500 pF ) , and so on . A m ark ing o f 224 wo uld be 22 wi thfour zeros (220 ,000 p F ) . Th e decim al p lace is a lways sh i f ted to the r igh t .

Al thou gh th is mark ing sy s tem is based on p icofarads , ev ery value can be expressed as micro farads (u F ) s imp ly by d iv id ing the p icofarad value by 1 mi l l ion . Fo rexa m ple, a 15 p F capa citor could be called a 0 .000015 uF (15/1,000 ,000) c apacitor.Of cours e, the re is no adv anta ge in ma rking such a small cap acitor in the micr ofaradrange wh en 15 pF is such a conve n ien t value , bu t the con vers ion is a s imple on e. T he15,000 pF capacitor also could be shown as 0 .015 uF (15,000/1,000,000). Capacitorswi th large p icofarad values are o ften e xpre ssed m ore effec t ively as micro farads . T oconf i rm your es t imates , you can mea sure the capa ci to r wi th a capaci tanc e meter.

Inductors

Like capaci to rs , inducto rs are energy-s to rage dev ices . Unl ike capaci to rs , inducto rs s to re energy in the fo rm of a ma gnet ic f ie ld . Before the in t roduct ion o f in tegra ted circuits, indu ctors ser ved a ke y role wit h capacito rs in the form ation of f i ltersand resonan t ( tu ne d) c i rcu i ts . Al tho ugh ad vances in so l id -s ta te e lec t ron ics h avema de indu cto rs v i rtual ly obso le te in t rad i t ional app l ica t ions , they rem ain invaluab lefor high-energy circuits such as power supplies. Inductors also are used in transfo rmers , motors , and re lays . Inductance i s measured in henrys ( H ) , bu t smal ler inducto rs can be found in the mi l l ihenry ( m H ) o r micro henr y ( u H ) range.

Coils are available in many shapes and sizes as shown in Fig . 2-21. Th e part iculars ize and shape wil l dep end on the amoun t o f energ y that must be s to red , and the m ag

netic characterist ics desired. Laser printers use coils in their ac, dc, or h igh-voltagepower supplies. Small coils also are available in surface-mount or leaded packages.A transformer i s ac tual ly a com binat ion o f inducto rs work ing in tande m. As F ig .

2 -22 shows, a t ransfo rmer i s com pos ed o f th ree impor ta n t e lem ents : a primary (o rinpu t) wind ing , a secondary (o r ou tp u t) wind ing , and a core s t ructu re o f som e typ e.Transfo rme rs are used to a lter (o r t rans fo rm ) ac vo l tage and curren t levels in a c i rcuit , as well as to isolate one circuit from another. An ac signal is applied to the primary winding. Because the magnitude of th is input signal is constantly changing, thema gne tic f ield i t ge ne rat es will cons tantly f luctuate as we ll . W he n this f luctuating




Encapsulatingmaterial \

Coil wraps

Core

Coil '

Conventional coil

2-21 Two typical inductor designs.

Core

Torroid coil

Core.

. \ \ \ \ \ \ \ \ \ \ \ \ \ \ % \ \ \ \

Current flow (7)-

Primarywinding

Secondarywinding

2-22 Diagram of a transformer.

field in tersects the secondary coil , another ac signal is created (or induced) acrossit . This principle is known as magnetic coupling. An y second ary ac s ignal wi l l dup l ica te the o r ig inal s ignal. P r imar y and second ary w ind ings are o f ten wou nd aroundthe same core s t ructu re that p rov ides a common s t ructu re and eff ic ien t magnet iccoup l ing f rom pr imary to secondary.

The actual amount o f vo l tage and curren t induced on a secondary co i l dependson the ra t io of the number o f p r imary wind ings to the num ber o f second ary w ind ings .This relat ionship is called the turns ratio. I f the secon dary co i l has m ore w ind ingsthan the p r imary co i l , then the vo l tage induced across the secondary co i l wi l l begreate r than the p r imary vo l ta ge . For exa m ple , if the t ransfo rm er has 1 ,000 p r ima rywindings and 2 ,000 secondary windings, the turns rat io is 1,000:2,000 ( 1 : 2 ) . With 10Vac app l ied to the p r imary, the second ary wi l l ou tpu t roug h ly

101

2

or 20 Vac

Such an ar rangement i s known as a step-up t ransfo rmer. If the s i tuation w er e reversed where the p r imary co i l had 2 ,000 wind ings wi th 1 ,000 wind ings in the sec-



Active components 27

or 15 Vac

This t ransfo rmer i s know n as a step-down t ransformer.Curren t a lso i s s tep ped in a transfo rmer, bu t opp osi te to the p ropor t ion o f v o l t

age s teps . If vo l tage i s s tep ped dow n by the facto r o f a tu rns ra t io , cu rren t is s tep pedup by the same factor. This relat ionship ensures that power out of a transformer isabou t equal to the po we r in to the t ransfo rmer.

Because inducto rs are energy-s to rage dev ices , they shou ld no t d iss ipate anypower by themselves . However, the wire res is tance in each co i l , combined wi th natural magnetic losses in the core, does a l low some po w er to be los t as heat . Hea tbu i ldup is the lead ing cause o f inducto r fa i lu re . Long- term exposure to heat caneven tual ly b reak down the tough enamel insu la t ing each wind ing and cause a shor t

circuit . Short circuits lower the overall resistance of the core so i t draws even morecurren t . Breakdown accelera tes un t i l the co i l i s des t royed .

Act ive componentsDiod es , trans is to rs , and in teg rated c i rcu its make up a mu ch b road er and m ore

powerfu l g roup o f active co m p o n en t s . S u ch co m p o n en t s a re r e fe r red to a s activebecause each par t ac tual ly does someth ing . Act ive componen ts use c i rcu i t energy toaccompl ish speci f ic , p ract ica l funct ions . The nex t par t o f th is chap ter p rov ides anoverv iew of common act ive componen ts , and shows you what each family o f par tscan do.

DiodesTh e c lass ical d iode i s a two- term inal sem icondu cto r dev ice that a llows c urren t

to f low in one direction (polarity) on ly, bu t no t in the o ther. Th is one-w ay p r ope r tyis know n as rectification. As dem onst r a ted in chap ter 6 , rec t i f ica t ion i s abso lu te lyessen t ia l to the bas ic operat ion o f ever y po we r supp ly.

Diod es are available in a w ide array of case styles as shown in Fig . 2-23. T he sizeand materials used in a diode case will depend on the amount of current that must becarried. Glass-cased diodes, normally made from sil icon, are generally used for low-p o w e r ( o r small signal) applications. Plast ic or cera mic cased diod es are typica llyused for low or medium power applications l ike power supplies, circuit isolation, orinductive flyback protection. Diodes also are available in surface-mount packages. Adiode has two terminals . The anode is its po sitiv e terrriinal, and the cathode is its ne gative terminal. Note that a d iode cathode is always marked with a solid str ipe or bar.

Whenever you work wi th rect i f ier d iodes ( regard less o f the case s ize o r s ty le) ,you shou ld be conc ern ed wi th tw o major d iode speci f ica t ions: fo rwa rd curren t (7 f)and peak inverse vo l tage ( P I V ) . C h o o se r ep lacem en t d io d es w i th 7 f and PI V values

ondary, the turns rat io would then be 2 ,000:1,000 ( 2 : 1 ) . If you a pp ly 30 Vac to t hepr imary, the secondary ou tpu t would be



28 Typical componen ts

Anode v , Cathode

Schematic symbol

Plastic-cased diode

2-23A sampling of diode case styles.

Glass-cased diode

I T U Surface-mount diode

that closely match the part to be replaced. When a si l icon diode is forward biased asshown in Fig . 2-24, the diode develops a constant voltage drop of about 0 .6 Vdc. Theremainder of applied voltage will drop across the current-l imiting resistor. Because adiode dissipates power, i t is important that you choose a current-l imiting resistorthat will restr ict forw ard curre nt to a safe leve l . If you do not restr ict curr ent to a safelevel , the d iode can be des t royed by excess heat .

Appliedvoltagesource

Forward-bias voltagedrop of 0.6 Vdc

tForwardcurrent

Current-limitingresistor

2-24 A forward-biased diode circuit.

A reverse-biased diode, such as the one shown in Fig . 2-25, acts much l ike anopen sw i tch— no curren t i s a l lowed to f low in the c i rcu i t . Th is character is t ic a lsodemonstrates the essential principle of rectif ication; d iode current will f low in only




Diode

Appliedvoltagesource

+Applied voltage

appears acrossreversed diode Currentlimitingresistor

2-25 A reverse-biased diode circuit.

one d i rect ion . Whatever vo l tage i s app l ied across the d iode wi l l appear across i t .

E v en i f t h e r ev e r se v o l t ag e l ev e l w ere in c reased , t h e d io d e w o u ld n o t co n d u c t . H o wever, the re are limits to the amou nt of rev ers e volta ge that a d io de can take. Th e l imitis called PIV. I f rever se-vo l ta ge e xce ed s PPV, the d iod e junct ion can rup tu re and fa ilas ei ther an open or short circuit . Typical PIV ratings can easily exceed 200 V.

Al though rect i f ier d iodes are no t mean t to be operated in the reverse-b iasedcond i t ion , the zener d iode i s a specia l species des igned exclus ively fo r reverse b iasing . F igure 2 -26 shows a common zener d iode c i rcu i t . No t ice the un ique schemat icsymbol used fo r zener d iodes . When app l ied vo l tage i s below the zener breakdownvol tage ( typ ical ze ner d iodes o pera te a t 5 , 6 , 9 , 12, 15 , o r 24 V d c ) , vo l tag e across thezener d iode wi l l equal the app l ied vo l tage , and no curren t wi l l f low in the d iode. Asap p l i ed v o l t ag e ex ceed s th e zen e r b reak d o w n v o l t ag e , cu r ren t b eg in s to f lo wthrough the d iode and vo l tage across the zener remains c lamped a t the zener level(that is, 5, 6, 9, 12 Vdc, e t c . ) . Any add i t ional app l ied vo l tage i s then d ropped across

Outputvoltage

A

Remainder of appliedvoltage is droppedacross the resistor

Zener diode

Current

limitingresistor Zener voltage isclamped across the

zener diode+Appliedvoltagesource

2-26 A basic zener diode regulator circuit.




the current-l irnit ing resistor. As long as applied voltage exceeds the zener breakdown vo l tage , zener vo l tage wi l l remain constan t . Th is zener ac t ion makes zenerdiodes perfect as simple regulators, and this action is the basis for most methods ofl inear voltage regulation.

Keep in mind that i t i s impossib le to d i fferen t ia te between rect i f ier and zenerd iodes by thei r ou tward appearance—both types o f d iodes appear iden t ica l in everyway. The only way you can tel l the two types of d iodes apart is to look up the part icu lar dev ice in a cross- referen ce manual , look a t the represe n ta t ion o f the com pon en tin a schematic, or look at the device si lk screening on i ts PC board. Rectif ier-typed iodes are typ ical ly labeled wi th a D p ref ix ( that i s , D32 , D27 , D3 , e t c . ) , bu t zenerd iodes o f ten use Z or ZD p ref ixes ( that i s , ZD5 , ZD201, e tc . ) .

Similarly, it is impossible to discern a faulty diode simply by looking at it, unlessthe d iode has bee n des t r oyed b y some sudden , severe over load . Such over loads arevirtually non existen t in periph erals such as printers, so you m ust use test instru men tsto confirm diode condit ion. Test instruments are discussed in the next chapter.

In a l l semiconducto r dev ices , e lec t rons must b r idge a semiconducto r junct ion

during opera tion. By mo difying the cons truction of a junc tion and encapsulating i tinside a diffuse plast ic housing, electron s mov ing acros s a jun ction w ill l iberate p hotons of v isible (o r infrar ed ) l ight . This action is the basic principle beh ind l igh t-em itt ing d iodes ( LE D s) . An LE D is shown in F ig . 2 -27 . No t ice that an LED is l i tt le m orethan a d iode—the wavy arrows ind icate that l igh t i s moving away f rom the dev ice .Al ter ing the chemical composi t ion o f LED mater ia ls wi l l a l ter the waveleng th o f

2-27Typical LED symbol andpackage styles.




emit ted l igh t ( i .e . yel low, o range, red , g reen , b lue , in f rared , e t c . ) . L ike o rd inaryd io d es , LEDs are in tended to be fo rward b iased , bu t LED vo l tage d rops are h igher(0 .8 to 3.5 V d c ) , and LED s o f ten requ ire 10 to 35 m A (mil l iam pe res) o f cu rren t togenerate the op t imum amount o f l igh t .

T h ere a re tw o o th e r d io d e -b ased d ev ices th a t y o u sh o u ld b e fam i l ia r w i th : t h eS C R ( s i l ico n -co n t ro l l ed r e c t i f i e r ) an d th e t r i ac . Yo u m ig h t en c o u n te r e i th e r o fthes e e lem ent s in you r p r in ter po w er supp ly. A n SCR is show n in F ig . 2 -28 . N ot i cethat SCRs are th ree- terminal dev ices . In add i t ion to an anode and a cathode, agate t e rm in a l is ad d ed to co n t ro l t h e S C R . A n o rd in a ry d io d e tu rn s o n w h e n e v eri t i s fo rward b iased . An SCR also must be fo rward b iased , as wel l as triggered b yap p ly in g a p o s i t i v e v o l t ag e (o r trigger signal) t o th e g a te . O n ce t r ig g e red , an S C Rwil l con t inue to condu ct as long as cu rre n t is f lowing th rou gh the SCR . Re m ovi ngthe gate vo l tage wi l l no t s top the SCR from conduct ing once i t has s tar ted . I f cu rren t s to p s a f t e r t h e g a te v o l t ag e i s r em o v ed , t h e S C R w i l l h av e to b e r e t r ig g e red .B ecau se S C R s a re th ree - t e rm in a l d ev ices , t h ey a re o f t en p ack ag ed in a f a sh io nv er y s im il a r t o tr an s i s to r s . Mo s t l o w o r m ed iu m p o w e r S C R s a re ev en p a ck ag ed in

su r face -m o u n t p ack ag es .

A n o d e

;j SOT-223jliiiiliiililiiii:: surface-mounti I I ii_J packag e

2-28 SCR schematic symbol and package styles.



32 Typical componen ts

A triac behaves very much l ike an SCR, but a tr iac can be tr iggered to conductcurrent in either d irection through the device instead of only one direction as in theSCR. Figure 2-29 shows a typical tr iac. Notice that a tr iac is g iven TWO anodes because i t can conduct in both directions. A tr iac will conduct once tr iggered by a voltage applied to i ts gate lead. The tr igger voltage must be the same polari ty as thevoltag e across the tr iac. For ex am ple, if the re is a posit ive voltag e from A l to A2 , thetrigger voltag e must be posit ive. If the re is a nega tive voltag e from A l to A2 , the tr igger voltag e must be nega tive. Once tr ig gere d, a tr iac condu cts until current stops flowing. Afte r cu rrent stops, the tr iac must be tr ig ger ed again befor e i t wil l cond uct again .

Anode 1

Gate

Anode 2

Triac in a 2-29TO-92 Triac schematic symbol andpackage package styles.

Triac in anSOT-223surface-mountpackage

Diode mark ingsAll diodes carry two very important markings as shown in Fig . 2-30: the part

number and the cathode mark ing . The cathode mark ing ind icates the cathode (o rnegat ive) d iode lead . Because d iodes are po lar ized dev ices , i t i s cr i t ica l that youknow which lead i s which . Otherwise , i f you incorrect ly rep lace a d iode, you migh tcause a circuit malfunction.

i Cathode

i m i 31^ 2-30i m i 31^ Typical diode markings

Part number 1

Unlike pass ive comp onen ts , d iode par t numbers con ta in no tang ib le in fo rmat ionon a diod e per form anc e specifications or l imits. Instead , the part num ber is an ind exor refe ren ce num ber that al lows you to look up the part icular specification in a ma n-




ufacturer 's or cross-reference data book. Classical d iode part numbers begin withthe p ref ix IN , fo l lowed by anyw here f rom one to four d ig i t s . Th e IN pre fix is use dby the JEDEC (Jo in t E lect ron Dev ices Eng ineer ing Counci l ) in the Uni ted S ta tes toind icate dev ices wi th one sem icondu cto r junct ion (d io de s) . C lass ical Japanese d iodepar t numbers beg in w i th the p ref ix 1SS, w h e r e SS m e a n s small signal.

You will almost certainly encounter d iodes with many unique and arcane markings. Fortunately, the re are m any clues to gu ide yo u along. Your first clue will be th ewhi te (o r g ra y) c a thode ban d— at leas t you can iden ti fy the par t as a d iode. The second identif ier will be the si lk-screen let tering on the PC b oard. D iodes a re usually assigned D or ZD number ing p ref ixes to den o te a rec t i f ier- type o r zener d iode. Onc eyou are confident that you hav e identif ied a diod e, use a cros s-refer enc e in dex to lookup the suspect part replacement or equivalent. The specifications you find for theequivalent part in a cross-reference manual will closely (if not exactly) match thosefor the original part . There are many semiconductor cross-references available.

TransistorsA transistor is a th ree- termina l semicondu cto r dev ic e wh ose ou tpu t s ignal i s d i

rectly controlled by i ts input signal . With passive components, a transistor can beconfigured to perform either amplification or switching tasks. Unfortunately, there isjust not enough room in th is book to discuss the theory and characterist ics of transistors, but i t is important that you know the most important concepts of transistors,and understand their various uses.

There are two major transistor families: b ipolar and FET (field-effect transist o r ) . B ipo lar t rans is to rs are com m on, inexpen sive , general -purpo se d ev ices that canbe made to handle amplification and switching tasks with equal ease. The three leadsof a bipolar transistor are the base, emitter, and collector. In most applications, thebase serves as the transistor input—that is where the input signal is applied. Theemitter is typically t ied to grou nd (usu ally throu gh one or m ore values of resist a n c e ) , and the collector pro vide s the outpu t signal . T he transistor also can be configured with the input signal supplied to the collector, the base is grounded, and theoutput appears on the emitter.

Th ere are two species o f b ipo lar t rans is to r : N P N and PN P For an N P N t ransisto r, base and co l lec to r vo l tages must be posi t ive (wi th respect to the emit ter ) . Asbase voltage increases, the transistor is turned on and current begins to f low fromcollector to emitter. As base voltage increases further, the transistor continues toturn on and allow more current in to the collector unti l the transistor f inally satu

rates. A satu rated transistor cannot be turn ed on any further. A P N P trans istor requ ires nega t ive base and co l lec to r vo l tage s (w i th resp ect to the em it te r ) to cause thet ransis to r to tu rn on and conduct cu rren t . As base vo l tage becomes more negat ive ,the transistor turns on harder unti l i t saturates. By far, NPN transistors are morecom mo nly used in smal l e lec t ron ics .

When a bipolar transistor is used in a switching circuit (also known as a drivercircuit) , the arrangement is often simpler as shown in Fig . 2-31. Unlike a transistoram plifier w ho se outp ut signal varies in direct p rop ortio n to its input, a switch ing circu i t i s e i ther on o r o ff—ver y mu ch l ike a mech an ical swi tch . Laser p r in ters make extensive use o f swi tch ing c i rcu its to operate L ED s, so leno id c lu tches , and motors . T he



34 Typical compon ents

+Vdc

Inputsignal

R l•o 2-31

A sim ple transistor driver circuit.~ Vdc IX

- L = J 0 VdcOutputsignal

typical c om pu ter svritching circuit uses a digital signal from an IC as the contro l signal . W he n the contro l signal is at a logic 0 , the transistor (a nd i ts loa d) re ma ins off .W he n a logic 1 signal is sup plied to the d river, the transistor turns on fully (satur a t e s ) . Current f lows through the load, in to the collector, through the transistor tothe emitter, then to ground. A base resistor is added to l imit current in to the base

from the signal source. An addit ional resistor might need to be placed in the collector circuit to limit current if the load is too large.

Phototransistors are a unique variat ion of bipolar transistors. Instead of an electrical signal being used to control the transistor, photons of l ight provide the basesignal. Light enters the phototransistor through a clear quartz or p last ic window onthe transistor body. Light that str ikes the transistor base l iberates electrons that become base curren t . The more l igh t that en ters the pho to t ransis to r, the more basecurren t i s p roduced , and v ice versa . Al though pho to t ransis to rs can be operated asl inear amplifiers, they are most often found in switching circuits that detect the absence or presence of l ight .

Al though pho to t ransis to rs can detect l igh t f rom a wide var ie ty o f sources , i t i snormal to use an LE D to supp ly a known, constan t l igh t source . Wh en a p ho to t ransistor and LED are matched together in th is way, an optocoupler (o r optoisolatof)i s fo rmed as shown in F ig . 2 -32 . No t ice th e ne w schem at ic sym bol fo r a pho to t ransistor. The wavy l ines indicate that l ight is entering at the base. Optocouplers areused in p r in ters to detect the p re sen ce o r absence o f paper, o r to dete ct wh eth er thepr in ter enclosures are c losed complete ly. Optocoup lers a lso can be fabr icated together on the same integrated circuit to provide circuit isolation.

©

LED source Phototransistorreceiver

©

Optoisolatorassembly

2-32 Cross-sectional diagram of an optoisolator.




Fie ld -effect t rans is to rs (F E T s) are const ructe d in a rad ical ly d i fferen t fash ionfrom b ipo lar t rans is to rs . Al though FETs make use o f the same basic mater ia ls andcan operate as e i ther ampl i f iers o r swi tches , they requ ire b ias ing componen ts o fmuch h igher value to se t the p roper operat ing cond i t ions . FETs are e i ther N-chan-nel o r P -channel dev ices as show n in F ig . 2 -33 . Th e d i fferen ce in t rans is to r type s depends on the mater ia ls used to const ruct the par t icu lar FET. An FET has th reeterminals: a source, a gate, and a drain. Th es e leads corre spon d to the emit ter,b ase , and collector of a b ipolar transistor. The gate is typically used for the input orcon tro l s ignal. Th e source i s norm al ly t ied to g round (som et im es th rou gh one o rm ore values o f res is tan ce) , and the d ra in supp l ies the ou tpu t s ignal.

N-channel FE T P-channel FE T

Drain Drain

Gate

Source Source

2-33 FET Schematic symbols.

W hen no vo l tage i s app l ied to an FE T g ate , cu rren t f lows f reely f rom drain tosource . Any necessary curren t l imi t ing must be p rov id ed b y inser t ing a res is to r in series in to the drain or source circuit . By adjusting the control voltage at the gate, cur

ren t f low in the d ra in and source can be con tro l led . For an N-channel FET, con tro lvo l tag e must be a nega t ive vo l tage . As gate vo l tage i s ma de m ore neg at ive , ch annelcurren t is res tr ict ed furthe r unti l i t is cut off entirely. Fo r a P-ch anne l FE T, a po sit ivecon tro l vo l tage is ne ede d . Higher p osi t ive gate vo l tage res t r ic ts channel cu rren t fu rther until the channel is cut off.

A v a r ia t ion o f th e F E T is t h e MO S F E T (m eta l -o x id e sem ico n d u c to r F E T ) . It isun l ikely that you wi l l ever encoun ter d iscre te MOSFET dev ices in your smal l computer, bu t many soph is t ica ted d ig i ta l in tegra ted c i rcu i ts mak e ex te nsive use o f MOS -FETs. One o f the few undesi rab le character is t ics o f FETs and MOSFETs is thei rsensi t iv i ty to damage by e lect ros ta t ic d ischarge (ESD). You can learn abou t ESD inchapter 4 .

Th ere are a var ie ty o f e lec t r ica l speci f ica t ions that descr ibe the perfo r ma nceand characterist ics of part icular transistors. When you discover that a transistormust be replaced, i t is always wise to use an exact rep lac em ent par t . Tha t way, youare su re the rep lacement par t wi l l behave as expected . Under some c i rcumstances ,how ever, an exac t rep lacem ent p ar t migh t take too long to get , o r no t even be avai lable. You can then use manufacturer 's or cross-reference data to locate substi tu teparts with specifications similar to the original part . Keep in mind that substi tu tingpar ts wi th a d i fferen t pa r t— ev en wh en speci f ica t ions are ve ry s imi lar— migh t ha vean unforeseeab le impact on the c i rcu i t . Do no t a t tempt to use "c lose" rep lacement




parts unless you have a kee n unde rstand ing of transistor princ iples and specifications, and you know exact ly w hat the par t d oes in a c i rcu it .

Transistors are available in a wide variety of case styles and sizes depending onthe po we r that must be han d led . F igure 2 -34 show s a se lect ion o f f ive popu lar cases ty les . Low-power, general -purpose dev ices are o f ten packaged in the smal l , p las t icTO-92 cases . The TO-18 metal case a lso i s used fo r low-power dev ices , bu t the TO-18 shown houses a pho to t ransis tor. N ote the quar tz win dow on the case top that a llows l igh t to en ter the dev ice . For regu lar t rans is to r app l ica t ions , the TO-18 " tophat" case is all metal.

TO-18 case(low power)

TO-92 case phototransistor(low pow er) opening shown

TO-220 case(medium tohigh pow er) SOT-223

surface-mountpackage 2-34

Typical transistor case styles.

SOT-143surface-mount

package

U Q.

rr~nMe d iu m -p o w er t r an s i s to rs u se th e la rg e r p l a s ti c TO -1 2 8 o r TO -2 2 0 cases . T h e

TO -1 2 8 u ses a th in m e ta l h ea t s ink m o ld ed in to th e to p o f th e d e v ice . T O -2 2 0cases use a large m etal moun t ing f lange/heat s ink loca ted d i rec t ly beh in d t hep las t ic case . T he f lange p r ov id es mec han ical s t reng th , as we l l as a secu re t herm alpath fo r an ex ternal heat s ink . An a l l -meta l TO-3 case (no t shown) i s used fo rh ig h -p o w er t r an s i s to r s . Tw o m o u n t in g h o les a re p ro v id ed to b o l t t h e d ev ice to achass is o r ex ternal heat s ink . Usual ly, case s ize i s p ropor t ional to the power capaci ty o f the t rans is to r.

Transistors also can be ma nufac tured in surface-m ount cases. Tw o typical surface-mount small-outl ine transistor (SOT) case styles are shown in Fig . 2-34. Due to




their small size, SMT transistors cannot dissipate very much power, but they areideal for small , low-power systems such as computers or printers.

Transistor markingsAs shown in Fig . 2-35, a transistor part number is merely an index or reference

number that al lows you to look up the equivalent components or specifications for apart in a data book or cross -refere nce manual. Th e num ber i tself contains no usefulinformation about the actual performance characterist ics or l imits of the part . Classical b ipolar transistor part numbers begin with the prefix 2N , fo llow ed by up to f ivedigits. The 2N p ref ix i s used by JED EC in the Uni ted S ta tes to deno t e dev ices w i thtw o sem iconduc to r junct ions . C lass ical Japanese t rans is tor par t num bers beg in wi thany of four prefixes: 2 SA ( h ig h - f r eq u en cy P N P t r an s i s to r ) , 2S B ( l o w - f req u en cy P N Ptransis to r) , 2S C ( h ig h - fr eq u en cy N P N t r an s i s to r ), an d 2S D ( l o w - f r e q u e n c y N P Ntransis to r) . JEDEC uses the p ref ix 3N t o d en o te F E Ts o r Ju n c tio n F E T s ( J F E Ts ) .T h e p re f ix es 2S J ( P -c h a n n e l J F E T ) , 2SK ( N - c h an n e l J F E T ) , a n d 3S K ( N o r P chan

nel MOSFETs) have been used in Japan .

JEDECtransistor

prefix

Part numbersuffix(up to five digits)

2-35 JEDEC transistor markings.

You also migh t encou nter m any transistors with arcane or nonstanda rd m arkings.In almost all cases, you can identify repla cem ent transistors and look up perfo rm anc especifications using manufacturer 's data or a cross-reference guide. Although thespecifications found in a cross-reference guide are for the replacement parts, theywill gene rally ma tch the original part specifications ve ry closely.




As with diodes, transistors rarely show any outward signs of fai lure unless theyhave mel ted o r shat tered f rom an ex t reme over load . You general ly must use tes tequ ipm ent to iden t ify fau l ty t rans is to rs . Test ing can be accom pl ished b y measu r ingthe dev ice whi le the c i rcu i t i s runn ing , o r removing the dev ice f rom the c i rcu i t andmeasuring i ts characterist ics out of circuit .

Integrated circuitsIn tegra ted c i rcu its ( IC s) are by far the most d iverse and powerfu l g roup o f e lec

t ron ic componen ts that you wi l l ever deal wi th . They have rap id ly become the fundamental bu i ld ing b locks o f modern e lect ron ic c i rcu i ts . Ampl i f iers , memories ,microprocessors, d igital logic arrays, oscil lators, t imers, regulators, and a myriad ofother complex functions can all be manufactured as ICs. Circuits that only a decadeago would have re qu ired an en t i re PC board in a p r in ter are now b eing fabr icated ona s ing le IC . Al thou gh you can o f ten es t imate the com plex i ty (and im por tan ce) o f aninteg rate d circuit from the num ber of p ins that i t has, i t is v irtually impos sible to p redict precisely what an IC does just by looking at i t . You will need a schematic of thecircuit or man ufacturer 's data of a part icular IC to de ter m ine w hat the IC do es.

Every in tegrated c i rcu i t—whether analog o r d ig i ta l—usual ly i s made up o f microscopic transistors, d iodes, capacitors, and resistors that are fabricated onto an ICdie . Many capaci to rs and inducto rs canno t be fabr icated on ICs, so conven t ionalparts can be at tached to an IC through one or more external leads. Your printer is almost en t i re ly a d ig i ta l sys tem . That i s , mo st o f the ICs are des ign ed to w ork w i th b inary s ignals . The microprocessor, memory, and most o f the con tro l ler ICs in yourlaser p r in ter, are d ig i ta l log ic com pon en ts . O ther ICs , how ever, are in ten ded t o w orkwith analog signal levels. Serial communication ICs and driver ICs are only some analog de vice s that you w ill f ind in a printer.

A logic gate is a circuit that prod uc es a binary result based on one or mo re binary inputs. A sing le in te gra ted circuit can hold as fe w as one logic gate or tho usand sof log ic gates depend ing on the complex i ty o f the par t icu lar IC . There are e igh t bas ic t y p e s o f l og i c g a t e s; A N D , O R , N A N D , N O R , I N V E R T E R , B U F F E R , X O R ( e x c l us ive O R ), and XN O R (exclu s ive N O R ). Each gate uses i ts ow n par t icu lar log ic sym bolas shown in Fig . 2-36. Beside each symbol is the truth table for the part icular gate.A t ru th tab le shows gate ou tpu t fo r every poss ib le combinat ion o f inpu ts . For thesake of simplici ty, no more than two inputs are shown, but individual gates can havefour, eight, or more inputs.

Flip-flops a re s l igh t ly mo re invo lve d than gener al -purpo se gates , bu t they are

such flexible logic building blocks, that f l ip-flops are usually considered to be logicgates. In the simplest sense, f l ip-flops are memory devices, because they can "remember" the various logic states in a circuit . Flip-flops also are ideal for workingwith logical sequ enc es. You will often find flip-flops used aroun d coun ter-t im er circuits. T he re are thre e classical varia tions of flip-flops: t he D flip-flop, th e SR flip-flop,and th e JK flip-flop.

As you look over the symbols in Fig . 2-36, you might notice that some of the inpu ts have a bubble a t the dev ice . W he ne ver you see a bubb le , i t ind icates ac t ive- lowlog ic . In conven t ional (ac t iv e-h ig h) B oolean log ic , a true s ignal ind icates an O N condit ion, or the p res en ce o f a volt ag e. In act ive-lo w logic, a true signal is OFF , and v olt-




sembly techn iques . Through-ho le assembly requ ires that a componen t be inser tedso that i t s leads p ro t rude th rough the PC board . The leads are so ldered in to p lacew h ere each lead p ro t ru d es . H o w ev er, t h e t en d en cy to p ack m o re p o w er fu l IC s o n tosmal ler PC boards has g iven r i se to an overwhelming number o f su rface-mount ICpackage styles. Figure 2-37 shows a small sampling of typical package styles.

Quad small-outlinepackage(QSOP)

2-37 Typical IC packa ge styles.

One o f the key advan tages o f su rface-moun t pa ckages i s that comp onen ts can beso ldered on to both s ides o f a PC bo ard — thu s, a lmost doub l ing the amoun t o f avai lab le PC b oard area . Th e SOIC (sma ll-outl ine IC ) design appear s ve ry similar to a DIP,but the SOIC is significantly smaller, and i ts f lat (or gull-wing shaped) pins arespaced together very c losely. The VSOP (ve ry smal l-ou t line pa ck ag e) and the QSOP(quad smal l -ou t l ine package) are des igned to package complex ICs in to ex t remelysmall packages with leads on two or four sides. Quad packages also are square (instead of rectan gu lar) w ith pins on all four sides. SOJ ( smal l -ou t l ine J- lead) packag es

rep lace regu lar gu l l -wing leads wi th leads that are ben t d ow n and unde r the dev icein a J sha pe.

F la t packs tend to be large , square ICs used p r imar i ly fo r more soph is t ica tedfunct ions such as microprocessors and specia l ized con tro l lers . A QFP (qu ad flatpack) and a TQFP ( th in quad flat pa ck ) offer as ma ny as 100 pins (25 pins pe r s i de ) .TQ FP s are handy in ex t rem ely t igh t spaces whe re regu lar QFP s migh t get in the w ayof o ther assembl ies . Ch ip carr iers are e i ther leaded (wi th leads) o r un leaded (wi tho u t l ead s ) . LL C ( lead less ch ip carr ie rs) s imply p ro v ide exp ose d con tacts that requ ire a ch ip -carr ier socket to guaran tee p roper connect ions . PLCC (p las t ic lead ed




chip car r ie r ) packages offe r J - shaped p ins tha t can e i ther be sur face mounted d irec t ly to a PC board or inser ted in to a ch ip-car r ie r socke t . Th e PG A (p in gr id a r ray )i s one of the mos t sophis t ica ted packaging schemes in use today. PGAs can providehundreds of leads on an IC. Sophist icated microprocessors such as Intel ' s 80486 arepacka ged in PG As e xcee din g 150 p ins . PG As a l so requi re the use of socke ts to en

sure proper contact for al l pins. Ta k e e x t r e m e c a r e t o p r e v e n t d a m a g e t o p i n sw h e n in s e r t i n g a n d e x t r a c t i n g P G A s .

I t is exc eptio na l ly rare for any typ e of IC to show out war d signs of fai lure, so i t isvery important for you to check careful ly any suspect ICs using appropriate testequipment and data while the IC is actual ly operat ing in the system. Gather al l theinformat ion you can about an IC before rep lac ing i t , because IC rep lace me nt a lwayscarr ies an element of r isk. You r isk damage to the PC board during IC removal , anddamage to the new IC during instal lat ion.



C H A P T E R

Soldering and

test instrumentsUp to now, you have seen th e features and sp ecifications of laser printers, with a variety of typical subassemblies. You know about their major electrical and mechanicalcomp onen ts (F ig . 3 -1 ) . Th is chap ter in t roduces you to the too ls and tes t equ ipmen tneeded for successful laser printer troubleshooting. If you are unfamiliar with testequ ipment , take some t ime and read th is chap ter carefu l ly. Exper ienced t rou-blesh ooters can skip th is chapter, but migh t nee d to refer to i t later for refe ren ce.

3-1 A Tandy LP400 laser printer.

4 2



Small tools and materials 43

Small tools and materialsI t might sound strange, but hand tools can often make or break your repair ef

fort . If you have ever started a repair and been delayed by one missing screwdriver,pliers , o r wren ch (you kn ew i t was there las t w e ek ) , then you know ho w much t im e

and frustrat ion can be sav ed by gath ering th e prop er tools before beg inn ing—not after. Check y our too lbox n ow If you d o not have a too lbox just ye t , no w is a go od t imeto consider the supplies and tools you need for mechanical and electronic repairs.

Hand toolsScrewdrivers are always go od tools to beg in with . Figu re 3-2 show s five types

of screw heads. A healthy variety of both regular and Phil l ips-type screwdrivers willgo a long way for basic assembly, d isassembly, and adjustment tasks. Avoid excessively large or unusually small scre wd rivers unless the re is a specific n ee d for th em .Several short-shaft screwdrivers can come in handy when working in confined ar

eas— prin ter assembl ies can be very densely packaged . Al len-head (hexag onal ho le)screws also are common, so include a set of Allen keys in your toolbox. Watch forspecialty screws. Spline and Torx-type screws are growing in populari ty as manufacturers seek to keep untrained personnel out of their equipment. Large mail-orderand retail hardw are stor es usually stock Sp line and Torx drivers .

Regular

Consider th ree t ypes of p liers. A gard en var iety pair of mechanic 's p liers is usefulfor keeping t ight hold on any nut, bolt , or o ther p esky p art . Tw o pairs of ne edle-n ose

pliers (one short nose, one long nose) will round out the collection. Needle-nose pliers are great for grabbing and holding parts in tight spaces.

A set o f smal l , e lec t ron ics-g rade, open-end wrenches work wel l to ho ld smal lnu ts dur ing assembly and d isassembly. W ren che s shou ld have th in bod ies ( fo rt igh t spaces) and shou ld be below Me inch (o r 8 mm for me tr ic s i ze s) . If you havethe cho ice between metr ic o r U.S . wrenches , get metr ic . Most laser p r in ters arema de in the Paci f ic Rim ( that i s , Japan , Kor ea , S inga pore , Hong K ong , and so o n)wh ere m etr ic par ts are s tandard . A smal l ad jus tab le wr en ch is a lways a go od addit ion to a toolbox.



44 Soldering and test instruments

Because you are work ing w i th e lect ron ic sys tem s, have on hand two s izes o f wirecu t ters (one smal l and one m ed ium ) to cu t jum per wires o r t r im rep lacem ent component leads. Include a separate wire str ipper to remove insulation from wires orcomponents. Resist the temptation to str ip insulation using wire cutters. Even if insulation is rem ov ed successfully, cutt ing blad es can often leav e a nick or pinch in the

conductor that might later fat igue and break. If you plan to make or repair connecto rs , you might need special crimp or insert ion tools to do the job. Use your judgment regard ing exact ly what too ls you need fo r conn ecto r repair.

Any wiring or circuit work is going to require a soldering iron. Invest in a goo d-quali ty solde ring pencil —in the range o f 20 to 25 W (30 W ma xim um ). Avo id heavy-duty, h igh-power soldering guns. Th ey migh t wo rk just fine for p lum bers, but thatmuch power can eas i ly des t roy p r in ted c i rcu i t boards , wire insu la t ion , and components. Always be sure to have one or two spare soldering t ips on hand.

Rem em ber that so lder ing irons exce ed 500°F a t the t ip . Not on ly are these temperatu res a serious hazard to your p ersonal safety, but the y are a serious fire hazardas well . Be sure to park a hot soldering iron in a strong, wire-frame holder. NEVERleave an iron unattended on a bench or table.

MaterialsTo use a soldering iron, you also are going t o nee d an ample supply of solder. U se

a 60% tin , 40% lead (60/40) solder containing a rosin cleaning agen t. N ev er us e pasteflux containing acids or solvents, or use solder containing ac id flux (so m etim escalled acid core or plumber's solder). Harsh so lven ts des t roy del ica te compo nen tleads and circuit traces .

A spool of hook-up wire can serv e a varie ty of uses ranging from prin ted c ircuitrepair and wire splices, to make shift test leads. Solid wire (18 to 20 gauge) is ofteneasiest to work w ith , but stranded w ire can be u sed just as we ll . You might f ind i t easier to keep several smaller spools of d ifferent-colored wire so you have color-codedwire for d ifferent purposes (for example, red for dc voltage, b lue for dc ground, andyel low fo r signal w ire s) .

Include a set of alligator leads. Alligator leads are available in a selection oflengths, colors, and wire gauge s. Alligator leads are handy for temp orary jum ping of signals during a repair, or to test the reaction of another signal in a circuit. A set of smallalligator leads and a set of large alligator leads will cov er you u nder m ost circum stance s.Individual alligator clips make excellent heat sinks to conduct heat away from c om ponents when you solder or desolder.

Heat-shrink tubing prov ides quick, clean, and effect ive insulation for exp ose dwiring or splices. Most general-purpose electronics supply stores sell heat shrinktubing in 3 foot length s, or in pack ages of assorted sizes. Use a heat gu n to shrink thetubing, al though you could use a blow drier. Heat-shrink tubing is available in a selection of colors.

These tools and materials are only a few more common supplies that you willneed to get s tar ted . Th e l is t i s by no means co mp lete . Hund reds o f general and special purpose tools are available to help you in your repairs—far too many to covercomplete ly. Ex per ienc e i s your bes t gu ide in decid ing wh ich too ls and mater ia ls arebest for you.



Soldering 45

SolderingSolder ing i s the mos t comm only used meth od of connec t ing wires and com po

nents in an electr ical or electronic circui t. In soldering, meta l surfaces (co m po nen tleads, wires , or pr inted circui t t races) are heated to high temperatures, then joined

with a layer of compatible molten metal . When done correct ly, soldering forms a lasting, corrosion-resis tant , intermolecular bond that is mechanical ly s trong and electr ical ly sound. All that is needed is an appropriate soldering iron and electronics-grade(60/40) solder. This sect ion covers both regular soldering and surface-mount soldering.

Solder ing backgroundBy str ict defini t ion, soldering i s a p rocess of bondin g meta l s toge ther. Th ere a re

thr ee dist inct types of soldering: brazing, s i lver soldering , and soft soldering. Brazingand si lver soldering are used for hard or precious metals , but soft soldering is thetechnique of choice for e lec t ron ics work .

To bond wire and compo nen t leads ( typ ica l ly made of cop pe r ) , a th i rd meta lmus t be adde d wh i le in it s mo l ten s ta te . The b onding meta l is known s im ply as sol

der. Seve ral differen t typ es of solde r are avai lable to han dle eac h solderin g technique , bu t the chosen so lder m us t be compa t ib le wi th the molecu la r charac te r i s ticsof the meta l s to be bon ded — oth erw ise a bond wi l l no t form. Lead and t in a re tw ocom mo n, inexpens ive meta l s tha t adhe re very w el l to copper. Unfor tuna te ly, n e i thermetal by i tself has the s trength, hardness, and melt ing-point character is t ics to makethem useful . Therefore, lead and t in are combined into an al loy. A rat io of approximately 60% t in and 40% lead yields an al loy that is pl iable , has reasonable hardness,and a low m elt ing poin t that is ideal for electron ics work . This mixtu re is the solde rthat you should use.

Al tho ugh so lder adheres very w el l to copper, i t does n o t adhere w el l a t a ll to theoxides tha t form na tura l ly on copper. Al though conductors might look c lean andc lear, som e amount of ox id iza t ion i s a lways presen t . Rem ove oxide s before so lder ingto ge t a goo d bond . Ap ply a ros in c lean ing agent (ca l led flux) to condu ctors beforeso lder ing . Al tho ugh ros in is chemica l ly inac t ive a t room tem pera tu re , it bec om es ext remely ac t ive when hea ted to so lder ing tempera tures . Act ive f lux combines wi thoxide and str ips i t away, leaving clean copper surfaces for molten solder. As a completed solder joint cools , any residual rosin cools and returns to an inact ive s tate .Ne ver use ac id or so lvent -based f lux to prepa re condu ctors . They can c lean away o xides as we ll as rosin, but acids and solvents rem ain act iv e af ter th e join t cools . Ov er

t ime , the res idue wi l l d i sso lve copp er wi res and conne c t ions , and eventua l ly cause acircui t fai lure. You can ge t rosin flux as a paste that can be brushe d into con du ctorsbefo re solderin g, but m ost electron ic solders have a cor e of rosin ma nufa ctured r ightinto the solder s trand itself. The rosin cleans the joint as solder is applied. Suchrosin-core so lder i s much more convenien t than workin g wi th f lux pas te .

Irons and tipsA soldering iron is l i t t le more than a resis t ive heat ing element bui l t into the end

of a long steel tube as show n in the cross-se ct ional d iagram of Fig. 3-3. W he n ac vol t-




3-3 Cross-sectional view of a typical soldering iron.

age is app lied to the heater, it wa rm s the base of a t ip . An y heat con du cted do wn thecool-down tube is d issipated harmlessly to the surrounding air. Dissipating the heatkeeps the hand le temp eratu re lo w enough to ho ld comfor tab ly.

Most of the heat is chan neled into a soldering t ip similar to the one s how n in Fig .3-4. Tips often have a core o f cop pe r that is p late d with iron. Th e t ip is coated with alayer of n ickel to stop high-temperature corrosion, then plated with chromium. A

chromium coating renders the t ip nonwettable —solder will not stick. Be cau se solder must st ick at the t ip end, that end is p lated with t in . A t in coating mak es the endwettable. Tips are available in a w ide ran ge of shapes and sizes. Be fore y ou select thebest t ip for the job, you must understand ideal soldering condit ions.

3-4 Soldering iron tip.

The very bes t e lec t ron ic so lder ing connect ions are made wi th in on ly a narrowspan o f time and temp eratu re . A so lder jo in t heated b etw een 500 to 550°F fo r 1 to 2seconds will make the best connections. You must select your soldering iron wattageand t ip to achieve these condit ions. The entire purpose of soldering irons is not tome lt solder. Instead, a soldering iron is suppo sed to d eliver he at to a jo i nt — the jointwil l mel t so lder. A larger jo in t (wi th m ore num erous o r larger condu cto rs ) requ ires alarger iron and t ip than a small jo int (w ith fe we r or sma ller con du cto rs) . If you use asmall iron to heat a large jo int , the jo int can dissipate heat faster than the iron candel iver i t , so the jo in t migh t no t reach an accep tab le so lder ing temperatu re . Conversely, using a large iron to heat a small jo int w ill over hea t the jo int . Ov erhe atingcan melt wire insulation and damage printed circuit board runs. Match wattage tothe application. Most general-purpose electronics work can be soldered using an ironwith a rat ing below 30 W.



Soldering 47

Becau se the en d of a t ip actually contacts t he jo int to be so ldere d, i ts shape andsize can assist heat transfer. Wh en he at must be ap plied across a wid e area (such asa wire splice), a wide area t ip should be used. A screwdriver (or flat blade) tip suchas show n in Fig . 3-5 is a go od choic e. If heat must be dire cte d with pinp oint accur acyfor small , t ight jo ints or prin ted c ircuits, a narrow blade or conical t ip is the best . T wotips for surface-mount deso lderin g also are shown in Fig . 3-5 .

Width Width

Flat tip (scr ew dr iver ) Conical tip

Width

Surface-mount desoldering tip Surface-mount desoldering tipfor small-outline ICs for chip com ponents such as

resistors, capacitors, andtransistors

3-5 An assortment of conventional and surface-mount tips.

SolderingS AF ET Y FIR ST — alw ay s park your soldering iron in a secure ho lder while it is on.

Never allow a hot iron to sit freely on a table or anything flammable. Make it a pointto ALW AYS w ear safety glasses wh en soldering. Activ e rosin or mo lten solder can easily f lick off the iron or jo int and d o perm anen t d am age to the t issue in your e yes .

Give your soldering iron plenty of t ime to warm up (5 minutes is usually ade

q u a t e ) . Once the i ron i s a t i t s work ing temperatu re , you shou ld coat the wet tab leportion of the t ip with fresh solder ( th is is known as tinning th e i r o n ) . Rub the t ipinto a spon ge soak ed in clean wa ter to wip e away any accum ulations of debris or carbon that might have formed, then apply a th in coating of fresh solder to the t ip end.Solder penetrates the t ip to a molecular level and forms a cushion of molten solderthat aids heat transfer. Re-tin the iron any t ime i ts t ip be co m es bla cke ned — per hap severy few minutes or after several jo ints.

Tin each condu ctor befo re actua lly mak ing the com ple te jo int . To tin a wir e, prepare i t by str ipping away Vw t o V u inch of insulation. As yo u strip insulation, be sure




not to n ick o r damage the conducto r. Heat the exposed copper fo r abou t 1 second ,then apply solde r in to the wir e— no t in to the iron. If the iron and t ip are app ropr iate,solder should flow evenly and smoothly in to the conductor. Apply enough solder tobond each exposed s t rand o f a s t randed wire . When t inn ing a so l id wire o r component lead, apply just enough solde r to l ightly coat the con du ctor su rface. C ond ucto rs

heat faster and solder f lows be tte r wh en all parts of a jo int are t inned in ad van ce.Making a comp lete solder joint is just as easy. Bring tog eth er e ach of your cond uc

tors as necessary to form the jo int . For exam ple, if you are soldering a com pone nt in toa printed circuit board, insert the component leads into their appropriate locations.Place the iron against all conduc tors to be hea ted as show n in Fig. 3-6. For a print ed circuit board, heat the printed trace and com ponen t lead together. After about 1 second,flow solder gently into the conductors—not the iron. Be sure that solder flows cleanlyand evenly into the joint. Ap ply solder for anothe r 1 or 2 seconds; then rem ove both solder and iron. Do not touch or m ov e the joint until mo lten solde r has set after several seconds. If the joint requ ires additional solder, reheat th e joint and flo w in a bit mo re solder.

3-6 Soldering a typical printed-circuit junction.

You can identify a go od s older jo int by its smooth, eve n, si lvery-gray app eara nce .An y charred or carb onized flux on the jo int indicates that you r soldering t em per ature is too high (or that he at is being applied for to o l o n g ) . Remember that so lder cannot flow unless the joint is hot. If it is not, solder will cool before it bonds. The resultis a rough, buil t-up, dull-gray or black mound that does not adhere very well . Themound is known as a cold solder jo int . You can usually corr ect a cold jo int by reheating the jo int pro perly and app lying fresh solder.

Surface-mount solderingConven t ional p r in ted-ci rcu i t boards use th rough-ho le com pone n ts . Par ts are in

serted on one side of the PC board, and their leads are soldered to printed traces onthe o ther s ide . Surface-mounted com ponen ts do no t penet ra te a PC board . Ins tead ,the components rest on only one side of the board as shown in Fig . 3-7 . Metal tabsrep lace comp onen t lead wires . Surface-mount com ponen ts range f rom d iscre te compone nts such as resistors and capacitors to active p arts such as transistors and inte-



Soldering 49

Small-outlineIC (SOIC)

3-7 Simplified vie w of a surface-mount PC board.

g ra te d c i rcu its . Ev en soph is t ica ted ICs like microp rocesso rs and ASIC s can be foundin su rface-mount packages .

During manufacture, surface-mount parts are glued into place on a PC board,then the board is brought quickly up to soldering temperature in a special chamber.Mol ten so lder is f lowed over the board whe re i t adheres to heated com pon en t leadsand PC t races . The remainder o f the board i s chemical ly and physical ly masked befo re so lder ing to p reven t mol ten so lder f rom s t ick ing e lsewh ere . Th e f in ished b oardis then coo led s lowly to p reven t thermal shock to the componen ts , masks ares t r ipped away, and the board can be tes ted (o r any th rough-ho le par ts can be a dd ed ) .This type of fabrication is called wave soldering, and it is similar to the prin cipleused to mass-so lder th rough-ho le PC boards . A d i fferen t manufactu r ing techn iqu eapp l ies a layer o f so lder pas te to a masked PC b oard befo re com pon en ts are app l ied ,then the board i s heated to f low so lder in to PC t race . Af ter componen ts are g luedin to p lace , the board i s qu ick ly reheated so so lder wi l l adhere to each componen tlead. The fin ished board is then cooled slowly. This process is known as reflow sol

dering. F igure 3 -8 shows a c lose-up v ie w of an SM T (surface-mou nt tec hn o log y)so lder connect ion .

3-8 Close-up view of an SMT solder con nection.

Discrete chipSmall-outline component

transistor (S O T ) (resistors,




Although the specific methods of surface-mount soldering will have l i t t le impacton your t roub leshoo t ing , you shou ld unders tand how surface-mount com pon en tsare assembled so that you can d isassemble them prop er ly du r ing you r repai r.

Desolder ing

Ideal ly, deso lder ing a connect ion invo lves removing the in termolecu lar bondthat has been formed during soldering. In reali ty, however, th is is v irtually impossible . The bes t that you can hope fo r i s to remove enough so lder to gen t ly b reak theconnect ion apar t wi thou t d es t roy ing the jo in t . Deso lder ing is a gam e o f rem oving asmuch solder as possible.

You wi l l find som e conn ect ions ver y easy to rem ove . For ins tance, a wir e inser ted in to a p r in ted c i rcu i t board can be rem ove d jus t by reheat ing the jo in t andgen t ly wi thdrawing the wire f rom i t s ho le once so lder i s mol ten . You can use deso ldering tools to clear away the solder i tself after the connection is broken.

Surface-mount componen ts p resen t a specia l p rob lem, because i t i s impossib leto move the part unti l i t is desoldered completely. By using special desoldering t ipsas shown in Fig . 3-5 , you can heat al l leads simultaneously so the part can be separated in one quick motion. There also are special t ips for desoldering a selection ofIC packages . Once a par t i s c lear, excess so lder can be re m ov ed w i th conven t ionaldesoldering tools such as a solder vacuum or solder wick.

Desoldering through-hole components is not as easy as i t looks. You must heateach solde r jo int in turn , and use a des older ing to ol to rem ov e as mu ch sold er as possible. Once each lead is clear, you w ill prob ably ha ve to break eac h lead free as sho wnin Fig . 3-9 . Grab hold of each lead and wiggle i t back and forth gently unti l i t breaksfree . A n a l ternate m ethod is to heat each jo in t whi le w i thdraw ing the lead w i th a pairof needle-nose pliers, then clean up any excess solder later. Unfortunately, th is

p rocess canno t be used wi th a ll com pone n ts . Exp er ien ce wi l l teach you the f inerpoints of desoldering.

Componentlead

3-9

Freeing a stubborn through-holesolder connection.

Needle-nosepliers



Multimeters 51

C o n s id e r tw o fac to r s w h e n y o u u se a m u l t im e te r. F i r s t , t h e m e te r m u s t b e se tto the des i red function ( v o l t a g e , cu r ren t , cap ac i t an ce , e t c . ) . S e c o n d , t h e rangemust be se t p roper ly fo r that funct ion . I f you are unsure what range to use , s tar tb y ch o o s in g th e h ig h es t p o ss ib l e r an g e . O n ce y o u h av e a b e t t e r i d ea o f w h a t r e adin g s to ex p ec t , t h e r an g e can b e r ed u ced to ach iev e a m o re p rec i se r ead in g . I fy o u r s ig n a l ex ceed s th e m e te r r an g e , an o v er r an g e w arn in g w i l l b e d i sp lay ed .Man y d ig i t a l m u l t im e te r s a re cap ab le o f se l ec t in g th e p ro p er r an g e au to m at i ca l ly( a u t o - r a n g i n g ) .

You can use a multimeter for two types of test ing: stat ic and dynamic. Dynamictests are made with power applied to a circuit , and static tes ts are ma de on unpow-ered c i rcu its o r com pone n ts . Measu reme nts l ike vo l tage , cu rren t , and f requency are

Mul t ime te rsTes t m e te r s can g o b y m an y n am es . S o m e p eo p le ca l l t h em multimeters o r

ju s t meters, and o ther s mig h t refer to the m as VOMs ( v o l t - o h m - m i l l ia m m e t e r ) o rmultitesters. R eg ard less o f w h a t n am e y o u ch o o se to ca ll t h em , m u l t im e te r s a re

the hand ies t and most versa t i le p iece o f tes t equ ipment that you wi l l ever use . I fy o u r to o lb o x d o es n o t co n ta in a g o o d -q u a l i ty m u l t im e te r y e t , n o w is a g o o d t im eto consider purchasing one.

Even the most bas ic mul t imeters can measure ac and dc vo l tage , cu rren t , andresistanc e. For less than $150, you can buy a go od digital multim eter that also includes features l ike a capacitance checker, continuity checker, d iode checker, andtransistor checker. Digital multimeters are easier to read, more to lerant of operatorerror, and more precise than analog multimeters. Figure 3-10 shows a typical d igitalmul t imeter.




dynamic tes ts ; most o ther tes ts such as res is tance/con t inu i ty, capaci tance , d iodeand transistor junc tion qua li ty are stat ic tests . T he fo llowin g is a re vie w of basic mult im e te r m easu rem en t t ech n iq u es .

Measuring vol tageMu l t im e te r s can m easu re b o th d c v o l t ag e (m ark ed D C V o r V d c) an d ac v o l t ag e

(m ark ed A C V o r Va c) . R em em b er th a t a ll v o l t ag e ( e i th e r ac or d c ) m u s t b e m easu redin parallel wi th the des i red c i rcu i t o r com pon en t . Ne ve r in ter rup t a c i rcu it and a ttempt to measure vo l tage in ser ies wi th o ther componen ts . Any such read ing wouldbe m ean ing less , and you r c i rcu i t migh t no t even funct ion .

Set your mul t imeter to i t s appropr ia te funct ion (DCV or ACV), then se lect theprop er ran ge. I f you are u nsure what ra nge to use , s tar t at the highest r an g e to p reven t poss ib le damage to the meter. An au to -rang ing mul t imeter wi l l se lect i t s ownrange. P lace you r tes t leads across ( in para l le l ) wi th th e par t under tes t as shown inF ig . 3 -11 , and read vo l tage d i rect ly f rom the d ig i ta l d isp lay. The dc vo l tag e read ings

are po lar i ty sensi t ive , so if you read +5 Vd c and th en reve rse t he tes t leads , you wi l lsee -5 V dc . Th e ac vo l tag e read ings are no t po lar i ty sensi t ive .

3-11 Measuring voltage.

Measuring currentMo st g en e ra l -p u r p o se m u l t im e te r s a l lo w y o u to m e asu re ac cu r ren t ( A C A o r

l a c ) an d d c cu r ren t (D C A o r Id c ) in a c i r cu i t , a l th o u g h th e re a re o f t en f ew ran g esto ch o o se f ro m . A s w i th v o l t ag e m e asu r em en t s , cu r ren t i s m easu red in a w o rk in gc i rcu i t w i th p o w er ap p l i ed , b u t cu r ren t m u s t b e m easu red in series wi th the c i r-



Multimeters 53

cu i t o r componen t under tes t . Inser t ing a meter in ser ies , however, i s no t a lwaysan easy task . In many c ases , yo u must ph ysical ly in ter rup t a c i rcu i t a t the po in tyou wish to measure , then connect tes t leads across the b reak . Al though i t migh tbe eas y to in ter ru p t a c ircu i t , keep in min d that you a lso must pu t the c i rcu i t backtogether, so use care when choosing a po in t to b reak . N e v e r t ry to read curren t

in para l le l . Curren t meters , by thei r natu re , exh ib i t very low res is tance acrossthei r leads (so m et i m es be low 0 .1 Q ) . P lacin g a cu rre n t met er in para l le l can causea shor t c i rcu i t across a componen t that can damage the par t , the c i rcu i t undertes t , o r your mul t imeter.

Set your mul t imete r to the des i red funct ion (D C A or A C A ) and se lect the appropriate range. If you are unsure about proper range, set the meter to i ts highestrange. You might need to plug one of your test leads in to a different current inputjack on the meter. Unless your multimeter is protected by an in ternal fuse, i t can bedamaged by excess ive curren t . Make su re that the meter can hand le the amount o fcurren t you are expect ing .

Tu r n o f f a l l p o w e r t o a c ir c u i t b e f o r e i n s e r t i n g a c u r r e n t m e t e r. T h i s p recau t ion p reven ts unpred ic tab le c i rcu it operat ion . If you wish to measure p ower-su pply curren t feedin g a circuit as show n in Fig . 3-12, op en the p ow er su pply l ine at anyconven ien t po in t (o f te n a t the supp ly o r c i rcu it board con ne cto rs) . Inser t the m eterand reapp ly powe r. Read curren t d i rect ly f rom the d isp lay. Th is p roc edu re a lso canbe used for measuring current within a circuit .

3-12 Measuring current.




Measuring f requencySom e mu l t imeters o ffer a f requ ency coun te r ( f o r H z) that can read f requ enc y

d irect ly. The ranges avai lab le depend on your par t icu lar meter. S imple hand-heldme ters can o f ten read up to 100 kHz, and ben ch- top m ul t imeters can hand le 10 MH zo r m o r e . F req u en cy m easu rem en t s a re d y n am ic r ead in g s m ad e in parallel across acomponen t o r c i rcu i t .

Set your mul t imeter to i t s f requency coun ter funct ion and se lect the appropr ia te range. I f you are unsure jus t what f requency to expect , s tar t a t i t s maximumfrequency range. Place your test leads across the signal source as shown in Fig . 3-13,and read freque ncy d i rect ly f rom the d isp lay.

Measuring resis tanceResis tance ( in oh m s) is the most com m on s ta t ic measu rem ent that you can take

with a mul t imeter. Measur ing res is tance i s a handy funct ion , no t on ly fo r check ingres is to rs , bu t fo r check ing o ther res is t ive e lem ents such as wires , conn ecto rs , motors , so leno ids , and some semiconducto r componen ts . Resis tance i s measured inparallel across com pon en ts wi th a ll c i rcu i t po we r OFF as show n in Fig . 3-14. You

migh t need to remove a t leas t one componen t lead f rom i t s c i rcu i t to p reven t in terconnect ions wi th o ther componen ts f rom causing fa lse read ings .

Ord inary res is to rs can be c hec ked s im ply by swi tch ing to a res is tance funct ionand se lect ing the p roper range. Many mul t imeters can re l iab ly measure res is tanceup to 20 MQ. P lace your tes t leads across the componen t and read res is tance d irect ly f rom the d isp lay. If res is tance e xce ed s the se lected range, the d isp lay wi ll indicate an over range or infinite resistance condit ion.

Continuity check s are ma de to ensure a re l iab le , low-res is tanc e conn ect ion between two po in ts . For example , you cou ld check the con t inu i ty o f a cab le betweentwo connecto rs to ensure that bo th ends are connected p roper ly. Set your mul t ime-



Multimeters 55

Measuring resistance.3-14

RH A M

te r to a low res is tance scale , then p lace your tes t leads across bo th po in ts to measure. Ideally, go od contin uity should be abou t 0 f l.

Che cking a capacitorThere are two methods o f check ing a capaci to r us ing your mul t imeter. I f your

m ete r has a bu i lt - in capa ci tance c hecker, a l l you n eed to do is se lect the capaci tanc efunct ion and se t the de s i red rang e. You migh t ha ve to p lace tes t p rob es in parallelacross the capaci to r under tes t , o r you migh t have to rem ove the capaci to r f rom thecircuit and insert i t in to a special f ix ture on the meter face. A capacitance checkerwi l l usually d isp lay capaci tance d i rect ly in micro farads (u F ) o r p icofarads (p f) . Aslong as your read ing is wi th in the to leran ce o f the mark ed value o f the capaci to r, youkno w the par t is good .

If your multimeter is not equipped with an in ternal capacitor checker, you coulduse the resistance ranges to approxim ate th e quality of a capacitor. This type of ch eckprovides a "quick and dirty " judg m ent of wh eth er the cap acitor is go od or bad. Th e principle behind this is simp le—a ll ohm m eter ranges use an in ternal battery to supply cur

rent for the comp onen t unde r test . W he n that current is supplied to a work ing capacitoras shown in Fig . 3-15, i t wil l charge the capacitor. Charge accu mulates as the ohm meter is left conn ected , and can be seen as changing resistance on the oh m me ter display.

Open

+0.0 Time

Capacitor Resistance readings versustime

3-15 Measuring capacitance with an ohmm eter.




When f i rs t connected across an ohmmeter, the capaci to r wi l l d raw a re la t ivelylarge amount o f cu rren t—th is ind icates low res is tance . As the capaci to r charges , i tdraw s less and less curre nt, so resistan ce ap pear s to increase . Ideally, a fully cha rge dcapacitor draws no current, so your resistance reading should climb to infinity. Whena capacitor behaves th is way, i t is probably good.

Und ers tand that you are no t ac tually measur ing res is tance o r capaci tance here ,but only the profi le of the charging characterist ic of a capacitor. If the capacitor isextremely small , or is open circuited, i t wil l not charge substantial ly, so i t wil l instantly read infinity. If a capacitor is part ial ly (or to tal ly) short-circuited, i t wil l notho ld a charge , so you migh t read zero ohms (o r res is tance can c l imb to some levelbe low in f in i ty and remain the re ) . In e i ther case , the cap aci to r is p roba b ly defe ct ive .I f you doub t your read ings , check several o ther capaci to rs o f the same value andcom pare read ings . Be su re to make th is tes t on a m ode rate to h igh res is tance scale .A low resistance scale can charge to infinity too quickly for a clear reading.

Semiconductor checksMany mul t imeters o ffer a semiconducto r junct ion checker fo r d iodes and t ran

sisto rs. Me ters eq u ipp ed w i th a d iod e range in thei r res is tance funct ion can be usedto mea sure the s ta t ic res is tance o f most com m on d iod es in thei r fo rwa rd o r r eve rse-biased condit ions as shown in Fig . 3-16.

. Set multimeter to a

Select the d iode range f rom your meter res is tance funct ion and p lace tes t leadsacross the d iod e in the fo rward d i rect ion . A wo rk ing s i licon d iode shou ld exh ib i t a res is tance betw ee n abou t 450 and 700 Q . tha t wi l l read d i rect ly on your m eter. Rev erseyour tes t leads to reverse-b ias the d iod e. Becau se a work ing d iode shou ld no t conduct at al l in the reverse direction, you should read infinite resistance.



Multimeters 57

A shorted diode will exhibit a very low resistance in the forward and reverse-biasd i rect ions . Th is symptom ind icates a shor ted semiconducto r junct ion . Be cer ta inthat at least one of the two diode leads is removed from the circuit before test ing.Th is wi l l p reven t i t s in terconnect ions wi th o ther componen ts f rom causing a fau l tyreading. An opened diode will exhibit very high resistance (usually infinity) in bothi ts fo rward and reverse d i rect ions . In th is case , the sem iconduc to r junct ion i s open-ci rcu i ted . I f you feel unsure how to in terp re t your m easurem ents , tes t several o thercomparab le d iodes and compare read ings .

Transis tors can be che cked in several ways . Som e m ul t imeters fea tu re a bu i lt - intransistor checker that measures transistor gain (or h {e) d irectly. If you r m ete r offersa transistor checker, insert your transistor in to the test f ix ture on the meter face ini ts co rrect lead o r ien ta t ion (emit ter, base , and co l lec to r ) . Manufactu rer 's speci f ications can tel l you w he the r a gain reading is correc t for a part icular part . A low (orze ro ) reading indicates a shor ted transistor, and a high (o r infinite ) read ing su ggestsan open-circuited transistor.

Your meter 's d iod e chec king fea ture also can be us ed to check a bipolar transistor base-emitter and base-collector junctions as shown in Fig . 3-17. Each junctionacts just l ike a diod e junc tion. Tes t one junction at a t im e. Set your m ultim eter to i tsdiode range, then place i ts test leads across the base-collector junction. If your transistor is NPN, place the posit ive test lead at the base. This arrangement should forwa rd bias the bas e-collect or junc tion and cause a norm al amount of d iod e res istance.Rev erse y our tes t leads across the base-co l lec to r junct ion . Th e t ransisto r shou ld n owbe reverse-b iased and show in f in i te res is tance . Repeat th is p rocedure fo r the base-emit ter junct ion .

Set multimeter to a diode• resistance scale to ensurebest performance

NO TE: Reverse test probesto check PNP transistors

Collector (N P N transistor)Base'

Base 7T

(PNP transistor)

' Emitter

3-17 Perform ing a transistor check.




I f your t rans is to r is PNP , your tes t lead p lacemen t mu st be rev erse d . For example , a fo rward-b iased junct ion in an N P N t ransis to r is reverse-b iased in a PN P t ransistor. You can refer to manufacturer 's specification sheets to determine which leadsin the transistor are the bas e, emitter, and collector.

As a f inal check, measure the resistance from emitter to collector. Note that you

should read infinite resistance in both directions. Although this is not a d iode junction, short circuits can develop during a transistor fai lure that might not appearacross normal junct ions . Rep lace any d iode wi th an open o r shor ted junct ion , o r ashort from emitter to collector.

Logic probesTh e p rob lem wi th mo st mul t imeters i s that they d o no t work we l l wi th d ig ita l

log ic c i rcu i ts . A mul t imeter can cer ta in ly measure whether a log ic vo l tage i s on o roff , but if that logic level changes quickly, a dc voltmeter function cannot track i tp roper ly. Log ic p rob es p rov id e a fast and easy mean s o f detect ing s teady-s ta te o r a lternating logic levels. Some logic probes can detect logic pulses faster than 50 MHz.

Logic p robes are ra ther s imple- look ing dev ices as shown in F ig . 3 -18 . A p robecan be powered f rom i t s own in ternal bat tery, o r f rom the c i rcu i t under tes t . Conn ec t t h e p ro b e g ro u n d l ead to a co n v en ien t c i r cu i t g ro u n d . If a p ro b e i s p o w er edfrom th e c i rcu i t und er tes t , a t tach it s po w er lead to a log ic su pp ly vo l t ag e in theci rcu i t . A smal l panel on the p robe body ho lds several LED ind icato rs and a swi tchth a t a l lo w s th e p ro b e to w o rk w i th tw o co m m o n lo g ic fam i l i e s : T T L ( t r an s i s to r-t r a n s i s t o r l o g i c ) a n d C M O S ( c o m p l e m e n t a r y m e t a l - o x i d e s e m i c o n d u c t o r ) . Yo umigh t f ind TTL and CMOS dev ices mixed in to the same c i rcu i t , bu t one family wi l l

usual ly dominate .

3-18 A B+ K DP-51 50 MHz logic probe.



Oscilloscopes 59

When the metal probe t ip is touched to an IC lead, i ts logic state is d isplayed onone o f the th ree L ED ind icato rs as shown in Tab le 3 -1 . Typ ical cho ices a re LO W,H IG H , an d P U L S E (o r C L O C K ) , b u t o th e r in d ica t io n s can b e p rese n ted as w e l l d epend ing on the soph is t ica t ion o f the p robe. Log ic p robes are most usefu l fo r t roubleshooting working logic circuits where logic levels and clock signals must bedetermined qu ick ly and accurate ly.

Table 3-1. Typical logic probe display patterns

Inpu t s ignal HIG H LED L O W L E D P U L S E L E DLogic 1 (T TL or CMO S) On Off OffLogic 0 (TTL or CMOS) Off On OffBad logic level or open circuit Off Off OffSquare wave (<200 kH z) On On BlinkSquare wav e (>20 0 kH z) On/Off On/Off BlinkNarrow high pulse Off On/Off Blink

Narrow low pulse On/Off Off Blink

Osci l loscopesOsci l loscopes o ffer a g reat adva n tage ov er mu l t imeters and log ic p robes . Ins tead

of reading signals in numbers or with l ighted indicators, an oscil loscope will showvoltage versus t ime on a visual d isplay. Not only can you observe ac and dc voltages,but i t enables you to watch digital voltage levels or o ther unusual signals occur in

real t ime. If you have used an oscil loscope in the past , you know just how useful i tcan be . Osci l loscopes such as the one shown in F ig . 3 -19 migh t appear somewhatoverw helm ing a t fi rs t , bu t many o f thei r operat ions work th e same way reg ard less o fwhat model you are work ing wi th .

3-19 A B+ K 2522A 20 MHz digital oscilloscope.




ControlsIn spite of their wid e variat ions of features and com plexity, most con trols are com

mo n to every o scil loscope. Controls fall in to four categ ories: horizontal ( t im e bas e) control , vert ical signal control , hous ekeep ing con trols, and optional (en ha nc ed ) controls.

Housekeeping con t ro ls hand le such th ings as osci l loscope power, t race in ten

sity, graticule in tensity, trace magnification, horizontal trace offset , vert ical trace offset , and trace finder. Any control that effects the quali ty and visibil i ty of a d isplay.

Becau se an osci lloscope d isp lays vo l tage versus t im e, ad jus ting e i ther vo l ta ge o rt ime sett ings will al ter the display. Horizontal controls manipulate the left- to-rightt ime appea rance (s w ee p ) o f the vo l tage s ignal . Your osci l loscope mas ter t ime base isadjusted using a TIM E/D IV knob or but ton. This sett ing establishes the rate at whichvo l tag e s ignals are swep t on to the screen . Smal ler se t t ings a l low shor ter ev en ts to b ed isp layed m ore c learly, and v ice versa . Remain ing h or izon ta l con tro ls include a horizon ta l d isp lay-mode se lecto r, sweep t r igger se lect ion and sensi t iv i ty, t r igger coup l ing se lect ion , and t r igger sou rce se lect ion . Your par t icu lar osci l loscop e m igh t o fferaddit ional controls.

An adjustment to an oscil loscope voltage sensit iv ity also will al ter your display.Ver t ica l con tro ls effect the def lec t ion (up- to -down) appearance o f your s ignal . Anosci l loscope ver t ica l sensi t iv i ty is con tro l led wi th the VOLTS/ DIV k nob . W he n sensit iv i ty i s increased (VOLTS/DIV becomes smal ler ) , s ignals wi l l appear larger ver t ically. Reducing sensit iv ity will make signals appear smaller vert ically. Other vert icalcon tro ls include coup l ing se lect ion , ver t ica l mode se lect ion , and a d isp lay inver terswitch.

Your osci l loscope migh t have any number o f op t ional con tro ls depend ing on i t scost and complex i ty, bu t cu rsor and s to rage con tro ls are som e of the most com mo n.Many scopes offer horizontal and vert ical on-screen cursors to aid in the evaluationof waveforms. Panel con tro ls a l low each cursor to be moved around the screen . Thed is tance between cursors i s then conver ted to a co rrespond ing vo l tage , t ime, o r f reque ncy value , and that numb er is d isp layed on the sc reen in appropr ia te un i ts . S to rage osci l loscopes a l low a screen d isp lay to be held r igh t on-screen , o r in memorywith in the scope to be recal led on demand .

Oscil loscope specificationsOsci l loscopes ha ve a var ie ty o f imp or tan t speci f ica t ions that you shou ld be fa

miliar with when choosing and using an oscil loscope. The first specification to knowis bandwidth. Band wid th repres en ts the range o f f reque ncies that the scop e can

work wi th . The bandwid th speci f ica t ion does no t necessar i ly mean that a l l s ignalswi th in that band wid th can be d isp layed accurate ly. Bandw id th i s usually ra ted f romd c to so m e m ax im u m f req u en cy (o f t en in m eg ah er t z— M H z) . F o r ex am p le , an in expensive osci l loscope m igh t cover dc to 20 MH z, and a m ore e xpe nsive m ode l migh tw o rk u p to 1 50 MH z o r m o re . G o o d b an d w id th is v e ry e x p e n s iv e— m o re so th an an yo ther fea tu re .

T h e vertical deflection (or vert ica l sen sit iv ity) is anoth er imp ortant sp ecification.De flection is l isted as the minimu m to maxim um VO LTS/D IV sett ings that are offered ,and the number of steps that are available within that range. A typical model mightpro vide ver t ical sensit iv ity from 5 mV/D IV to 5 V/DIV bro ken d ow n into 10 steps.



Oscilloscopes 63

the d isp lay when an unknown s ignal is f irst applied. If your s ignal does happen to

leave the visible display, you cou ld redu ce sensi t iv ity ( inc rease the V O LT S /D IV se tt i n g ) to m a k e the t race visible a gain .

For example , suppose you were measur ing a +5 Vdc power supp ly ou tpu t . If

VOLTS/DIV is set to 5 V/DP7, each m ajor ver tical d ivision rep res ent s 5 V, so y o u r +5 Vdc

signal should appear 1 full division ab ove y ou r baseline (5 V/DPV x 1 division = 5 V) as

shown in Fig. 3 -21 . At a VOLTS /DIV sett ing o f 2 V/DIV, the same +5 V s ignal wou ld now

appear 2.5 d ivisions abo ve yo ur baseline (2 V/DIV x 2.5 divisions = 5 V ) . If your inputsignal wer e a negat ive vo l tage , the t race would appear b elow t he baseline, but it wou ldread t he same way.

v 6 v

5 Volts/division scale 2 Volts/division scale

3-21 Measuring dc voltages with an oscilloscope.

You also can read ac s ignals d irectly from th e o sc i l loscope. Swi tch your inpu tcoup l ing con tro l t o the ac pos i t ion , then set a basel ine jus t as you w o u l d for dc m e asu rem en t s . If you are unsure ho w to set the vert ical sensit iv ity, start with a low sensitivity (a la rge VOLTS/DIV set t ing) , then s lowly increase th e sens i t iv i ty ( reduce the

VOLTS/DIV scale) af ter a signal is c o n n e c t e d . K e e p in mind that ac v o l t ag e m easu rem en t s on an o sc i l loscope wi l l not m a tch ac vo l tag e read ings on a mul t imeter. An

oscil loscope displays instantaneous peak values for a w a v e f o r m , and ac v o l tm e te r sm easu re in rms ( ro o t m ean sq u are ) v a lu es . To co n v er t an rms va lue to peak , m ul t ip ly rms by 1.414. To co n v er t a peak vo l tag e read ing to rms, d iv ide peak by 1.414.

Wh en ac tu a l ly m easu r in g an ac s ignal , it m i g h t be n e c e s s a r y to adjust the os

c i l lo sco p e t r ig g e r l ev e l co n t ro l to ob ta in a st ab le ( s t i l l ) t r ac e . As Fig. 3-22 sh o w s ,s ignal vo l tages can be m e asu re d d i r ec t ly f ro m th e d isplay. Fo r e x a m p l e , the s inuso id a l w a v efo r m of Fig. 3-22 var ies f rom -10 to +10 V. If s co p e sen s i t i v i ty w ere set

to 5 V/DIV, its p e a k s w o u l d be two d iv is ions above and b e l o w th e b ase l in e . Be

cause th is is a p e a k m e a s u r e m e n t , an ac v o l t m e t e r w o u l d s h o w the s ignal as

peak /1 .414 (10 /1 .414) or 7.07 V rms.

5 Volts/division scale 2 Volts/division scale

3-22 Measuring ac voltages with an oscilloscope.




Tim e and f requency measurem entsAn oscil loscope is perfect for measuring cri t ical signal parameters such as pulse

wid th , du ty cycle , and f requency. The hor izon ta l sensi t iv i ty (TIME/DIV) con tro lcomes in to p lay wi th t ime and f requency measurements . Before making any measurements, you must f irst obtain a clear baseline as you would for voltage measure

me nts. Wh en a baseline is established and a signal is con ne cte d, adjust the TIM E/D IVcontrol to display one or two complete cycles of the signal.

F igure 3 -23 shows two typ ical per iod measurements . With VOLTS/DIV set to5 ms/DIV, the sinusoidal waveform repeats every 2 divisions. This wave represents aper iod o f 10 ms (5 ms/DIV x 2 d iv is ions) . Because f requ ency is the s imple recip roca lo f the per iod , you can calcu la te f requency d i rect ly f rom per iod . A per iod o f 10 msrepr esen ts a frequ enc y of 100 Hz (1/10 m s ). Th is proces s also wo rks for squ arewaves and other waveforms that are not sinusoidal . The square wave in Fig . 3-23 repeats ev ery 4 divisions. A t a TIME /DIV sett ing of 1 ms/DIV, i ts perio d is 4 ms. Th is per iod corresponds to a f requen cy o f 250 Hz.

5 ms/div 1 ms/div

10 m s = 100 Hz

5 m s/division scale

3-23 Measuring timing with an oscilloscope.

I t

4 ms = 250 Hz

1 ms/division scale

Instead of measuring the entire period of a pulse cycle, you also can read thet ime bet we en any tw o po in ts o f in teres t . For the square wav e in F ig . 3-23 , you cou ldread i ts pulse wid th to be 1 ms . You also could rea d the lo w por tion of th e cyc le as apu lse wid th o f 3 ms (ad de d to geth er fo r i t s to ta l per iod o f 4 m s) . A s ignal duty cycleis simply the rat io of a signal ON time to i ts to tal period expressed as a percentage.For ex am ple , a square wav e on fo r 2 ms and o ff fo r 2 ms wou ld have a du ty cy cle o f50% [2 ms/(2 ms + 2 m s) x 1 0 0 % ) . For an on t ime of 1 ms an d an off t im e of 3 m s, itsdu ty cycle would be 25% [1 ms/ ( l ms + 3 m s) x 100%] . U s e cau t io n in d u ty -cy c le

m e a s u r e m e n t s .



4C H A P T E R

Service gu idelines

Elec tron ic troub leshoo ting is a strang e pursuit ; i t is an activ ity that fal ls so m ew he rebe tw een ar t and sc ience. Success in t roub leshoo t ing dep end s largely on a thorough ,logical troubleshooting approach and the right type of test equipment, as well as anelement of in tuit ion and luck. This chapter shows you how to evaluate and determine p r in ter p rob lem s, locate techn ical data, and p resen t a ser ies o f serv ice gu idelines that can ease your work.

The troubleshooting cycleRegardless of how complex your part icular circuit or system might be, a rel iable

troubleshooting procedure can be broken down into four basic steps as shown in Fig .4-1: (1) define your symptoms, (2) identify and isolate the potential source (or loca-

Define yoursymptoms

Define yoursymptoms

IIdentify and

isolate

I

4-1The universal troubleshootingprocess.

Re-test

Return toservice

Repair orreplace

6 5



66 Service guidelines

t ion) of the problem, (3) replace or repair the suspected component or subassembly,and (4) re-test the system thoroughly to be sure that you have solved the problem. Ifyou have not solved the problem, begin again from step 1. Th e p ro cedu re i s a un iversal procedure you can use for any troubleshooting—not just for laser printers.

Define your symptomsSooner or later, a laser printer is going to break down. The problem might be as

simple as a st icky gea r or as com plicat ed as an extens ive e lectro nic fai lure. H ow eve r,befo re you o pen the too lbox , you must have a f irm unders tand ing o f the sym ptom s.You must do more than to simply say, "It 's busted." Think about i ts symptoms carefully. Ask yourself what is (or is not) happening. Consider when i t is happening. Ifth is instal lat ion is new , ask yourse lf if the com pu ter is set up prop erly, or if the rightcables are being used, or if DIP switches are set up correctly. If you have used yourprinter for a while, do you remember the last t ime you cleaned and lubricated i t? Isthe print l ight , dark, or completely missing? Is the paper advancing freely? By recognizing and understanding your symptoms, you will f ind i t easier to trace a problemto the appropr ia te subsect ion o r componen ts .

Use your senses and wri te do wn as many sym ptom s as you can — wh ate ver yousmell , see, or hear. Writ ing symptoms might sound tedious now, but when you are upto your e lbows in repai r work , a wri t te n reco rd o f sym ptom s and c i rcumstances wi l lkeep you focused on the task a t hand . Wri t ing symptoms is even more impor tan t i fyou are a nov ice t roub leshoo ter.

Identify and isolateBefore you try to isolate a problem in the laser printer, f irst be sure that the

printe r is , in fact , causing the prob lem . In ma ny circum stanc es, printer pro blem s w illbe obvious, but there are some si tuations that might appear ambiguous (no printwith power on, errat ic printing, not enough contrast , e t c . ) . A lw ay s r em em b er th a t aprinter is just a subse ction of a larger s ystem including yo ur c om pute r, laser printer,and interconnecting cable. Especially in new instal lat ions, a computer fai lure, softwar e incompat ib il ity, o r cab le p rob lem migh t be causing your sym ptom s.

An easy app l ica t ion o f the un iversa l t roub leshoo t ing p rocedure fo l lows. Onceyou have carefully identif ied your symptoms, isolate the printer. You can isolate aprinter by removing i t from its communication cable. You can replace i t by test ing i ton ano ther computer sys tem wi th a work ing p r in ter (one that you know is work ingw e l l ) . A f r iend o r co l league m igh t le t you tes t your p r in ter on thei r comp uter sys tem.

Because various computers can be set up to communicate in different fashions, youmight have to al ter the in ternal sett ings of your printer to match those of the working printer. When printing from an operating system such as Microsoft Windows, youmight have to select and configure a new printer driver to support your printer. Ifyour p r in ter exh ib its the same symptom s on ano ther computer, there i s an exc el len tchance that the problem is within the printer. You can then proceed with specifict roub leshoo t ing p rocedures . I f , however, those symptoms d isappear and yourpr in ter works p roper ly, you shou ld suspect a p rob lem in your computer, so f twareconfiguration, DIP switch sett ings, printer driver, or in terconnecting cable.



The troubleshooting cycle 67

An oth er tes t is to t ry a work ing p r in ter on your com pu ter sys tem . As befo re , youmigh t need to se lect ano ther p r in ter d r iver to operate the work ing p r in ter p roper lyon your exist ing system. If another printer works properly, i t verif ies that the computer, softwa re co nfiguration, and cab le are in tact . If a wo rkin g printe r fai ls to wo rkon your sys tem, check the compu ter comm unicat ion in ter face , so f tware se t t ings ,and in terconnect ing cab le . Com plete th is check in add i t ion to tes t ing your quest ionable printer on another system.

W he n you are confiden t that the printe r is at fault, you can begin to identify anypossible problem areas. Start at the subsection level . You might recall from chapter 1that a laser printer consists of several major su bsections. Your printer fault wil l be located in at least one of these subsections or subassemblies. The troubleshooting proced ures of chapters 6 throug h 9 will aid you in deciding w hich subsections are at fault .On ce you have identif ied a potential pro blem , you can begin the actual repair proc ess.In man y cases, you r repair will involve replacing a defe ctiv e sub assembly. Fo r skil ledtechnicians, the repair might include tracking a defect to the component level .

Repair or replaceOn ce you have an unders tand ing o f wha t is wro ng and w he re to look , you migh t

beg in the actual repai r p rocedures that you th ink wi l l co rrect the symptoms. Someprocedures requ ire on ly s imple ad jus tments o r c lean ing , and o thers migh t requ irethe exch ang e o f e lec t r ica l o r mech an ical par ts . A l l p r o c e d u r e s a r e i m p o r t a n t a n dsh o u ld b e fo l l o w ed v e ry ca re fu l ly.

Parts are usually classified as components or subassemblies. A comp onen t par tis the smallest possible individual part that you can work w ith . Com pone nts can ser veman y different purp oses in a printer. Resistors, capacitors, gears, belts , mo tors, and integra ted circuits are just a few type s of com pon ent parts. Usually, com pon ents contain

no serv iceab le par ts— the co mpon en ts them selves must be rep laced . A subassembly iscomposed of a variety of individual components. Unlike components, a complete subassemb ly serves a single, specific pu rpose in a printer, but i t too can be repa ired b y locating and replacing any faulty com pon ents . Repairing a defe ctive subassembly simplyby m stalling a new one in the printer is certainly an accep table solution.

Al l techn ic ians must mak e the cost /perfo rmance t rade off wh en perfo rm ing a repair. Com pon en t par ts are mu ch less expens ive than subassembl ies , bu t com pone n tsare often specialized and can be difficult to get . You also might need test equipmentand t ime to t roub leshoo t to the componen t level . Rep lacing subassembl ies i s fas terand eas ier than t rac ing componen t fau l ts , even though assembl ies are more expensive . Subassembly s erv ice mak es good sense fo r ind iv iduals who lack the t ime, ex per ience, o r tes t equ ipm ent to worry abou t com pone n t- level fau lts .

Rep lacement e lec t ron ic componen ts migh t o f ten be purchased f rom several dif

ferent sources, but keep in mind that many mechanical parts and fi t t ings might onlybe available through the manufacturer or d istr ibutor. Many of the mail-order companies listed at the en d of th is book will send you their co m plet e ca talogs or produ ct l istings at your request . Going to the manufacturer for subassemblies or components isoften so me wh at of a calculated risk — the y might do business only with their affi l iatedservice centers, or refuse to sell parts d irectly to consumers. If you find a manufac-




turer will ing to sell you parts, you m ust often k now t he manu facturer 's exac t partnumber o r code. Remember that many manufactu rers are i l l equ ipped to deal wi thconsum ers directly, so be patient and be prepa red to mak e several d ifferent calls .

During a repair, you might reach a roadblock that requires you to leave thepr in ter fo r a day o r tw o (o r lon ge r) . Th e delay i s typ ical when you hav e d iagnosed afailure and are wait ing for parts. Make i t a point to reassemble the printer as muchas possible before leaving i t . Place any loose parts in to plast ic bags and seal themshut. Reassembly will prevent a playful pet , curious child , or well-meaning spousefrom accidentally misplacing or discarding parts while the printer si ts on your workbenc h. Making loose parts secure is tw ice as imp ortan t if you r work sp ace is in a well-traveled or family area. You also will remember how to put i t back together later on.Make no tes to remind yoursel f what par ts go wh ere .

Re-testWhen repair is complete, carefully reassemble the laser printer and test i t before

conn ecting i t to a com puter . Run a thor oug h self-test to check printe r operat ion. T he

self-test checks the image-formation system, paper pickup and registrat ion, fusingassembly, power supply, and much of the printer electronics. If symptoms persist ,you wi l l have to re-evaluate them and narrow the p rob lem to ano ther par t . If norm aloperat ion i s res to red (o r s ign i f ican t ly improved) , tes t the p r in ter wi th a computerand in terconnect ing cab le . When you can ver i fy that your symptoms have s toppedduring actual operation, the printer can be returned to service.

Do not be discouraged if the printer st i l l malfunctions. Simply walk away, clearyour head, and start again by defining your symptoms. Never continue with a repairi f you are t i red o r f rus t ra ted—tomorrow is ano ther day. For techn ic ians t roub leshoo t ing to the componen t level , a lso real ize that there migh t be more than onebad com pone n t to deal wi th . Rem em be r that a laser p r in ter i s jus t a co l lec t ion o f assembl ies , and each assembly i s a co l lec t ion o f componen ts . Normal ly, every th ingworks togeth er, bu t wh en on e par t fa il s, i t migh t cause one o r mo re in terc onne ctedparts to fai l as well . Be prepared to make several repair at tempts before the printeris repa ired c om pletely .

Gathering technical dataTechn ical information is perha ps your m ost valuable tool in tackling a printer re

pair. Just ho w mu ch information yo u actually ne ed will dep end on the part icular problems you are facing. Simple adjustments and cleaning might be accomplished withli t t le or no specific technical information (except your own observations and commonsense jud gm en t) , bu t complex e lect ron ic t roub leshoo t ing migh t requ ire a com pleteset of schematics. Parts lists will be ne ed ed to orde r ne w mec hanical com pon ents andall types of subassemblies. More in tricate repair procedures generally need morecomprehensive technical l i terature. Luckily, there are some avenues of information.

Your user 's manual is always a goo d p lace for basic prin ter inform ation. A user 'smanual describes how to set up and operate the printer, outl ines i ts important specifications and comm unica tion inter face, and points out i ts major asse mb lies and con-



Electricity hazards 69

trols. If you are unfamiliar with the printer or unaccustomed to changing i ts configurations, a user 's manual can keep you out of trouble. Some user 's manuals also presen t a shor t se lect ion o f very bas ic t roub leshoo t ing and main tenance p rocedures ,bu t these are a lmost a lways re la ted to the p r in ter se tup and oper at ion — not to it s internal circuitry or mechanics.

You can find techn ical informa tion on many individual com pon ent s on data sheetspub l ished by the comp onen t m anufactu rer. Fo r exam ple , if you wan t a p in d iagram ofan IC manufactured by Motorola, you could refer to a Motorola data book containinginformation on that part icular component. The data book tel ls you what the part is ,what i t does, what purpose each pin performs, and what i ts electrical specificationsare. Although data books bear no direct relat ionship to your part icular printer, theycan give you much insight on the purpose and functions of individual components.

Ho we ver, i f you in tend to pursue deta i led e lect ron ic re pai rs , you w i ll need a se to f schemat ics . A complete se t o f schemat ics can qu ick ly and eff ic ien t ly gu ide youth rough even the most compl icated p r in ter. I f you are work ing on an o lder p r in ter,t h e re m ig h t b e a co m p le te d o cu m en ta t io n p ack ag e p u b l ish ed b y H o w ard W. S am s &

Co. T h e i r co m p reh en s iv e Sams Photofact series has long be en an indispen sable parto f the e lect ron ic serv ice indust ry. The address and phone number fo r Howard W.Sams & Co. are l isted in appendix C. A manufacturer 's maintenance manual also offers parts l ists and mechanical d iagrams that clarify how the printer is assembled.

Your printer manufacturer can be a key source of information, but not al l manufacturers are will ing to sell technical information to individuals or private organizat ions . S tar t by check ing d i rect ly wi th the manufactu rer. Thei r phone number i susually l isted so m ew he re in the user 's manual. If no user man ual is available, you canprobab ly f ind the m anufactu rer in app end ix C . You can t ry to con tac t thei r tech n icall i tera tu re , par ts o rder, o r serv ice depar tments to o rder a serv ice o r repai r manual .Serv ice in fo rmat ion can be expensive (as much as $50 o r more) so be p repared .

If you can not get satisfaction from the manu facturer, ch eck with a local de aler(not a retai l store) that sells for that manufacturer. The Yellow Pages of you r localte leph one bo ok can g ive you go od leads . A rep u tab le de aler can get par ts and technical information that you cannot. Finally, try contacting a service organization thatrepai rs your typ e o f p r in ter. The y migh t b e wi l ling to o rder a copy fo r you , bu t so meorganizations prefer that you bring the printer in for their repair services.

Electricity hazardsNo mat ter how harmless your p r in ter migh t appear, a lways remember that po

ten t ia l shock hazards ex is t . Once th e p r in ter i s d isassemb led , there can be s everal locat ions where l ive ac vo l tage i s exposed and eas i ly access ib le . Domest ic e lec t ron icequ ipm ent opera tes on 120 Vac a t 60 Hz. Som e Eu ropea n cou n tr ies use 240 Vac at50 Hz. When voltage of th is level establishes a path through your body, i t causes aflow of current that might be large enough to stop your heart . Because i t only takesabou t 100 m A (mil l iam per es) to t r igger card iac ar res t , and a typ ical p r in ter fuse i sra ted fo r 1 o r 2 A, fuses and c i rcu i t b reakers wi l l N O T p ro te ct you . Th e h igh vo l tageavailable in laser printers also present serious shock hazards.




gras p bo th ends o f a l ive ac l ine to com ple te a s t rong c urre n t pathth ro u g h y o u r h ea r t .

4 . Inspect your test probes carefully before test ing high-voltage circuitry.S t a n d a r d " o f f - t h e - s h e l f " p r o b e s d o n o t n e c e s s a r i l y h a v e t h ei n s u l a t i n g p r o p e r t i e s ( c a l l e d d i e l e c t r i c s t r e n g t h t o p r o t e c t y o u . Ifyou m ust make po we red tes ts on a h igh-vo l tage c i rcu it , be su re to use tes tleads that offer sufficient pro tec tion .

5 Wo r k d r y Do no t work w i th w et hands o r c lo th ing . Do no t wo rk in w et o rdam p env ironm ents . Make su re that any availab le f i re ex tmg uish ing

equipment is suitable for electrical f ires.6 . Treat e lec t r ic i ty wi th the p roper respect . Whenever e lec t ron ic c i rcu i t ry i s

exp ose d (es pecia l ly pow er supp ly c i rcu i t r y ) , a shock hazard ex is ts .Re m em ber that i t i s the f low of cu rren t th roug h your body, no t the vo l ta gepotential , that can injure you. I n s u l a t e y o u r s e l f a s m u c h a s p o s s i b l ef r o m a n y e x p o s e d w i r i n g .

Static electricityAnother troubleshooting hazard can come from stat ic voltages accumulated on

your bo dy or tools. If you have eve r walked across a carp eted floor on a dry win ter day,you have p robab ly exp er ienced the effects o f E SD (electrosta t ic d isch arg e) f irst handwhile reaching for a me tal object . Unde r the right condit ions, you r bo dy can accu mulate stat ic charge potentials greate r than 20,000 V. W hen y ou pr ovid e a conduc tive p athfor electrons to f low, that buil t-up charge rushes aw ay from your b od y at the point closest to the object. The result is often a brief, stinging sh ock. Such a jolt can be startlingand annoying, but is generally harmless to people. Semiconductor devices, however,are highly susceptible to damage from ESD while you handle or replace circuit boardsand components. This section introduces you to stat ic electrici ty, and shows you howto prevent ESD damage during your repairs.




Static formationWhen two d iss imi lar mater ia ls are rubbed together, the fo rce o f f r ic t ion causes

elect rons to move f rom one mater ia l to ano ther. The excess (o r lack) o f e lec t ronscause a charge to develo p on each mater ia l . Beca use e lect rons are no t f lowing , thereis no current, so the charge is said to be static. Ho we ver, the charg e doe s exh ib i t a

vo l tage po ten t ia l . As mater ia ls con t inue to rub together, thei r charges increase—som et imes to po ten t ia ls o f thousands o f vo l ts .

In a human , s ta t ic charges can be dev elop ed b y normal eve ryd ay act iv i t ies suchas walk ing on a carpet . F r ic t ion betw ee n the carp et and shoe so les cause opp osingcharges to be developed . The charge on the shoe induces an equal (bu t opposi te)charg e in you r body, wh ich a cts as a capac itor. Slid ing across a viny l car seat , pull inga swea ter on or off , or taking clothes out of a dry er are just som e of the ways that astat ic charge can appear in the body.

Device damage

ESD poses a ser ious th reat to v i r tual ly a l l modern semiconducto r dev ices . Hugesta tic vo l tages that bu i ld up in the env ironm ent (o r in your b od y) can find thei r w ayinto al l types of advanced ICs. If that happens, the result for the component can becatas t roph ic . S ta t ic d ischarge can damage b ipo lar t rans is to rs , TTL dev ices , ECL(emit ter-coup led log ic)dev ices , operat ional ampl i f iers , SCRs (s i l icon-con tro l led rect i f iers) , and JFE Ts ( junct ion f ie ld -effect t rans is to rs) . Cer ta in ly the mos t suscep t ib leco m p o n en t s a re th o se f ab r i ca t ed u s in g MO S (m eta l -o x id e sem ico n d u c to r ) t ech n o logy. A typica l MO S transistor is shown in Fig . 4-3 .

MO S d ev ices (P MO S , N MO S , H MO S , C MO S , e t c . ) h av e b eco m e th e co rn e r s to n eo f h ig h -p e r fo rm an ce IC s as m em o r ies , h ig h - sp eed lo g ic , m ic ro p ro ces so r s , an do th e r ad v an ced d ig i t a l co m p o n en t s . MO S tech n o lo g ies o ffe r h ig h - sp eed , h ig hco m p o n en t d en s i ty, an d lo w p o w er co n su m p t io n . Ty p ica l MO S IC s can eas i ly c ramove r 1 mi l l ion t ransis to rs on to a s ing le IC . Ev er y par t o f the t rans is to r mu st b em a d e co n t in u a l ly sm a l le r t o k ee p p ac e w i th th e d em a n d s fo r h ig h e r l ev e l s o f i n t eg ra t io n . A s eac h p a r t o f t h e t r an s i s tor sh r in k s , h o w ev e r, b reak d o w n v o l t ag e s d ro p ,an d E S D d am ag e p ro b lem s esca la t e .




A typ ical MOS transistor brea kd ow n is show n in Fig . 4-4 . No tic e the areas of posit ive and negative semiconductor material that forms i ts three-terminals: source,gate, and drain . The gate is isolated from othe r parts of the transistor by a th in fi lmof si l icon dioxide (sometimes called the oxide layer). Unfortun ately, th is layer is extremely th in , and i t can be easily overcome by high voltages l ike those from stat ic

discharges. Once this happens, the oxide layer is punctured. This renders the entiret ransis to r (and the whole IC) permanen t ly defect ive .

Source Drain

Silicondioxide

A typical MOSgate junction

Electrostatic discharge(ESD) occurs atjunction

Permanently damagedMOS junction

4-4 The effect of MOS breakdown.

Controlling static electricity

Do no t underes t imate the impor tance o f s ta t ic con tro l dur ing your p r in ter repairs. Withou t realizing i t , you co uld des troy a new IC or circuit assem bly befo re youeven have a chance to insta ll i t— and you wou ld neve r eve n kno w that sta t ic dam agehas occurred. All i t takes is the careless touch of a charged hand, tool , or p iece ofclothing. Take the necessary steps to ensure the safe handling and replacement ofyour sensi t ive (and expensive) e lec t ron ics .

One way to control stat ic is to keep charges away from boards and ICs. This stat ic control is often a part of de vice pack aging and the sh ipping container. ICs are usual ly pack ed in a specia l ly -made c ondu ct ive foam. Carbon g ranu les are com pou nde dright in to the polyethylene foam to achieve conductivity of about 3 ,000 Q/cm (ohmsper ce n t im ete r) . Foam p reven ts ben d ing o f IC leads , absorbs v ib ra t ions and shocks ,



Reassembly and disassembly hints 75

only long enough to follow your part icular troubleshooting and test ing procedures,then unplug it again.

Laser printers contain a wide array of electrical connectors handling everythingfrom ac line voltage to laser control signals. During disassembly, you might have to remove one or more connectors to free a circuit board or o ther subassembly. Never r e

move a connector by yanking on i ts wires—many connector shells use keys or latchesto hold them in place. Alw ays r em ove a connec tor by holding i ts shell . Take careful n oteof each connector location and orientation. Some connectors are keyed so they canonly be reinserted in their proper orientation, but o ther type s of connec tors might notbe th is foolproof. Make a sketch of wh ere everyth ing go es befo re taking things apart.

Take careful note of physical parts as well , especially when you must d isassemble complex drive trains of gears or pulleys. I t wil l help you tremendously when i tcomes t ime to reassemble the sys tem. Mark your par ts befo re d isassembly wi th anindelible fel t- t ip marker. Feel free to use any kind of markings that are clear to you,but marks should show how each part is mounted in relat ion to i ts adjacent parts.Fig . 4-5 is just one simple exa m ple .

ReassemblyW he nev er you m ust rep lace ICs on a th rough-ho le p r in ted c i rcu i t board , a lways

solder an IC holder in its p lac e, then plug the rep lace me nt IC into its holder. Prin tedci rcu i t boards (especia l ly complex boards) are very del ica te , and p r in ted t races canbe damaged by excessive or repeated heating. If you instal l an IC holder in i ts p lace,you will never have to desolder those points again . To replace that IC in the future,just unplug i t and instal l a new one. Keep in mind there might not be enough roomto instal l an IC sock et if the boa rd is loca ted close to an enclosu re of anoth er a ssembly. Use your best judgment to decide if there is enough room for a socket.

Alwa ys doub le check your con necto r locat ions and o r ien ta t ions befo re app ly ingpower. If a connector is engaged backward or is skipping pins, your circuits can beser iously damaged . I f you have made o r ien ta t ion marks on the connecto rs befo redisassembly, they should be a snap to install p roper ly.

Gear train

Alignmentmarks

4-5 Making alignment marks before disassembly.




Metal shields or shrouds are often added to l imit RF ( r ad io - f r eq u en cy ) in t e r f e rence between c i rcu i ts . Swi tch ing-regu la ted power supp l ies and h igh-speed dev icessuch as microp rocesso rs are o f ten sh ie lded thorough ly. Th is p reven ts no ise gen erated in one circuit from causing false signals in ano ther circuit . If you have e ve r see nor heard rad io o r te lev is ion recep t ion near a comp uter, then you have p roba b ly w i t

nessed the effects o f RF no ise . Because a p r in ter uses many o f the same e lect ron iccomponen ts that a computer does , i t too can generate no ise . Be cer ta in that a l l RFnoise shielding is instal led and secu red p roper ly. You can add me tal or p last ic guard sto protect physical parts such as drive trains. Be sure you replace al l protective covers before re-test ing your laser printer.



C H A P T E R

Electrophotographic

Elect roph o tograp h ic ( E P ) p r in ters are d i fferen t f rom o ther types o f p r in ters thatyou migh t be familiar wi th (such as do t matr ix o r ink je t p r in te rs) . Those conve ntional printers develop dots as a one-step process using impact, heat , or ink. EPprinters (Fig . 5-1) are not nearly as simple. EP images are formed by a complex anddelicate in teraction of l ight , stat ic electrici ty, chemistry, pressure, and heat—allgu ided by a soph is t ica ted e lect ron ic con tro l packa ge ( E C P ) . Th is chap ter deta i ls thebackground o f EP techno logy, and exp la ins the operat ion o f the image-fo rmat ion

system of your laser printer.

technology

77



78 Electrophotographic technology

The electrophotographic approachAs a resu lt o f the in teract ion o f e lem ents , E P p r in t ing i s accom pl ished th rough a

process ra ther than th rough a p r in t head . The co l lec t ion o f componen ts that performs the EP printing process is called an IFS ( im ag e- fo rm at io n sy s t e m ) . A n IF S is

mad e up of e igh t d is t inct ive areas (see F ig . 5 -2 ) : a pho tose nsi t ive d ru m ( 1 4 ) , c leaning b lade, erasure lamp ( 3 ) , p r imary corona (4 ) , wr i t ing mecha n ism (5 and 6 ) , toner,transfer coron a ( 1 3 ), and fusing rollers (18 and 1 9). Eac h of the se parts p lay an important role in the proper operation of an IFS.

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®oj ®@ ® © ®@®® ®® ©

1. Delivery assembly 11. Registration rollers2. Face-down tray 12. Transfer corona roller3. Erase lamp assembly 13. Transfer corona assembly4. Primary corona 14. Photosensitive EP drum5. Beam-to-drum mirror 15. EP drum prote ctive shield6. Laser/scanning assembly 16. Feed guide assembly7. Main body covers 17. Lower main body8. Paper tray 18. Upper fusing roller9. Separation pad 19. Low er pressure roller

10. Feed roller assembly 20. Face-up output tray (closed)

5-2 Cross-sectional diagram of a laser printer. Hewlett-Packard co

A photosensit ive drum, such as the one shown in Fig . 5-3 , is considered thehear t o f any IFS . An ex t ru ded a luminum cy l inder i s coate d wi th a non t ox ic o rgan iccom poun d that exh ib i ts pho tocon duc t ive p ro per t ie s . Tha t is , the coat ing wi ll conduct e lec t r ic i ty when exposed to l igh t . The a luminum base cy l inder i s connected toground o f the h igh-vo l tage po we r supp ly.



The electrophotographic approach 7 9

I t is the drum that actually receives an image from a writing mechanism, d evelops the image wi th toner, then t ransfers the developed image to paper. Al thoughyou might th ink that th is consti tu tes a print head because i t delivers an image to paper, the image is no t yet permanen t—other operat ions must be perfo rmed by theIFS . Complete image development i s a s ix -s tep p rocess that invo lves a l l e igh t IFScomponents: cleaning, charging, writ ing, developing, transfer, and fusing. To understand the IFS, you should know each of these steps in detail .

Clean ingBefore a new pr in t ing cycle can beg in , the pho tosensi t ive d rum must be physi

cally cleaned and electrically erased. Cleaning might sound l ike an unimportant step,bu t no t eve n the bes t d rum wi l l t ransfer e very micros cop ic g ran u le o f toner to a pageevery t ime. A rubber cleaning blade is applied across the entire length of the drumto gently scrape away any residual toner that might remain from a previous image. Ifres idual toner wer e no t c leaned , i t cou ld adhe re to subsequen t p ages and appear asrandom b lack speckles. Tone r that is rem ove d f rom the d rum is dep osi ted in to a debris cavity as shown in Fig . 5-4 . Keep in mind that cleaning must be accomplishedwithou t scra tch ing o r n ick ing the d rum. An y dam age to the pho tosen si t ive d rum sur-

Rubber cleaning ,blade

Debriscavity

Photosensitiveouter layer

Waste

5-4Simplified diagram of a drumcleaning assembly.



80 Electrophotograp hic technology

f ace w o u ld b eco m e a p e rm an en t m ark th at w o u ld ap p ea r o n ev e ry su b seq u en t p ag e .Som e EP p r in ter des igns actual ly re tu rn scrap toner back to the toner supp ly for reuse . Th is k ind of recycl ing techn iqu e can ex t en d the li fe o f you r e lec t rop ho tog raph ic(E P ) car t r idge and e l iminate the ne ed fo r a large debr is cav i ty.

Images are wri t ten to a d rum surface as hor izon ta l rows o f e lec t r ica l charges

that co rrespo nd to the imag e being p r in ted . A do t o f l igh t causes a re la t ively posi t ivecharge a t that po in t . The do t o f l igh t co rres pond s to a v isual do t in the c om plet ed image. Absence o f l igh t a l lows a re la t ively negat ive charge to remain and no do ts areg en era t ed . T h e ch a rg es cau sed b y lig h t m u s t b e r em o v ed b e fo re an y n e w im ag es canb e w r i t t en — o th er w ise im ag es w o u ld o v e rw r i t e an d su p er im p o se o n o n e an o th e r.

A series of erase lamps are plac ed near the drum surface. The ir l ight is f il teredto a l low on ly effect ive waveleng ths to pass . Erase l igh t b leeds away any chargesalong the d rum. Charges are carr ied to g round th rough the a luminum cy l inder asshown in F ig . 5-5. Af t er erasure , the d ru m surface is com plete ly ne u t ra l— it con ta insno charges at all.

Controlcircuits

Staticelectriccharges

5-5 Discharging the EP drum with an erase lamp.

ChargingThe neutral drum surface is no longer receptive to l ight from the writ ing mecha

nism. New images cannot be writ ten unti l the drum is charged again . To charge (orcondition) the drum, a uniform electrical charg e must be app lied eve nly across i ts en

tire surface. Surface charging is acco mp lished by applying a trem end ous neg ative vo ltage (o ften m ore than -6 ,00 0 V ) to a solid wir e called a primary corona located c loseto the d rum. Because the d rum and h igh-vo ltage po we r supp ly share the same g round ,an electrical f ield is established b etw ee n the corona w ire and dru m as in Fig . 5-6 .

A t low vo l tages , the a ir gap be tw ee n a co rona wire and d rum w ould act as an insulator. With thousands of volts of potential , however, the insulating strength of airb reaks down and an e lect r ic corona fo rms. A corona ion izes any a i r mo lecu les su rround ing the wire , so negat ive charges migrate to the d rum surface .

Th e t roub le w i th ion ized gas is that it exh ib i ts a ve ry lo w res is tance to cu rren tf low. Once a co rona is es tab l ished , there i s essen t ia l ly a shor t c i rcu i t between the



The electrophotographic approach 81

Primary corona

High-voltagepower supplyassembly

Photosensitivedrum

Ionized air

Primary grid.

- 6 k V

-6 0 0 V

Use of a control gridplaces a -6 00 V chargeon the drum's surface

Chargingvoltage

Regulatinggrid voltage

5-6 Conditioning the drum with the primary corona.

wire and d rum . Th e shor t c i rcu i t i s no t goo d fo r a h igh-vo l tage p ow er supp ly. A pri

mary grid (par t o f the p r imary corona asse m bly) is add ed be tw ee n the wir e anddrum. By app ly ing a neg at ive vo l tag e to the g r id , charg ing vo l tag e and curren t to thedrum can be carefu l ly regu la ted . Th is regulating grid voltage ( o f t en -6 0 0 to -1 ,0 0 0V ) se ts the charge lev el ac tually app l ied to the d rum that i s typ ical ly equal to the regu la t ing vo l tage ( -600 to -1 ,000 V) . The d rum is now ready to receive a new image.

To fo rm a la ten t image on a d rum surface , the un i fo rm ch arge that has cond it ioned the d rum must be d ischarged in the p recise po in ts where images are to beproduced . Images are wri t ten us ing l igh t . Any po in ts on the d rum exposed to l igh twi l l d ischarge to a very lo w level (ab ou t -1 00 V ) , and any areas lef t unex pos ed re ta inthei r cond i t ion ing charge ( -6 00 to -1 ,00 0 V ) . Th e dev ic e that p rod uces and d i rectsl ight to the drum surface is called a writing mechanism.

Because images are fo rmed as a ser ies o f ind iv idual do ts , a h igher number o fdo ts per area a llows finer reso lu t ion (and h igher q ual i ty ) o f the image. For exam ple ,

sup pose a writ in g mec han ism can place 300 dots per inch on a single horizontal l ineon the d rum, and the d rum can ro ta te in increments o f <*> of an inch. Using thesespeci f ica t ions , your p r in ter can develop images wi th a reso lu t ion o f 300 x 300 do tsper inch ( d p i ) . Current EP printers are reaching 600 x 600 dpi.

Lasers have been t rad i t ionally used as wri t ing m echan ism s ( thus the nam e" laser p r in t er" ) , and are s ti ll u sed in ma ny E P p r in ter des igns , bu t new p r in ters arereplacing lasers with bars of l ight-emitt ing diodes (LEDs) or arrays of l iquid crystalshu t ters (L CS s) to d i rect ligh t as ne ed ed . (Wri t ing mechan isms are cov ere d m oreex tens ively in th is chap ter. ) O nce an image has be en w ri t ten to a d rum, that im agem u s t b e d ev e lo p ed .

Wri t ing



The electrophotographic approach 83

No tice tha t an ac bo ost er b ias (mo re than 1 ,500 V ) is ad de d in series to the dcintensity bias. The ac causes strong fluctuations in the toner charge level . As the acs ignal goe s posi t ive , the in tensi ty level increases to help toner par t ic les ove rco m e att rac t ion o f the perm anen t m agne t o f the cy l inder. As the ac s ignal goe s neg at ive , intensi ty levels decrease to pu l l back any toner par t ic les that migh t have fa lse ly

jumped to unexposed areas . Th is techn ique improves p r in t densi ty and image contras t . The developed image can now be app l ied to paper.

TransferAt th is po in t , the developed toner image on the d rum must be t ransferred on to

paper. Because toner i s now at t rac ted to the d rum, i t must be p r ied away by app lying an even larger a t t rac t ive c harge to the p age. A transfer corona wire charge s thepage as shown in F ig . 5-8. The theo ry beh ind the o perat ion o f a t ransfer co ron a is exact ly the same as that fo r a p r imary co ron a— ex ce p t that the po ten t ia l i s no w posit ive . Th is p laces a pow erfu l p osi t ive charge on paper, which a t t rac ts the n egat ive lycharged to ner par t ic les . Re m em be r that th is i s no t a per fect p roc es s— no t a ll toneris transferred to paper. This is why a cleaning process is needed.

Photosensitivedrum

Paper(no netcharge)

High-voltagepower supply

assembly

5-8 Transferring the dev eloped image to paper.

Cau t ion is needed here . Because the negat ively charged d rum and posi t ivelycharged paper a t t rac t each o ther, i t i s poss ib le that paper cou ld wrap around thedrum. Even though the smal l -d iameter d rum and natu ra l s t i ffness o f paper tend topreven t wrapp ing , a s ta t ic charge e l iminato r (o r eliminator comb) is inclu ded tocoun teract posi t ive charges and remove the a t t rac t ive fo rce between paper and




drum immediate ly af ter toner i s t ransferred . The paper now has no net charge . Thedrum can be c leaned and p repared fo r a new image.

FusingOnce the toner image has reached paper, i t i s on ly held to the page by g rav i ty

and weak e lect ros ta t ic a t t rac t ion—toner i s s t i l l in i t s powder fo rm. Toner must bef ixed permanen t ly (o r fused) to the page befo re i t can be hand led . Fusing is accomplished with a heat and pressure assembly l ike the one shown in Fig . 5-9 . A high-intensity quartz lamp heats a nonstick roller to about 180°C. Pressure is applied witha p l iab le rubber ro l ler. When a developed page passes between these two ro l lers ,heat f rom the top ro l ler mel ts the toner, and p ressure f rom the bo t tom ro l lersque ezes mo l ten toner in to the pape r f ibers . In the fiber, the toner coo ls and adh erespermanently. The fin ished page is then fed to an output tray. Note that both rollersare called./ksmc7 rollers, eve n thou gh only the he ate d top roller actually fuses.

5-9 Fusing the transferred image to paper.

To pr eve nt t one r part icles from sticking to a fusing roller, i t is coa ted w ith a nonstick material such as Teflon™. A cleaning pad is added to wipe away any toner thatmigh t yet adh ere. Th e pad also applies a th in coating of si licon oil to pre ve nt furthersticking.

F u s in g t em p e ra tu re m u s t b e ca re fu l ly co n t ro l l ed . O f t en a th e rm is to r is u sedto r eg u la t e cu r ren t t h ro u g h th e q u a r t z l am p to m a in ta in a co n s tan t t em p era tu re .A snap -act ion therm al swi tch a lso i s inclu ded as a safe ty in ter lock to p rev en td am ag e in case th e lam p t em p era tu re sh o u ld ri se o u t o f co n t ro l . I f t e m p e r a t u r ei s n o t c o n t r o l l e d c a r e f u l l y, a f a i l u r e c o u l d r e s u l t in p r i n t e r d a m a g e , o re v e n a f i r e h a z a r d .

Wri t ing mechanismsAfte r charg ing , the pho tosens i t ive d rum con ta ins a un i fo rm elect ros ta t ic charge

across i ts surface. To form a latent im age , the dru m mus t be d ischa rged at any po intsthat comp rise the image . L igh t i s used to d ischarge th e d rum as need ed . Such a wri ting mech anism is sho wn in Fig . 5-10.



Writing mechanisms 85

Writing mechanism

5-10 Simplified diagram of a gen eric writing mechanism .

Images are scanned onto the drum one horizontal l ine at a t ime. A single passacross the drum is called a trace or scan line. Light is d irec ted to any points along thescan l ine wh ere dots are required. W he n a scan l ine is com plete d, the drum increm entsin preparation for another scan line. I t is up to the printer con trol circuits to break do wnan ima ge into individual scan l ines, then direct the w rit ing m echanism accordingly.

LasersLasers have been around s ince the ear ly 1960s , and have dev elop ed to the p o in t

where they can be manufactu red in a g reat var ie ty o f shapes , s izes , and power ou tpu t levels . To unders tand why lasers make such a usefu l wr i t ing mechan ism, youmust unders tand the d i fference between laser l igh t , and o rd inary whi te l igh t asshow n in Fig . 5-11.

Ord inary w hi te l igh t is ac tual ly no t whi te . Th e l igh t you see i s com pos ed o f m anydifferen t waveleng ths , each t ravel ing in i t s own d i rect ions . When these var iouswave leng ths com bine, they do so v i r tually at random . Th is makes eve ryda y l igh t verydifficult to direct and almost impossible to control as a f ine beam. As an example,take a f lashlight and dir ect i t at a far wall . You will see just ho w m uch w hite l ight canscatter and disperse over a relat ively small d istance.

Th e natu re o f laser l igh t , how ever , is mu ch d i fferen t . A laser bea m con ta ins on lyone major wavelength of l ight ( i t is monochromatic). Ea ch ray travels in the sam edirect ion and comb ines in an add i t ive fash ion (kn ow n as coherence). T h ese ch a racterist ics ma ke laser l ight easy to d irect at a targ et as a hair-th in beam , wit h almos t noscat ter (divergence]. Older EP p r in ters used hel ium-neo n (H e N e ) gas lasers , bu tstrong semiconductor laser d iodes have essential ly replaced gas lasers in almost al llaser printing applications.




l igh t pow er. Th is t rade o ff i s usual ly wo r th wh i le fo r the smal l s ize , l igh t w eig h t ,and h igh re l iab i l i ty o f a semiconducto r laser.

G en era t in g a l a se r b eam i s o n ly th e b eg in n in g . T h e b eam m u s t b e m o d u la t ed( tu r n ed o n an d o ff ) w h i l e b e in g sw ep t ac ro ss th e d ru m su r face . B e am m o d u la t io ncan b e acco m p l i sh ed b y tu rn in g th e l a se r o n an d o ff a s n e ed ed (u su a l ly d o n e w i thsem ico n d u c to r l a se r d io d es ) a s sh o w n in F ig . 5 -1 3 , o r b y in t e r ru p t in g a co n t in uo u s b eam w i th an e l ec t ro -o p t i ca l sw i t ch ( ty p ica l ly u sed w i th g as l a se r s th a t a red i ff icu l t to swi tch on and o ff rap id ly ) . Mirro rs are used to a l ter the d i rect ion o f thel ase r b eam , an d len ses a re u sed to fo cu s th e b ea m an d m ain ta in a lo w d iv e rg e n c eat a l l po in ts a long the beam path . F igure 5 -13 is one i l lus t ra t ion o f a laser wri t ingm ech an i sm , b u t i t sh o w s th e co m p lex i ty th a t i s i n v o lv ed . T h e w e ig h t o f g l a s sl en ses , m i r ro r s , an d th e i r sh o ck m o u n t in g s h av e k ep t E P l a se r p r in t e r s b u lk y an de x p e n s i v e .

To external device

1Interface PCA

BD VDO VSTNC

7 rDC controller

BDPD LDRV SCNCONT

Scanningmirror

Scannermotor

Focusinglenses

Beam detect mirror

Mirror

Photosensitive drum

-100V Chargewhere laserstrikes

-600V Charge

where laserdoes not strike

5-13 Diagram of a laser/scanning system. Hewlett-Packard co

A l ig n m en t h as a lw ay s b een an u n av o id ab le p ro b lem in co m p lex o p t i ca l sy stems such as F ig . 5 -13 . Consider what migh t happen to the beam i f any op t ica lco m p o n en t sh o u ld b e co m e d am ag ed o r f al l o u t o f a l ig n m en t— fo cu s an d d i r ec t io np ro b lem s co u ld r en d e r a d ru m im a g e u n in te l l ig ib l e . R ea l ig n m en t o f o p t i ca l sy s t em sis v i r tual ly imp ossib le wi th ou t sp ecia l a l ignme nt too ls , and is be yo nd th e scop e o f




th is book . F inal ly, p r in t ing speed is l imi ted by the speed o f moving par ts , and thera te a t which the laser beam can be modulated and moved .

L E D sFortunate ly, a pho tose nsi t ive d rum is rece p t iv e to ligh t from m any d i fferen t

sources . Even l igh t f rom l igh t -emit t ing d iodes (LEDs) can expose the d rum. By fabricating a series of micro scop ic L ED s into a single scan line as sho wn in Fig . 5-14, anLED can be p rov ided fo r every poss ib le do t in a scan l ine . For example , the ROHMJE3008SS02 is an LED print bar containing 2 ,560 microscopic LEDs over 8 .53inches . Th is number o f LEDs equates to 300 do ts per inch . Each LED is jus t50 x 65 micro me ters (u m ), and they are spaced 84 .6 um apar t .

5-14 Diagram of an LED print bar in operation.

Th e op eration of an LE D p rint bar such as the on e show n in Fig . 5-15 is remarkablystraightforward. An entire series of data bits corresponding to each possible dot in ahorizonta l line is shifted into intern al digital circuitry within th e print bar. Do ts that willbe visible are rep res en ted by a logic 1, and dots that are not visible will remain at logic0. For a de vice such as the JE3008SS0 2, 2,560 bits must be en tered for each scan l ine.

Afte r a comp lete l ine o f data has bee n loaded th roug h the D IN (da ta in ) p in , theLED s must be f i red . Th is is per fo rm ed in segm ents to reduc e the pow er su rges thatwould be generated i f every LED were f i red together. The JE3008SS02 is d iv idedinto four segments of 640 dots. A tr igger signal (or strobe) can be app l ied to STR 1through STR4. The signal passes data to driver circuits of each segment. LEDs thati l luminate will d ischarge latent points on the drum surface. LEDs that do not l ightwill have no effect . E ach strob e is f ired sequ ential ly unti l all four seg me nts ha ve b ee nstrob ed. All 2 ,560 dots can be sca nned in und er 2 .5 mill isecon ds (m s ). Th e drum isi n c r e m e n t e d lA m of an inch, and a ne w scan l ine can be load ed in to the pr int bar.

You can probably see the advantages of an LED print bar system over a laser approach . The re are no mo ving par ts invo lved in l igh t del ive ry— no mirro r moto r to jamor wea r ou t . Th e p r in ter can operate a t much h igher sp eeds becau se i t does no t haveto ov er co m e the dynam ic limitat ions of mo ving parts. Th er e is only one focusing lens




5-15 Partial schematic of an LED print bar. ROHM Corporation.

be tw ee n the p r in t bar and d rum. Th is g reat ly s impl i f ies th e op t ics assembly, and removes substan t ia l weigh t and bu lk f rom the p r in ter. An LED system overcomes a lmost a l l a l ignment p rob lems, so a defect ive assembly can be rep laced o r a l ignedquickly and easily.

LCSsA print bar does not necessari ly have to generate i ts own l ight. Liquid crystal

shu t ters (L CS s) con tro l the t ransfer o f l igh t f rom a s ing le f luorescen t lamp sourc e tothe d rum su rface as sho wn in Fig . 5-16. Inste ad of L ED s, an array of individual l iquid

crystal shutters is fabricated in to a single scan l ine—one shutter for every possible

Fluorescent light

5-16 Diagram of an LCS print bar in operation.




dot in a scan line. W he n a shutter is on (o p e n ), light is al low ed th roug h at that p oint .If the shu tter is off ( cl os ed ) l ight at that point is b lo ck ed off .

Data i s loaded and t r iggered in an LCS pr in t bar in much the same way as fo rLED systems. An entire horizontal scan l ine of data is loaded into the print bar. Onebit of data is supplied for each shutter. Visible dots can be represented by a logic 1 ,

and invisible dots can be represented by a logic 0 . The scan l ine is tr iggered in segments by a series of strobe signals sent by the printer ECP.

At the t ime of th is writ ing, l iquid crystal shutters suffer from several importantdrawbacks. First , the l ight source is very cri t ical to proper drum exposure. If the fluorescent l ight source becomes old or d irty, i t might shed l ight unevenly, so l ight intensity can vary along i ts length . Un eve n l ight d irectly results in un eve n drumexposure—even i f the LCS pr in t bar i s work ing f ine . L iqu id crys ta l has a re la t ivelys low response t ime ( the t ime fo r a shu t ter to fu l ly open o r c lose) compared to lasero r LED scann ing . As a resu l t , ma x imu m p ract ica l p r in t ing spe ed is sever ely l imi tedto just a few pag es p er rninute. Finally, resolution also is l imite d. Cur rent l iquid c rystal fabrication techniques al low no more than 300 dots per inch. Unless these l imitat ions are overcome, LCS techno logy wi l l never reach the accep tance that LED pr in tbars have en joyed .

The electrophotographic cartridgeElect ros ta t ic p r in ters mandate the use o f ex t remely t igh t manufactu r ing to ler

ances to ensure p recise , consis ten t operat ion . A d efect o f even a few thousan d ths o fan inch cou ld cause unaccep tab le image fo rmat ion . Ev en the effects o f normal mechanical wear can have an adverse effect on print quali ty. Many key IFS componentswould have to be rep laced every 5,000 to 10,000 pag es to maintain acce pta ble p erfo rmance. Clear ly i t would be undesi rab le to send your p r in ter away fo r a complete(and t ime-consuming) overhau l every 10 ,000 pages .

To ease ma nufactu r ing d i fficu l t ies and p r ov ide fast , a ffo rdab le m ain te nan ceto every ES p r in ter user, cr i t ica l componen ts o f the IFS , as wel l as a supp ly o fto n e r, a r e a s sem b led in to a r ep lac eab le e l ec t ro p h o to g r ap h ic (o r E P ) ca r t r id g e . A sFig . 5-17 show s, a typ ica l E P ca r t r idg e con ta ins the ton er ro l ler, ton er supp ly, debr is cav i ty, p r imary corona (and p r imary g r i d ) , p h o to sen s i t iv e d ru m , an d c l ean in gb lad e as sem b ly. A l l n ecessa ry e l ec t r i ca l co n n ec to r s an d d r iv e g ea r s a re in c lu d ed .B y assem b l in g sen s i t i v e co m p o n en t s in to a s in g le r ep laceab le ca r t r id g e , p r in t e rr e l i ab i l i t y i s su b s t an t i a l ly im p ro v ed b y p rev en t in g p ro b lem s b e fo re th ey b eco m e

not iceab le . The cost o f an EP car t r idge i s low enough to consider i t a d isposab lepar t .

A ty p ica l E P ca r t r id g e can p ro d u ce 2 0 0 to 5,000 p r i n t e d p a g e s . T h e e x a c tn u m b er v a r i e s d ep e n d in g u p o n ju s t h o w m u ch to n e r i s av a i l ab le , an d w h ich c r it ica l p a r t s a re p l aced in th e ca r t r id g e— h ig h ly in t e g ra t e d E P ca r t r id g es w i l l l as tlonger than s imple car t r idges . Because toner i s comprised par t ia l ly o f o rgan ic mater ia ls , i t has a l imi ted usefu l l ife (o f te n s ix mo nths af ter th e car t r id ge i s re m ov edf ro m i t s sea l ed co n ta in e r ) .



The electrophotographic cartridge 91

5-17

B An EP cartridge.

Protecting an EP cartridgeAs you migh t imag ine , the p recis ion com pon en ts in an EP car t r idge are sensi t ive

and del ica te . Th e pho tosensi t ive d rum and tone r supp ly are par t icu lar ly sen si t ive tolight and environmental condit ions, so follow the handling and storage guidelines.

The pho tosensi t ive d rum is coated wi th an o rgan ic mater ia l that i s ex t remelysensi t ive to ligh t . Al thou gh a meta l sh roud covers the d rum w hen the car t r idge i s exposed , l igh t migh t s t il l pen et ra te th e sh roud and cause exposu re (a lso know n as fog

ging). Dea ctivating the printer for a t im e will often elim inate mild fogging . D o notremove or in terfere with the proper action of the shroud in open l ight unless ab-




Rotate the cartridge slowly and

gently. NEVER shake it violentlyor stand it on either end

solutely necessary, and then only for short periods. Defeating the shroud will certainly fog the drum. You might need to place a seriously fogged cartr idge in a darkarea fo r several days . Ne ver exp ose the d rum to d i rect s un l igh t— direct sun l igh t canpermanen t ly damage the d rum coat ing .

Avoid ex t remes o f temperatu re and humid i ty. Temperatu res exceed ing 40°C can

permanen t ly damage an EP car t r idge. Ex treme humid i ty i s near ly as dangerous . Donot a l low the car t r idge to become exposed to ammonia vapors o r o ther o rgan ic so lven t vapors— they b reak down the d rum coat ing very qu ick ly. F inal ly, keep a cart r idge secure and level . Nev er a l low it to be d rop ped o r abused in any way.

As the toner supply diminishes, i t might be necessary to redistr ibute remainingtoner so that i t reaches the toner roller. Because toner is available along the entirecartr idge, i t must be redistr ibuted by rocking the cartr idge back and forth along i tslong axis as show n in Fig . 5-18. If you t ip a cartr idg e up right, rema ining t one r will fallto one end and cause uneven distr ibution.



C H A P T E R

Power supplies

All electrical and electronic components in your laser printer (Fig . 6-1), as well asevery o ther p iece o f e lec t ron ic equ ipment , requ ire e lec t r ica l energy to funct ion . Energy is always supplied in the form of voltage and current, and there must be adequate amounts o f bo th to ensure the p roper operat ion o f each componen t .Unfortunately, ac power available in your home, shop, or office, is not d irectly compat ib le wi th the com pone n ts in your p r in ter. As a resu l t , l ine po w er must be m anipulated and converted in to values of voltage and current that are suitable for yourspecific equipment. Conversion is the task of a power supply. You will f ind thre etypes of power supplies in your laser printer: a dc supply, an ac supply, and a high-voltage supply.

T h e n a m e power supply is mislead ing. A PS (p ow er supp ly ) does no t ac tuallycreate pow er. Ins tead , i t conver ts com mer cia l ly gen erat ed ac po we r in to one o rmo re vo l tage levels bet te r su i ted to part icu lar tasks and comp onen ts . Pow er supp l ies

93



94 Power supplies

are general ly rugged and re l iab le dev ices—so much so that they are o f ten overlooked or disregarded as possible problem sources. Luckily, most supplies are reasonably simple to follow, and can be repaired or replaced with ease. B e f o r e y o ua t t e m p t a p o w e r - s u p p l y r e p a i r , b e s u r e t o r e v i e w t h e h a z a r d s o f h i g h - v o l tag e e l ec t r i c i ty o u t l in ed in ch ap te r 4 .

Many different PS circuits exist . Each is designed to best suit the needs of thespecific circuits that they must supply. In spite of the array of PS arrangements,there are on ly two typ ical operat ing modes— l i n e a r an d switching. B o th m o d es r efer to the way a supply controls i ts output ( s ) . Th is chap ter shows you the const ruction, operation, characterist ics, and repair considerations for both power supplyt ype s . Background for high-voltage supplies is not d iscussed here because of thedangers and s t r ingen t componen t requ irements associa ted wi th h igh-vo l tage syst e m s , but troubleshooting information for the supplies is presented.

Power supplies—ac and linear dcT h e t e rm linear mea ns line or straight. A s shown in the block diagram of Fig.

6-2, a linear PS operates in essentially a straight line from ac input to dc output. Components and po we r capacity can vary bet we en m anufacturers and mode ls, but all lineardc power supplies will contain the same three basic subsections: a transformer, recti

fier, an d filter. T h e regulator block also is found in the majority of linear po w er supplies, but it is not mandatory for a minimum w orking supply. Alternating-current po we rsupplies are generally simple transformers to provide one or more levels of ac to heatthe fusing assembly.

Fuse

Transformer Filter

6-2 Block diagram of a linear power supply.

TransformersA transformer is used to al ter the ac volt age and curren t charact erist ics of ac

input power, al lowing ac to be converted in to more useful levels of voltage and curren t . Th is impor tan t p roces s o f t ransfo rmat ion is accom pl ished th rough the p r incip les o f magnetic coupling as show n in the sche ma tic of Fig . 6-3 .

Transformers use two coils of solid wire wrapped along opposite sides of a commo n metal s t ructu re (ca l led a c o r e ) . Although Fig. 6-3 only shows two leads for eachcoil , ma ny transform ers offer available leads (or taps) f rom bo th the p r imary andsecondary coils. The core is often buil t from laminated plates of permeable material



Power supplies—ac and linear dc 95

Fuse Transformer

ac outputvoltage

/ j Vac 6-3Principles of transformeroperation.

ac inputvoltage

(m eta l that can be m ag ne tiz ed ). Th e co re ser ves not only as a physical base, but it iscri t ical in concentrating magnetic f ields around the transformer as well .

An ac voltage ( or primary voltage), usually 120 Vac, is app lied across the p rimarywmding of a transformer. Alternating current voltage will cause current to fluctuatethrough the primary winding. In turn, this sets up a varying magnetic force field in theprimary. The core concentrates this magnetic field and helps to transfer magnetic forceto the secondary winding. Note that a solid core is not man datory —m agnetic couplingbetween two coils can occur across an air gap—but solid cores make coupling muchmore efficient. This fluctuating magnetic field in the core cuts across the secondarywmding, where i t induces a secondary ac voltage between i ts terminals. Figure 6-3shows an ac voltm eter m easuring this voltage and show s the transformer principle.

Voltage across the secondary winding is d irectly proportional to the rat io of prima ry-to-sec onda ry w inding s. For ex am ple, if the re are 1 ,000 turns of wir e in the primary coil and 100 turns of wire in the secondary coil , the rat io (called turns ratio)wou ld be 10 :1 . Because the re are few er secondary w ind ings than p r imary w ind ings ,

that transformer will be known as a step-down t ransformer. If 120 Vac w er e appliedacross the primary, i ts secondary would ideally yield

If the rat io were reversed with 100 primary turns and 1 ,000 secondary turns, thetransform er w ould b e a 1:10 step-up de vice . An input of 12 Vac to th is prima ry w ouldresult in an output of

across the secondary. An isolation transform er has a 1:1 turns rat io— the num berof primary and secondary turns is equal, so secondary voltage will ideally equal primary vo l tage .

A transform er also steps current, but current is ste pp ed in rev ers e of the volta gerat io. If voltage is stepped up, current is stepped down by that same ratio , and viceversa. In th is way, power taken from a transformer secondary will roughly equal thepower provided to i ts primary.

12 Vac120 Vac

/10 \

120 Vac12 Vac/ 1 \



96 Power supplies

A s an exa m ple , suppose the t ran sform er of Fig. 6-3 has 120 Vac at 0 .1 A supp liedto i t s p r imary. Pr imary power would then be (P=I x V) o r

12 W (120 Vac x 0.1 A ) .

Wi th a 10 :1 s tep-d own t ransformer, second ary vo l ta ge wou ld be 12 Vac , bu t

On paper, power ou tput a lways equa ls the power input . In rea l i ty, however, ou tputpower is a lways s l ight ly less than input power due to losses in the core and coi l res i s tance . Severe losses can cause excess ive h ea t ing in the t ransformer. T he ra t io o fou tpu t pow e r t o i npu t pow e r (PJP^) is kno wn as efficiency. Most so l id -core t ransformers can reach 80 to 95% eff ic iency — but neve r 100%.

You might wonder why t ransformers wi l l no t s tep dc vo l tages . Af te r a l l , dc canproduce a s t rong magnet ic f ie ld in so lenoids ( impac t p r in t wi res for example) . Althou gh dc can pro du ce a f ield, a ma gne tic f ie ld mu st fluctuate as a funct ion of t im eto induce a potential (vo l ta ge ) on another conductor. Di rec t cur ren t wou ld cer ta in lyma gnet ize the pr imary winding , bu t wi thou t cons tan t fluc tua t ion , no vo l ta ge wou ldbe induced across the secondary winding .

Th e ac po we r suppl iesN ow tha t you unders tand h ow a t ransform er work s , you have the essen t ia l

kno wled ge to unders tand the conce pts of an ac pow er supply tha t i s l i t t le mor e than

the output from a t ransformer. As you saw in the last chapter, heat is needed to fusetoner to the paper. Hea t i s typ ica l ly prov ided by a quar tz lamp inser ted in to the u pper of two fusing rol lers . Heat requires substant ial power. Transformers are eff icientdevices capable of handl ing la rge amou nts of power, so the hea t ing lamps a re a lmos ta lways powered by ac provided f rom a t ransformer. Keep in mind tha t ac might besuppl ied by a s tand-a lone t ransformer, o r taken f rom a tap off the t ransform er powering your printers dc supply. Because ac sources and dc supplies are so often integra ted in to the same assem bl ies , sym ptom s and so lu t ions to ac supply problem s w i l lbe covered wi th dc l inear supply problems.

RectifiersVol ta ge across the t ransform er se cond ary i s s t il l in an ac form — tha t i s , the p o

l a r it y sw ings be tw e e n p os i t i ve a nd ne g a t i ve vo l t a ge s . A l t e r na t i ng c u r r e n t mus t bec onve r t e d i n to dc be f o r e i t c a n be u se d by mos t e l e c t r on i c c ompone n t s . T h i s c onvers ion i s known as rectification, wh ere on ly on e po la r i ty of the input i s a l lo we dto reach the ou tput . Al though a rec t i f ie r ou tput var ies grea t ly, the po la r i ty of i t ss igna l wi l l a lways remain wi th in one po la r i ty—thus the te rm pulsating dc. D iode sare ideal for use in rect i f ier c ircui ts because they only al low current to f low in onedi rec t ion . You wi l l encounter th ree c lass ica l types of rec t i f ie r c i rcu i t s : ha l f wave ,fu l l wave , and d iode br idge .

of cur ren t wou ld be ava i lab le. This resu lt s in a seconda ry p ow er of

12 W (12 Va c x 1 A ) .



Power supplies— ac and linear dc 9 7

A half-wave rect ifier circu it is sho wn in Fig. 6-4. It is the sim plest and m oststraightforward type of rectif ier circuit because i t only requires one diode. As secondary ac voltage exceeds the turn-on voltage of the diode (about 0 .6 V for a si l icond i o d e ) , i t begins to conduc t current. Th e curren t gen era tes an output that mim ics theposit ive half of the ac input. If the diod e we re re ver sed , i ts output w ould be rev erse d.Th e disad vantage of th is typ e of rectif ier is that it is inefficien t— it only deals with halfof i ts ac input—the other half is basically ignored. The result ing gap between pulsesresults in a low er ave rage output and a higher amoun t of ripple (ac no is e) con ta inedin the final dc signal. Half-wa ve rectif iers are rarely used in mo der n p ow er supplies.

Transformer

6-4 A half-wave rectifier circuit.

Full-wave rectif iers such as the one shown in Fig . 6-5 offer som e substantialperfo rmance advan tages over the hal f -wave des ign . By us ing two d iodes in the configuration shown, both polari t ies of the ac secondary voltage input can be rectif iedinto pulsating dc. Because a diode is at each terminal of the secondary, polari t ies at

each diod e will be op pos ite as show n. Wh en th e ac signal is pos it ive, the up per diod econducts , bu t the lower d iode i s cu t o ff . When the ac s ignal becomes negat ive , thelower d iode conducts , bu t the upper d iode i s cu t o ff . One d iode i s a lways conducting, so there a re no gaps in the final output signal . Rip ple leve ls are low er and th e average dc output voltage is h igher. The disadvantage to a full-wave rectif ier is i tst ransfo rmer requ irement . A cen ter- tapped secondary i s needed to p rov ide a g roundrefe ren ce for the supply, wh ich often takes a larger transformer. La rge transform ersare not popular with printer designers trying to reduce weight and bulk.

Pulsating dc

6-5 A full-wave rectifier circuit.



98 Power supplies

Diode bridge rectif iers use four rectif ier d iode s to pro vide full-wave rectif ication without the troubles of a center-tapped transformer. Figure 6-6 shows a typicalbridge rectif ier stage. Alternating current from the transformer secondary is conne cte d to a series of d iod es arrang ed in a Wheatstone bridge fashion. Dio des D l andD2 provide the forward current paths, and D3 and D4 offer isolation between sec

ondary vo l tage and a common reference po in t that serves as g round . When ac vo l tage i s posi t ive , d iode Dl conducts because i t i s fo rward b iased , and D4 p rov idesiso la t ion versus g round . As ac vo l tage be com es ne gat ive , D2 condu cts whi le D3 supp l ies i so la t ion versus g round . Th e c om plete b r idge gen erates a fu l l-wave pu lsa ting dcoutput. Bridges are by far the most popular type of rectif ier circuit .

Pulsating dcoutput voltage

ac input

Transformer Diodes

6-6 A bridge rectifier circuit.

FiltersBy strict techn ical definit ion, pulsating dc is dc becau se volta ge polari ty remains

consistent (even if i ts magnitude does change). Unfortunately, even pulsating dc isunsuitable for any type of electronics power source. Voltage levels must be constantover t ime to operate e lec t ron ic dev ices p roper ly. A filter is used to ach ieve asmoothed dc voltage as shown in Fig . 6-7 .

Capacitors are typically used as f i l ter elements because they act as voltage storage de vices —a lmo st l ike l ight-duty batteries. W he n pulsating dc is applied to a capac-itive filter as in Fig. 6-7, the ca pacito r char ges wit h curre nt sup plied from the rectifier.Ultimately, the capacitor charges to the peak value of pulsating dc. When a pulse fallsoff its peak (back tow ard z e r o ) , th e capacitor will continue to sup ply current to a load.This action tends to hold up the output voltage over t ime—dc is f i l tered.

However, f i l tering is not a perfect process. As current is drained away from thecapac itor by i ts load, volta ge across the fil ter also will drop . Voltag e con tinues to dro punti l a new pulse of dc recharges the fi l ter for another cycle. This repeti t ive chargeand discharge results in regular f luctuations of the fi l ter output. These fluctuationsare known as ripple. R ipp le is an undesi rab le com pon en t o f a sm oothed dc ou tpu t .

Figure 6-7 also shows a graph of voltage versus t ime for a typical f i l ter circuit .Th e ideal dc ou tput w ould simply b e a constant, f lat l ine at al l points in t ime. In reali ty, there will always be some amount of f i l ter r ipple. Just how much ripple dependsupon the load. Fo r a l ight load (a high resistance that draw s relat ively l i t t le cur re nt ) ,d ischa rge is less be tw ee n pu lses, so ripple also is lower. A large load (a low resistance



Power supplies— ac and linear dc 99

Pulsatingdc inputvoltage

Smoothed,dc output

voltage RiPPle

1

I d e a l d cVoltage

\ Pulsating) dc input

Time

6-7 Effects of a capacitive filter stage.

that draws substantial current) requires greater current, so discharge (and ripple) isg reate r bet we en pu lses . Th e re la t ionsh ip o f dc pu lses is show n fo r re feren ce.

Addit ional f i l tering can be accomplished by adding more capacitance to the fi l terstage. More cap acitance holds more charge, so load can be supplied with less discharge.

As a general rule, more capacitance results in less ripple and vice versa. Although thisis true in theory, ther e are som e practical limits to just ho w m uch cap acitan ce can b eused in a po w er filter. Size is always a big conce rn. C apacitors lar ger than 4,700 uF a relarge and cumbe rsom e. Ab ov e 10,000 uF, a f i lter can accept so much charging currenton its initial charge (known as inrush current) that it migh t see m like a short circuit.Exce ssive inrush current can blow a fuse or eve n dam age the rectif ier stage.

B e f o r e a t t e m p t i n g t o w o r k o n a p o w e r s u p p ly, y o u m u s t u n d e r s t a n d t h ep o t e n t i a l f o r a f i l t e r s h o c k h a z a r d . P o w e r c a p a c i t o r s a c c u m u l a t e a s u b s t a nt i a l am o u n t o f e l ec t r i ca l ch a rg e an d h o ld i t fo r a l o n g t im e . I f y o u to u ch th el e a d s o f a c h a rg e d c a p a c i t o r, c u r r e n t w i l l f l o w t h r o u g h y o u r b o d y. A l th o u g hthis is almost never dangerous, i t can be very uncomfortable or result in a sl ightburn.

To rem ove any store d charge in your f il ter stage, the charge must be b led away ina contro lled fashion as shown in Fig. 6-8. A larg e-value res istor (c alled a bleeder resistor) can be con ne cte d across the filter. Th e resistor will slowly drain awa y any rem aining charge. Note that some fi l ter capacitors might already be buil t with a bleederresistor. If a load rem ains con ne cte d across the filter, that also will discharge the filterafter power is removed. Never at tempt to discharge a capacitor using a screwdriver orwire. Th e sudden release of en ergy can actually weld a wire or screw driver blade rightto the capacitor terminals, as well as damage the capacitor internal structure.



10 0 Power supplies

RegulatorsA transformer, rectifier, and filter are the essentials of every linear power supply.

These parts combined will successfully convert ac into dc that can drive most basicelectrical and electronic com ponen ts. The re are several troubles with these simple unregulated supplies that make the m undesirable. First, ripple is always pre sen t at a filter output. Under some circumstances, this can cause erratic operation in even themost forgiving ICs (in tegra ted circuits) . Second, output voltage varies with load. Whileload is fairly light, this effect m ight be negligible, but the effects of heavy loads also cancause unpredictable circuit performance. The filter output must be stabilized to elimi

nate effects of ripple and loading. Stabilizing the output is the task of a regulator.Linear regulation is as i ts name implies—current f lows from the regulator input

to the outputs as shown in Fig . 6-9 . Voltage that is supplied to the regulator inputmust be somewha t h igher than the des i red ou tpu t vo l tage (usual ly by several vo l ts ) .Internal circuitry within the regulator m anipulates input voltage to pro du ce a steady,consistent output level over a fair ly wide range of loads and input voltages. If inputvoltage drops below some minimum value, the regulator fal ls out of regulation. Inthat case, the output signal follows th e input signal—inclu ding ripple.

6-9 Block diagram of a gene ric regulator.



Power supplies—ac and linear dc 101

To maintain a constan t ou tpu t vo l tag e , th e l inear regulating circuit (or IC) willt h ro w aw ay ex t r a en e rg y p ro v id ed by th e filter in the fo rm of heat . To car ry away the

energy, most regu la to rs are o f ten a t tached to large metal heat sinks. Heat sinkscarry heat to the su rround ing air. Al tho ugh l inear regu la t ion p rov id es a s imple and

re l iab le method of o p e ra t io n , it also is v e ry w as te fu l and inefficient . T ypical l inearregu la to rs a re only up to 5 0 efficient . Fo r e v e r y 10 W of p o w e r p r o v i d e d to the supply, only 5 W is p r o v i d e d to the load . Much of th is waste occurs in the regu la t ionprocess . Swi tch ing regu la t ion is mu ch mo re eff ic ien t , but that subject is co v eredlater in th is c hapter.

Yo u m ig h t en co u n te r m an y v a r io u s ty p es of r eg u la to r c i r cu i t s . F ig u re 6-10

s h o w s a v e r y s im p le se r i e s v o l t ag e r eg u la to r co n s t ru c ted w i th d i sc re t e p a r t s . In

p u t v o l t ag e is a p p l i e d to the ze n e r d i o d e ( Z d ) t h r o u g h a cu r ren t - l im i t in g r e s i s to r( R z ) . Th e z e n e r d i o d e clamps v o l t a g e to its z e n e r l e v e l . In t u rn , t h is zen e r p o t ential turns on the p o w er t r an s i s to r t h a t a llo w s lo ad cu r ren t to f low. Outpu t vo l t ag eeq u a l s zen e r v o l t ag e m in u s a sm a l l v o l t a g e d ro p (u su a l ly 0.5 to 0.7 V ) f ro m the

t r an s i s to r b ase -em i t t e r j u n c t io n . You can set th e o u t p u t v o l t a g e by ch an g in g the

zen er d io d e .

Current limitingresistor ( R z )

dc inputvoltage ( 7 t a )

Powertransistor

(Q)

A WZener diode -

V be = 0.6 V dc

dc outputV z voltage ( 7 0 U t )

1 v out =v z-v be

* V z Zener voltage

V be Base-emitter voltagedrop in a silicon transistor

6-10 Diagram of a discrete series voltage regulator.

F o r th e e x a m p l e of Fig . 6-10, su p p o s e th a t i n p u t v o l t ag e is 10 V and yo u are

using a 5.6 V z e n e r d i o d e . W h e n p o w e r is ap p l i ed to the c i rcu i t , zener vo l tage wi l lb e c l a m p e d at 5.6 V. B ecau se in p u t v o l t ag e is 10 V, the d i f f e ren ce of 4.4 V ( 10 V -

5.6 V ) wi l l appear across the cu r ren t - l im i t in g r e s i s to r Rz. Z en er v o l t ag e sa tu ra t e sthe t rans is to r, so its o u tp u t w i l l be 5.6 V m in u s t he t r an s i s to r b ase -em i t t e r d ro p of

0.6 V, or 5.0 V d c . A s l o n g as i n p u t v o l t a g e r em ain s ab o v e the zen e r v o l t ag e , o u tp u tv o l t ag e sh o u ld r em ain s t ead y r eg a rd less of l o a d — o u t p u t s h o u ld be r eg u la t ed .L o a d c u r r e n t can be substan t ia l , so you wi l l o f ten f ind a p o w er t r an s i s to r u sed as

the regu la t ing t ransis to r.R eg u la to r c i r cu i t s can eas i ly be f ab r i ca t ed as i n t eg r a t ed c i r cu i t s as s h o w n in

F ig . 6 -11 . A d d i t io n a l p e r fo rm an c e f ea tu res su ch as au to m at i c cu r ren t l im i t in g and



102 Power supplies

Filtered dc inputvoltage V ^

* High-frequency filter capacitors

6-11 An IC voltage regulator.

o v e r- t em p era tu re sh u td o w n c i r cu i t ry can b e in c lu d ed to im p ro v e r eg u la to r r e l i ab i l i ty. Inpu t vo l tage must s t i l l exceed some min imum level to ach ieve a s teady ou tpu t , bu t IC regu la t o rs are mu ch s impler to use . On e add i t ional consid era t ion fo rIC regu la to rs is the add i t ion o f h igh -f requ enc y f i l ter capaci to rs a t bo t h the inpu tand ou tpu t . The capaci to rs f i l ter ou t any h igh-f requency no ise o r s ignals thatcou ld in ter fere wi th regu la to r operat io n . / /FX high-f requen cy) f i l ters are gen era l lysm al l -v a lu e , n o n p o la r i zed cap ac i to r s (0 .0 1 u F o r 0 .1 u F ) . A co m p le te l i n ea r p o w ersupply is shown in Fig . 6-12.

6 -12 Schematic of a basic linear dc pow er supply.



Troubleshooting ac and linear dc supplies 103

Troubleshooting ac and linear dc suppliesUnder most c i rcumstances , l inear power supp l ies are reasonab ly s imple and

st ra igh t fo rward to t roub leshoo t . A sound p rocedure i s to use your vo l tmeter (o r osci l loscope) to t race the p resence o f vo l tage th rough the supp ly. I f an ou tpu t has

failed, start your measurements at the output and work back unti l you find the appropr ia te s ignal again . The fo l lowing symptoms and t roub leshoo t ing p rocedurespresen t more deta i l s . I f y o u s h o u l d d e t e r m i n e a p o w e r s u p p l y t o b e d e f e c t i v e ,r e m e m b e r t h a t it i s a l w a y s a c c e p t a b l e t o r e p l a c e t h e s u p p l y o u t r i g h t .

S y m p t o m 1 Pow er supply is completely dead. Laser printer does not oper

ate and no power indicators are lit. Before you beg in to d isassemble the p r in ter,check to make su re that you are receiv ing an appropr ia te amount o f ac l ine vo l tagein to the pow er supp ly. Use your mu l t imeter to m easure ac vo l tag e a t the wall ou t le tpow ering yo ur p r in ter. Normal ly, you shou ld read betw een 105 and 130 Vac (2 10 to240 Vac in Eu ro pe ) fo r a po we r supp ly to operat e p roper ly. Mo re o r less ac l ine vo l tage can cause the pow er sup p ly to malfunct ion . E x e r c i s e e x t r e m e c a u t i o n w h e ne v e r m e a s u r i n g a c l i n e v o l t a g e l e v e l s . R e v i e w t h e h a z a r d s o f e l e c t r i c i t y a sd i scu ssed in ch ap te r 4 .

W he n you de term ine that an appro pr ia te am ount o f ac is avai lab le a t the p r in ter,the fault probably exists at the printer. Check the printer power switch to be surethat i t is turned on. Even though i t sounds si l ly, th is really IS a common oversight.Next, check the printer main l ine fuse that is often located closely to the ac l ine cordconnecto r. Unplug the p r in ter before rem ovin g the fuse for exam ination . You shouldfind the fusible l ink in tact , but it is not always possib le to see t he entire l ink. Use you rm ultim eter to measu re continuity across the fuse. Norm ally, a wo rkin g fuse shouldread as a short circuit (0 Q ) . If you re ad infinite res istanc e, the fuse is de fec tive and

shou ld be rep laced . U s e c a u t i o n w h e n r e p l a c i n g f u s e s . U s e o n l y f u ses o f t h es a m e r a t i n g . If a new fuse fai ls immediately when replaced, i t suggests a seriousfailure (such as a short circu it) else wh ere in the po w er sup ply or printer. D o not continue to rep lace fuses if the y continu ally fai l.

If ac voltage and the fuse is in tact , you must d isassemble your printer and workon the power supp ly. Ta k e a l l p r e c a u t i o n s t o p r o t e c t y o u r s e l f f r o m a c a n dh i g h - v o l t a g e h a z a r d s . Check all connectors and wiring leading to or from the supply to rule out a broken wire or loose connector.

Turn on p r in ter pow er and use your mul t imeter to m easure dc ou tpu t vo l tage (s )from the supply. Most printed circuit markings will g ive you some indication of what

voltage should exist at each resp ective output. A low or nonexiste nt outp ut indicates aproblem. Make sure that the output is not being shorted by i ts load. Disconnect thesupply from its load and meas ure i ts outp ut (s) again . If you r readings cl imb up to a normal level , there m ight be a short circuit som ew her e in the printer e lectronics . If readings remain low, you will have to troubleshoot the supply. If you do not have theinclination or skill to test the sup ply in m or e detail, repla ce the e ntire su pply assembly.

For th is pro ced ure , refer to the diagram of Fig . 6-12. If the volta ge ou tput is complete ly zero , check fo r the p resence o f a dc ( low vo l t age ) p ro tec t ion fuse in the ou tput circuit . Some might be normally sized fuses in the power supply, but o ther fusesmight be instal led in the printer electronic co ntrol packa ge ( E C P ). If supply outpu ts



10 4 Power supplies

measure okay but the printer still does not function, look carefully for any subminia-ture or pico fuses (resem bling carbon film resisto rs) that might be d efec tive.

Of al l the components in your supply, the regulator has the greatest stress. Useyour multimeter to measure the dc input to the regulator. You should read severalvo l ts g reater than the exp ect ed ou tpu t . For e xam ple , a regu la to r wi th an ou tpu t o f

5 Vdc requ ires an inpu t o f +7 o r +8 Vdc. When the regu la to r inpu t vo l tage i s co rrect , but i ts outpu t is not , the regu lator is prob ably d efe ctiv e and shou ld be rep lace d.A lo w or nonexis tent r egulat or input suggests a faulty fi l ter or rectif ier.

A shorted power fi l ter capacitor can pull down the output from a rectif ier. Unplug the printer, remove at least one capacitor lead from its circuit , and test the capacitor as discussed in the test equipment section of chapter 3 . Replace any fi l tercapaci to r that appears open o r shor ted . Any power capaci to r that appears ho t o rsmells strange is a clear indication of trou ble.

Inspect the rectif ier circuit carefully. A faulty rectif ier d iode can completely disable your supply. Unplug the printer and test each rectif ier d iode as discussed in thetest equ ipm ent section of cha pter 3 . W he n a brid ge rectif ier fails , you w ill usuallyf ind the two fo rward d iodes op en c i rcu i ted . Rep lace any d iodes that appear open o rshort circuited. If your rectif ier is buil t in to a potted bridge module, the entire module must be replaced.

Finally, turn on printer power and check the ac voltages at the transformer primary and secondary wind ings . U s e c a u t i o n w h e n m e a s u r i n g a c . You should findabou t 120 Vac across the p r imary and som e lowe r amount o f ac (usual ly be tw ee n 8and 30 Vac) across the secondary. An open circuit in ei ther winding can prevent anysecondary output. Be careful to check for shorted transformer windings. Be suspicious of a transformer that becomes very hot after a short period of use, or one thatem its an audible 60 Hz hum . Such a trans forme r m ight be de velo pin g a short circuit .

Consider the poss ib i l i ty o f a PC board fa i lu re , especia l ly i f the laser p r in terfa i led af ter being d ropped o r abused . Fau l ty so lder ing a t the facto ry (o r on a p rev ious repai r ben ch ) a lso can cause a PC bo ard p r ob le m . As F ig . 6 -13 show s, the reare th ree d i fferen t k inds o f p rob lem s that can p lagu e a p r in ted c i rcu i t : lead pu l l -th rough , t race b reak , and board crack .

Lead pull-through occurs wh en a com pon en t lead o r wire i s r ipped away f romits through hole. Often, the soldering at the printed circuit pad might appear perfectly n ormal, but there w ill be a hole in the m iddle w he re th e lead wa s. Th e lead alsomight remain within i ts printed circuit hole, but not be fully connected. This kind ofpro blem can easily result in bizarre , in term ittent be havior, but i t can be repaire d simply by rehea ting the solder jo int to recon ne ct the lead .

Trace break occurs com mo nly around large o r awk ward com pone n ts that aretoo wel l anchore d to the PC board to exp er ien ce lead pu l l- th rough . Ins tead , physicalforce w ill break the solder pad aw ay from its trac e. Tra ce breaks are difficult beca usethey are usually so fine and clean that you might not see them upon a visual inspection. You have to spot them by wiggling each lead individually. When a solder padm o v e s , bu t it s t race does no t , you wi ll see the b reak locat ion . Jum per bet we en tw oadjacent solder pads to rel iably repair th is fai lure. Do not at tempt to solder orjumper across the b reak itself. Chemical coatings applied to printed circuit boardsprevent solder from sticking to trace areas.



10 6 Power supplies

you migh t no t be ab le to see t race b reaks on v isual inspect ion . You migh t h ave to wigg le each so lder pad ge n t ly to revea l any t race b reaks . Boa rd cracks are fa i rly obv iousprob lems, bu t large cracks can sever many t races—it rea l ly depends on the cracksize and board complex i ty. Usual ly, b reaks o r cracks can be corrected by so lder ingjumpers between the so lder pads across each defect . Of course , you migh t decide to

rep lace the supp ly ou t r igh t ra ther than t ry to repai r PC board p rob lems .Consider the poss ib i l ity o f thermal in termi t ten ts i f your p r in ter works f ine wh en

it is f irst turned on but fai ls after some period of operation. Often, the printer mustthen si t for a t ime with al l power off before i t can be used. Test for thermally in termi t ten t c om pone n ts by sp ray ing suspect par ts wi th a liqu id ref r igeran t (avai lab lef rom almost any e lect ron ics par ts s to re o r mai l -o rder house) .

Beg in by exposing the po we r supp ly. Ap ply p ow er and oper ate the p r in ter un ti l i tfai ls . Use your multimeter to measure each supply output before AND after i t fai ls , soyou will know which outputs are fai l ing. When you have identif ied a faulty output,check i ts regu la to r fo r excess ive heat . Nev er touch l ive comp onen ts that migh t be h o tor carrying high vol tag es — it is a certain opport unity for in jury Instead , smell aroundthe regulator for any trace of smoke or unusually heated air. Spray the regulator withrefr igeran t , wai t a mo me nt , and recheck your ou tpu t vo l tag e . I f normal vo l tage returns temporari ly, you have isolated the problem. Replace the faulty regulator. Keepin mind that you might have to spray a component several t imes to cool i t properly.

F i l ter componen ts and rect i f ier d iodes are rare ly sub ject to thermal p rob lems.Transfo rm er w ind ings can open o r shor t due to exces s ive hea t , bu t on ly af ter a longper iod o f b reakdown. Use your mul t imeter to measure vo l tages th rough the remainder o f the supp ly to t rack do wn any fu r ther p rob lem s. I f y o u r t e s t s a r e i n c o n c l us iv e , y o u m ig h t d ec id e to r ep lace th e su p p ly.

S y m p t o m 3 Laser printer is not operating properly. It might be function

ing erratically or not at all. Powe r indicators might or might not be lit. U se y o u rmul t imeter to measure the ac l ine vo l tage reach ing the p r in ter. Under normal c i rcum stances , you shou ld meas ure 105 to 130 Vac (s om e Euro pean coun tr ies use 210to 240 Va c) . On the avera ge, 120 Vac shou ld be avai lab le . If line vo l tage d rops be low105 Vac, pow er supp ly outputs can be gin to fall out of regulation. As a result , pr interc i rcu i ts migh t no t receive enough vo l tage o r cu rren t to ensure p roper operat ion ,which can cause er ra t ic operat ion that can d isab le (o r even damage) the p r in ter.High inpu t vo l tages (o ve r 130 Va c) can fo rce mo re curren t in to the supp ly than des i rab le . Add i t ional cu rren t genera tes heat that can cause p rema ture b r eakd own s inthe power supp ly.

Unplug the laser printer and check for any loose connectors or wiring that mightbe in ter rup t ing c i rcu i t operat ion . The connecto rs migh t have been improper ly instal led at the factory, or you might have re-instal led them incorrectly during a previous repair effort . Turn on printer po w er and use you r multim eter and measu re v oltag eat each supply output. If you locate a defective output, t roubleshoot the supply fromits output, back to i ts transform er as discussed b elow. If al l supply outp uts ap pear correct , there might be a fault in the printer electronic control package, so troubleshootyour printer electronics according to the procedures outl ined in chapter 9 .

I f you d etec t a fau l ty su pp ly ou tpu t , use your m ul t imete r to meas ure inpu t vo l tage a t that regu la to r. It shou ld be sev eral vo l ts h igher than the ex pe cte d regu la to r



Construction and operation of dc switching supplies 107

output. When a regulator input appears normal, but i ts output does not, t ry replacing the regu la to r. Lo w regu la to r inpu t vo l tag e (o r no inpu t vo l tag e) m igh t be cau sedby a fault in the fi lter or rectif ier stages. Unp lug th e printer an d chec k you r f il ter capacitors) for open or short circuits as discussed in the test equipment section ofchap ter 3 . Re pla ce any fi lter capac itors that app ear de fec tive . If the fi lter ca paci

t o r ^ ) app ear in tact , check each rect i f ier d iode. Rep lace any rect if ier d iodes that appear faulty. You also might replace the power supply outright.

S y m p t o m 4 Fusing quality is intermittent or poor (toner smudg es easily),or fuser fails to reach op erating temperature within 60 to 120 seconds. Tonermust be heated to about 180°C to bond to paper properly. Normally, voltage suppliedfrom th e ac pow er su pply will al low the quartz lamp to heat . If ac is low or in te rmitten t ,the quartz lamp m ight not be able to reach or hold i ts operating te m pera ture . Ideally,you should be able to run you r f inger t ips across the surface of a printe d pa ge withou tsmudging the page. If the print smudges, the toner is not being heated enough. Theremight be several reasons for th is. Th e quartz lamp m ight be fail ing, the electronic temperature control or sensor might be faulty, or the ac supply might be low.

Unplug the p r in ter, expose the ac supp ly area , and make su re that a l l connect io n s a re secu re . A p p ly p o w er to th e p r in t e r an d u se y o u r v o l tm e te r t o m easu rethe ac vo l tage being p rov ided to the quar tz lamp. U s e e x t r e m e c a u t i o n to a v o i dr e c e i v i n g a n e l e c t r i c s h o c k o r a b u r n fr o m t h e q u a r t z l a m p — r e m e m b e rt h a t t e m p e r a t u r e s c a n a p p r o a c h 1 8 0 ° C . The correct ac vo l tage level wi l l usual ly be marked on the t ransfo rmer ou tpu t . I f vo l tage i s co rrect , the p rob lem isp rob ab ly in the lamp or tem pe ratu re con t ro l ( re fer to chap ter 7 fo r m ore d eta i l s ) .I f the ac vo l tage level i s low or absen t , measure the ac inpu t that shou ld be approx imately 120 Vac (220 Vac in many European coun tr ies) . I f ac inpu t i s low, o ri f ac inpu t i s co rrect bu t ac ou tpu t i s low or absen t , rep lace the ac power module

or t ransfo rmer.If the ac supply is absent, the quartz lamp has failed, or the electro nic tem per atu re

control or sensor has failed completely, the fuser will not even approach its operatingtemp erature. Th is problem will usually result in an error cod e being displayed on thelaser printer control panel during initialization. Chapter 7 deals with fuser problems inmore detail.

Construct ion and operat ionof dc switching supplies

The g reat d isadvan tage o f l inear power supp l ies i s thei r t remendous waste . Atleast half of al l power provided to a l inear supply is l i terally thrown away as heat—most of th is waste occurs in a regulator. Ideally, if there were just enough energysupplied to the regulator to achieve a stable output voltage for any given load, regulator waste could be reduced almost entirely.

Ins tead o f th rowing away ex t ra inpu t energy, a swi tch ing power supp ly sensesthe ou tpu t vo l tage p rov ide d to a load , then swi tches the ac p r imary (o r seco nda ry)voltage on or off as needed to maintain steady levels. A block diagram of a typicalswitching power supply is shown in Fig . 6-14. There are many configurations possi-



Construction and operation of dc switching supplies 109

i l lSwitchingtransistor

ac inputvoltage

+ _ _ j ~ dc output—i— voltage

Sensing/switching

circuit

Voltage senseHigh-frequency

switchingpulses

6-15 Schematic diagram of a basic switching pow er supply.

which increases the vo l tage p roduced by the secondary wind ing . Outpu t vo l tagecl imbs back up again to it s des i red value— outp u t vo l tag e i s regu la ted .

Th e reve rse wi l l happen as load decreases on the secon dary c i rcu i t ( less cu rren tis drawn by the l o a d ) . A smaller load will tend to make output voltage climb. Again,the same actions happen in an unregulated supply. The sensing/switching circuit detects th is increase in voltage and reduces the switching duty cycle. As a result , thedu ty cycle fo r chopped dc decreases , and t ransfo rmer secondary vo l tage decreases .Outpu t vo l tage d rops back to i ts des i red value— outp u t vo l tage remains re gu la ted .

Consider the advantages of a circuit such as Fig . 6-15. Current is only drawn inthe primary circuit when i ts switching transistor is on, so very l i t t le power is wastedin the primary circuit . Th e secon dary circuit wil l supp ly just enou gh po w er to ke epload vo l tage constan t ( regu la ted) , bu t very l i t t le power i s wasted by the secondaryrectif ier, f i l ter, or switching circuit . Switching power supplies can reach efficiencieshigher than 85 %, wh ich is 35 % mo re efficient than most com para ble l inear supp lies.More eff ic iency means less heat i s generated by the supp ly, so componen ts can besmaller and packaged more t ightly.

Unfortunately, there are several d isadvantages to switching supplies that youmust be awa re of. First , switching supplies tend to act as radio transm itters. Th eir 20to 40 kHz operat ing f requencies can in ter fere w i th rad io and te lev is ion rece p t ion andwith circuits inside the printer. This potential problem is prevented in most switching supplies by covering or shielding with a metal casing. I t is important that you rep lace any sh ie ld ing removed dur ing your repai r. S t rong EMI ( e l ec t ro m ag n e t i cin ter feren ce) can d is tu rb the p r in ter. Second , the ou tpu t vo l tage wi l l a lways con ta insome high-frequency ripple. In many applications, th is is not enough noise to present in terfere nce to the load. In fact , a gre at man y printers use switch ing po w er supplies. Finally, a switching supply often contains mo re co mp one nts and is m ore difficultto troubleshoot than a l inear supply. This disadvantage is often outweighed by thesmaller, l ighter packaging of a switching supply, but replacement rather than repairis usually an econom ical decision .



110 Power supplies

Sensing and switching functions can be fabricated right onto an in tegrated circuit . IC-based switching circuits al low simple, inexpensive circuits to be buil t asshown in Fig . 6-16. Notice how similar th is looks to a l inear supply. Alternating-curren t l ine vo l tage i s t ransfo rm ed (usual ly s tep ped d ow n ) , then i t i s rec t i f ied a nd f iltere d befo re reach ing a swi tch regu la t ing IC . Th e IC chops dc vo l tage a t a du ty cyc le

that wi l l p rov ide adequate power to the load . Chopped dc f rom the swi tch ing regulator is f i l tered by the combination of choke and output f i l ter capacitor to reform asteady dc signal at the output. The output voltage is sampled back at the IC that constantly adjusts the chopped dc duty cycle.

ac inputvoltage

Fuse

Primary filter

dc outputvoltage

Switchregulating

IC

T

Choke

JWf\_

Outputfilter'

1I

6-16 Schem atic of a simple IC-based switching supply.

Troubleshooting dc switching suppliesTro u b lesh o o t in g a sw i t ch in g p o w er su p p ly can b e a co m p lex an d t im e-co n

suming task . Al though the operat ion o f rec t i f ier and f i l ter sect ions are reasonab lys t ra igh t fo rward , sensing /swi tch ing c i rcu i ts can be complex osci l la to rs that are dif

f icu l t to fo l low wi thou t a sche ma t ic . Sub assem bly rep lac em en t o f dc sw i tch ingsupp l ies are common. For th is t roub leshoo t ing d iscuss ion , consider the IC-basedswitching supply of Fig . 6-17.

The STK7554 is a swi tch ing regu la to r IC manufactu red as a 16-p in SIP ( s ing lein - line pa ck ag e) . It o ffers a dual ou tpu t o f 24 Vd c and 5 Vd c. Not ic e that BO TH ou t

pu t wavefor ms f rom th e STK 755 4 are 38 V square wa ves , bu t i t i s the duty cycle o fthose square waves that se ts the des i red ou tpu t levels . The square wave ampl i tudesimply provides energy to the fi l ter circuits. Fil ters made from coils (or chokes) andh igh-value po lar ized capaci to rs sm ooth the square -wave inpu t (ac tual ly a fo rm ofpu lsa ting dc ) in to a s teady source o f dc . The re wi l l be som e smal l amou nt o f h igh-f requency r ipp le on each dc ou tpu t . Smal ler, nonpo lar ized capaci to rs on each ou tpu tact to f i l ter out h igh-frequency components of the dc output. Finally, note the resisto r-capaci to r-d iode combinat ions on each ou tpu t . The combinat ions fo rm a su rgeand f lyback p ro tecto r that p rev en ts en ergy s to re d in the choke f rom re-en ter ing th eIC and damaging i t . Refer to Fig . 6-17 for the following symptoms.



Troubleshooting dc switching supplies 111

Parts listDl toD 6 Rectifier diodesLI Surge supressorTl Transformer12,13, L4 ChokesRl lOkfl 'AWR2, R3 47kfl 1WCI 6000 |iFC2 22 (iFC3, C4, C7

0.01 uFCB, C6, C8 1000 uFC9 1 M-FFl FuseIC1 Switching regulator

STK 7554

6-17 Schematic of a dual-output, IC-based switching DC pow er supply.

S y m p t o m 1 Power supply is completely dead. Laser printer does not oper

ate, and no power indicators are lit. As with l inear supp lies, chec k the ac l ine voltage en ter ing the p r in ter befo re beg inn ing any major repai r work . Use your mu l t imeterto measure the ac l ine voltage available at the wall outlet powering your printer. U s e

e x t r e m e c a u t i o n w h e n e v e r m e a s u r i n g a c l i n e v o l t a g e l e v e l s . R e v i e w t h eh a z a r d s o f e l e c t r i c i t y a s d i s c u s s e d i n c h a p t e r 4 . Normally, you should read betw ee n 105 and 130 Vac to ensu re prop er supp ly ope ration . If you find eithe r ver yhigh or low ac voltage, try the printer in an outlet that provides the correct amountof ac vo l tage . Unusual l ine vo l tage levels migh t damage your power supp ly, so p roceed cautiously.

If ac l ine voltage is normal, suspect the main power fuse at the printer. Mostpower fuses are accessible from the rear of the printer near the ac l ine cord, butsome fuses might only be accessible by disassembling the printer. Unplug the printerand remove the fuse from its holder. You should find the fusible l ink in tact , but useyour mu l t imeter to measure con t inu i ty across the fuse. A good fuse shou ld m easureas a short circuit (0 Q ) , and a fai led fuse will measu re as an open circuit ( infi nit y).Rep lace any failed fuse and re-te st the p rinter. If the fuse continua lly fails , th ere is aser ious defect e lsewhere wi th in the power supp ly o r o ther p r in ter c i rcu i ts . I f yourprinter has an ac selector switch that sets the supply for 120 Vac or 240 Vac operation, be su re that switch is in the p rop er posit ion.

Unplug the p r in ter and d isassemble i t enough to expose the power supp lyclearly. Turn on the printer and measure each dc output with your multimeter or oscil loscope. If each output measures correctly, then your trouble l ies outside the supp ly—perhaps in some connecto r o r wir ing that p rov ides power to the p r in ter



1 1 2 Power supplies

elect ron ics package. A low ou tpu t vo l tage suggests a p rob lem wi th in the supp ly i tself. Check each con necto r and a ll in terconn ect ing wir ing lead ing to or f rom the supply. Many switching supplies must be a t tached to a load fo r p ro per sw i tch ing tooccur. If the load circuit is d iscon nec ted from its supply, the vo ltag e signal could shutdown or oscil late wildly.

When supp ly ou tpu ts con t inue to measure incorrect ly wi th a l l connecto rs andwiring intact , chances are that your problem is inside the supply. With a l inear supply, you begin test ing at the output, then work back toward the ac input. For aswitching supply, you should begin test ing at the ac input, then work toward the defective output. You could also replace the supply module outright.

Measure the p r imary ac vo l tage app l ied across the t ransfo rmer. U s e e x t r e m ec a u t i o n w h e n m e a s u r i n g h i g h - v o l t a g e a c . The value shou ld be approx imately120 Vac. If voltage has been interrupted in that primary circuit , you will read 0 Vac.Check the primary circuit for any fault that might in terrupt power. Measure secondary ac voltage supplying the rectif ier stage. I t should read higher than the highest ou tpu t vo l tage that you expect . For the example o f F ig . 6 -17 , the h ighestexpected dc ou tpu t i s 24 V, so ac secondary vo l tage shou ld be several vo l ts h igherthan th is ( fo r example , 28 Vac) . I f p r imary vo l tage reads correct ly and secondaryvo l tage d oes no t , you migh t ha ve an ope n c i rcu i t in the p r imary o r secondary t ransfo rmer w ind ing . Rep lace the t ransfo rmer o r rep lace the en t i re supp ly.

Nex t , check the p reswi tched dc vo l tage supp ly ing the swi tch ing IC . Use yourmu l t imeter o r osci l loscope to measure th is dc level . You shou ld read app rox im atelythe peak value o f wh atev er s econdary ac vo l tage you jus t measured . For F ig . 6 -17 , asecondary v o l tage o f 28 Vac shou ld y ie ld a dc vo l tage o f abou t 38 Vd c (28 Vac rms x1.414) Vdc. If th is voltage is low or nonexistent , unplug the printer and check eachrect i f ier d iode, then inspect the f i l ter capaci to r. Componen t tes t ing techn iques are

shown in chapter 3 .Use your osci l loscope to measure each chopped dc ou tpu t s ignal . You shou ld

f ind a h igh-f requency square wav e a t each ou tpu t (20 to 40 kH z) w i th an ampl i tudeapprox imately equal to the p reswi tched dc level (38 to 39 V in th is c a s e ) . Set youroscil loscope to a t ime base of 5 or 10 us/DIV and start your VOLTS/DIV sett ing at10 VOLTS/D IV. Onc e you hav e establishe d a clear trace , adjust the t ime base andvertical sensit iv ity to optimize the display.

I f you d o no t read a cho ppe d dc ou tpu t f rom th e swi tch ing IC , e i ther the IC is defect ive , o r one (o r more) o f the po lar ized ou tpu t f i l ter capaci to rs migh t be shor ted .Unplug the printer and inspe ct each ques tionab le f i lter cap acitor as discus sed inchapter 3 . Replace any capacitors that appear shorted. Usually, f i l ter capacitors fai lmore readily in switching supplies than in l inear supplies because of h igh-frequencyelectrical stress, and the smaller physical size of most switching supply components.If al l f i l ter capacitors check out correctly, replace the switching IC or replace the entire supply. Use care when desoldering the old regulator. Instal l an IC socket ( if poss ib le) to p reve n t rep eat so lder ing w ork , then jus t p lug in the ne w IC.

S y m p t o m 2 Supply operation is intermittent. Laser printer operation cutsin and out with the supply. Beg in by inspect ing the ac l ine vo l tag e in to yourprinter. Be sure that the ac l ine cord is secured properly at the wall outlet andprinter. Make sure that the power fuse is instal led securely. If the printer comes on



Troubleshooting dc switching supplies 113

at al l , the fuse must be in tact . Unplug the printer and expose your power supply. Inspect every connecto r o r in terconnect ing wire lead ing in to o r ou t o f the supp ly. Aloose o r imp roper ly ins ta l led connect o r can p lay havoc wi th a p r in ter operat ion . Paypar t icu lar a t ten t ion to any ou tpu t connect ions . Most o f ten , a swi tch ing p ow er supp lymust be con ne cte d to its load circuit to oper ate . W ithout a load, the supp ly might cu tout or oscillate wildly.

Often , in termi t ten t o perat ion m igh t be the resu l t of a PC b oard p ro b lem such asthe ones show n in Fig . 6-13. PC b oard pr oblem s are often th e result of physical abu seor impact, but they also can be caused by accidental damage during a repair. Leadpu l l- th rough occurs whe n a wire o r com pon en t lead is pu l led away f rom i ts so lderjoint , usually through i ts hole in the PC board. This type of defect can easily be repai red by re- inser t ing the pu l led lead and p roper ly reso lder ing the defect ive jo in t .Trace breaks are hairl ine fractures between a solder pad and i ts printed trace. Suchbreaks can usually ma ke a circuit inop erat ive, and they are almost imp ossible to spotwithout a careful v isual inspection. Board cracks can sever any number of printedt races , bu t they are o f ten very easy to spo t . The bes t method fo r repai r ing t race

breaks and board cracks is to so lder jum per w ires across the dam age be tw een tw oadjacent solder pads. You also might simply replace the power supply outright.

Some fo rms o f in termi t ten t fa i lu res are t ime o r temperatu re re la ted . I f yourprinter works just f ine when first turned on, but fai ls only after a period of use, thenspontaneously returns to operation later on (or after i t has been off for a while), youmigh t be faced wi th a thermal ly in termi t ten t com pon en t . A com pon en t m igh t workwh en coo l , bu t fa il la ter on after reach ing o r exce ed in g i t s work ing tem pera tu re . After a p r in ter qu i ts , check fo r any unusually ho t comp onen ts . Ne ve r touch an operating circuit with you r f ingers—injury is almost cer tain . Instead , smell around t hecircuit for any trace of burning semiconductor or unusually heated air. If you detectan overheated component, spray i t with a l iquid refrigerant. Spray in short bursts forthe bes t coo l ing . I f normal operat ion re tu rns , then you have i so la ted the defect ivecomponen t . Rep lace any componen ts that behave in termi t ten t ly o r rep lace the entire supply. If operation does not return, test any other unusually warm components.

S y m p t o m 3 Laser printer is not operating properly. It might be functioning erratically or not at all. Power indicators might or might not be lit. Use yourmultimeter and check the ac l ine voltage first . Normally, domestic U.S. laser printersrequ ire a l ine volt age b et w ee n 105 to 130 Vac to ensu re prop er op eratio n. If l ine voltage i s low, the supp ly ou t pu t(s) migh t no t be ab le to main ta in regu la t ion . As vo l tag efalls , circuitry in the printer might begin to behave erratically or not at al l—it couldeven damage some del ica te p r in ter c i rcu i ts .

Check all wiring and connectors leading to and from the power supply to be surethat everything is t ight and instal led correctly. Pay part icular at tention to connectororientation. Loose, missing, or incorrectly inserted connectors can easily disableyour p r in ter, o r a t leas t cause unpred ic tab le operat ion . Swi tch ing power supp l iesusual ly requ ire a load c i rcu i t to be connected . Otherwise , i t s ou tpu t (s) migh t osci llate out of control or shut dow n totally.

Man y sw i t ch in g p o w er su p p l ie s a re co n ta in ed in a m e ta l en c lo su re o r sh ro u dth a t is w i re d to ch ass is ( ea r th ) g ro u n d . T h e g ro u n d b lo ck s (o r attenuates) anye lec t ro m ag n e t i c in t e r f e ren ce (E MI) g en era t ed b y th e su p p ly. Mak e su re th a t a l l



11 4 Power supplies

or ig inal sh ie ld ing is in p lace and securely wired to g round . I f i t i s no t , EMI migh tin ter fere wi th the operat ion o f o ther p r in ter c i rcu i ts to cause er ra t ic o r randombehav io r.

Use your mul t imeter o r osci l loscope to measure each supp ly ou tpu t . I f a l l ou tpu ts measure correct ly, then your t roub le i s most l ikely in the p r in ter e lec t ron ics .Refer to the t roub leshoo t ing p rocedures fo r e lec t ron ic c i rcu i ts con ta ined in chap ter9. Outpu ts that are low or nonex is ten t suggest a p rob lem in the po we r supp ly itself.

Trace the supp ly f rom ac inpu t to dc ou tp u t (s) o r rep lace the supp ly.Measure the ac vo l tage across your t ransfo rmer p r imary wind ing . Under normal

ci rcumstances , it shou ld read app rox im ately 120 Vac. Check the seco ndary ac vo l tage f rom the t ransfo rmer. You shou ld measure an ac level that i s h igher than yourlarges t expected dc ou tpu t . For F ig . 6 -17 , the secondary vo l tage shou ld be abou t 28Vac. I f the second ary vo l tage a ppears low or nonex is ten t , the t ransfo rm er p r im ary o rsecondary wind ings migh t be defect ive .

Measure the fi l tered dc voltage entering the switching circuit . You should read avo l tage lev el approx im ately equal to the peak second ary vo l ta ge . In F ig . 6-17, your dc

reading should be about 39 Vd c (28 Vac x 1.414). If th is volt ag e is low or n on-ex istent ,unplug the printer and check all rectif ier d iodes and the fi l ter capacitor as discussedin chapter 3 . Replace any rectif ier or f i l ter components that you find defective or replace the supply; then re-test the printer.

Use your osci l loscope to observ e any cho pp ed dc ou tpu t (s ) f rom the swi tch ingIC . You shou ld f ind h igh-f requency square waves (20 to 40 kHz) wi th an ampl i tudeapp rox im ately equ al to the p resw i tche d dc level . Th is level would be abou t 38 V fo rFig. 6-17. Set you r oscil los cop e to a t ime bas e of 5 to 10 us/DIV, with a vert ica l sensit iv ity of at least 10 VOLT S/DIV. Ad just t hes e sett ings as neces sary t o establish aclear trace.

If you do not read chopped dc, then either your switching regulator IC has fai led,or one (or more) polarized fi l ter capacitors have shorted. Unplug the printer and testeach fi l ter capacitor as shown in chapter 3 . Replace any capacitor that appears to beshorted, then retest the supply. If al l f i l ter capacitors check correctly, replace theswi tch ing IC . Use care wh en deso lder ing the o ld IC to p rev en t any acciden ta l PC boarddam age. Solder in an IC hold er ( if pos sib le), then just p lug in the n ew s witching IC.

High-voltage supply troubleshootingA h igh-vo l tage pow er supp ly i s ne ed ed to p rov ide the exci ta t ion that en erg izes

the p r in ter p r imary and t ransfer co ronas , as wel l as the development ro l ler. Th isbook does no t cover h igh-vo l tage supp ly background because o f the safe ty dangersinvo lved . The e lect r ica l s t ress on h igh-vo l tage componen ts requ ires ra ther specia lized componen ts . For the purposes o f th is book , a defect ive h igh-vo l tage supp lyshou ld be rep lace d ra ther than repa i red .

S y m p t o m 1 Laser p rinting is too light or too dark. As you saw in chap ter 5 ,h igh vo l tage i s a cr i t ica l e lement o f the laser p r in ter image-fo rmat ion sys tem. Highvo l tage i s needed on the p r imary corona to p rov ide a un i fo rm charge to the d rumsurface. If h igh voltage drops off or fai ls completely, the result ing image will be l ighto r nonex is ten t . Th e same th ing is t rue o f the dev elop m ent ro l ler and transfer co rona



High-voltage supply troubleshooting 115

assem blies. If h igh voltag e fai ls , ton er will not transfer to the dru m, and l it t le ( if an y)ton er that jum ps to the drum w ill actually transfer to the pa ge.

As a sanity chec k, try adjusting the contrast set t ing. An ex tre m ely lo w sett ingmigh t appear to have these symptoms. I f the p rob lems pers is t wi th a h igh con tras tse t t ing , rep lace th e h igh-vo l tage po w er supp ly.

S y m p t o m 2 Cannot control laser printer contrast. In m ost cases, you shouldbe able to optimize the printer contrast by adjusting the contrast control . When ane w EP car t r idge i s ins ta lled , con tras t shou ld be s l igh t ly reduc ed to com pen sate fo rthe abundan t supp ly o f f resh toner. As th e EP car t r idge i s consum ed , con tras t can beincre ased to m ake up for the gradual loss of toner. If contrast canno t be adjusted, th econ tras t con tro l knob migh t be defec t ive . Bec ause the con tras t con tro l is in tegrate dwith the h igh-vo l tage supp ly, rep lacing the h igh-vo l tage supp ly wi l l rep lac e the contrast control .



7C H A P T E R

Image-formation system

As d iscussed in chap ter 5 , e lec t rop ho tog raph ic (E P ) p r in ters (F ig . 7 -1 ) use a complex combination of l ight , stat ic electrici ty, heat , chemistry, and pressure, al l guidedb y a co m p lex ECP (e lec t r on ic con tro l pack ag e) . Th ere is no s ing le par t responsib lefor applying print—EP printers use a series of individual assemblies that make up i tsIFS ( image-fo rm at ion s ys tem ). Becau se EP imag e fo rmat ion uses a p rocess ra therthan a print head, there are many more variable condit ions that will affect the ult ima te p r in t qual ity and appearanc e. You have read abou t ho w E P pr in ters wo rk ; th ischap ter p resen ts deta i led exp lanat ions and t roub leshoo t ing p rocedures speci f ica l lyfo r EP (LED or laser) p r in ters .

7-1A Hewlett-PackardLaserJet IV printer.

116



System start-up problems 117

Your printer IFS ( image -fo rmat ion sy s te m ) is com pos ed o f e igh t majo r com ponents as shown in Fig . 7-2: a photosensit ive drum and developer assembly, cleaningb lade, erase lamps, a p r imary c orona assembly, a wri t ing m echan ism (a laser b eamor LED array), transfer corona, stat ic el iminator teeth , and a fusing assembly. Eachelem ent has to work p rope r ly to p rod uce h igh-quali ty p r in t . W he n a fau lt occurs in

any of thes e areas, the result ing print will be ad vers ely affect ed.

Primary corona

Erase lamps

Writingmechanism

0

Fusing rollers

Photosensitive drum

Developer roller

Transfer corona

Static eliminator

7-2 Components of an image-formation system

System start-up problemsEP printers perform a self-test on start up to ensure that the ECP is active and

respond ing normal ly. The se l f - tes t a lso checks communicat ion between the p r in terand host computer. After the self-test is passed, the fusing assembly must reach upwar ds of 180°C within 90 seco nds . W he n a self-test and wa rm up occ ur normally, thepr in ter is genera l ly ready to opera te as long as pap er and toner are dete cted . Unfortunately, printer start ups are not always so smooth. This part of the chapter detailsthe symptom s and so lu t ions fo r the most p erp lex ing s tar t -up p rob lem s.

S y m p t o m 1 Nothing happens when power is turned on. You should hearthe p r in ter resp ond as soon as po we r i s tu rned on . You shou ld see a po we r ind icato ron the control panel (alphanumeric displays will typically indicate a self-test) . Youalso should hear and feel the printer cooling fan(s) in operation. If the printer remains dead, there is probably trouble with the ac power. Check the ac l ine cord forp roper connect ion wi th the p r in ter and wal l ou t le t . Also check the p r in ter main acfuse. When the ac and fuse check properly, there is probably a problem with thepr in ter po we r supp ly. Refer to chap ter 6 fo r power-supp ly t roub leshoo t ing .



11 8 Image-formation system

I f the p r in ter fan (s) and pow er ind icato r op erate , you can be su re the p r in ter i sreceiv ing power. I f the con tro l panel remains b lank , there migh t be a p rob lem wi ththe dc power supp ly o r ECP. Fo l low the t roub leshoo t ing p rocedures o f chap ter 6 tocheck the p r in ter dc power supp ly. When the power supp ly checks p roper ly, thet roub le i s l ikely somewhere in the ECP or con tro l panel assembly itself. R e m o v e

po we r f rom the p r in ter and check the con tro l panel cab le . If there are no ind icato rsat al l on the control panel, replace the control-panel cable. If problems remain, tryrep lacing the ECP. I f you wish to t roub leshoo t the EC P in m ore deta i l, refer to the ins t ruct ions o f chap ter 9 . W he n on ly one o r a few ind icato rs appear on the con tro lpanel , t ry rep lacing the con tro l panel cab le . If p rob lems remain , rep lace th e con tro lpanel . If you wish to t roub leshoo t th e con tro l panel assembly in more deta i l, refer tochap ter 9 .

S y m p t o m 2 Your printer never leaves its warm -up mode. There is a con

tinuous WAR MING UP status code or message. T he initial self-test usually takes nom ore than 10 seconds f rom the t ime po w er i s f irs t app l ied . The fus ing-ro l ler assembly then must warm up to a work ing tem pera tu re , and is typ ical ly accep tab le wi th in90 seconds from a cold start . At that point , the printer will establish communicationwith the host computer and s tand by to accep t data , so i t s WARMING UP codesh o uld ch an g e to an O N -L IN E o r R E A D Y co d e .

If the printer fai ls to go on-line, the problem is often a faulty communication interface, or a control-panel problem. Turn the printer off , d isconnect i ts communication cable, and restore power. If the printer f inally becomes ready without i tscomm unicat ion cab le , check the cab le it se lf and it s connec t ion betw ee n the com pute rand printer. The cable might be faulty, or you might have plugged a parallel printerinto the computer serial port (or v ice versa). There also might be a faulty in terface inyour host comp uter o r p r in ter. Try a work ing p r in ter (on e that you k now is work ing

w el l ) wi th the compu ter to ensure that the com pute r por t is work ing correct ly.I f the p r in ter s t i l l fa i l s to become ready wi th the communicat ion cab le d iscon

nected , unp lug the p r in ter and check that the con tro l -panel cab les o r in terconnecting wir ing are a t tached p roper ly. Try reseat ing o r rep lacing the con tro l panel cab le .Check the control panel to see that i t is operating correctly. Try replacing the controlpanel. If problems persist , replace the ECP, which usually contains the control panelin ter face c i rcu i t ry. Depend ing on the complex i ty o f your par t icu lar p r in ter, the inter face/ fo rmat ter migh t be a separate p r in ted c i rcu i t p lugged in to the main log icboard , o r it s funct ions m igh t be inco rpora ted r igh t in to the main log ic board itself. Ifyou wish to t roub leshoo t the con tro l panel o r ECP fu r ther, refer your t roub leshoo t

ing to chapter 9 .S y m p t o m 3 You see a CHECK SUM ERRO R message indicating a fault has

been detected in the ECP program ROM . Du ring a self-test , the EC P will test i tsp rog ram RO M to see that i t i s work ing p rop er ly. Th is tes t is typ ical ly a checksum tes to f ROM con ten ts . I f the calcu la ted checksum does no t match wi th the checksumrecorded on ROM, an er ro r i s generated . A checksum erro r usual ly ind icates a fa i lu re o f the EC P RO M de v ice . Try the p r in ter po we r o ff . Wai t several minu tes ; then res to re power. I f the p rob lem pers is ts , rep lace the ECP or i t s in ter face/ fo rmat termo dule . I f you wa n t to a t tem pt a more d eta i led repai r, refer to ch ap ter 9 and t ry rep lacing the ROM itself.



Laser-delivery problems 119

S y m p t o m 4 You see an error indicating communication problems betweenthe printer and computer. Th e p r in ter and com put er are no t comm unicat ing p roperly. This symptom is typical in serial communication setups when baud rates or serial transfer pro toco ls do not match exa ctly (r efe r to cha pter 9 for detailed informa tionon ser ia l communicat ion and t ransfer p ro toco ls) . Check your ser ia l communicat ioncable f irst . Make sure that the ca ble is instal led prope rly. Also m ake sure that i t is thecorrect type and is wired properly for your printer. Keep in mind that p ins 2 and 3 onthe p r in ter cab le migh t ne ed to be reve rsed fo r p r ope r operat ion . I f the p ins must b ereverse d , use a null modem (avai lab le f rom almost any consum er e lect ron ics s to r e)on the printer end of the cable. Also be aware of the cable length. Serial communication cables are typically l imited to 15 m ete rs (5 0 fee t) , and Centron ics (p ara llel) cables are l imited to 3 meters (10 f e e t ) . Try a shorter cable if necessary.

There are f ive communicat ion parameters that must match between the hostcomputer and p r in ter : start b i ts , stop b i t s , data b i t s , parity t y p e , an d baud rat e. Ifany one o f these param eters do no t match , com mu nicat ion wi l l no t take p lace . A t th epr in ter end , there are usual ly DIP swi tch se t t ings o r con tro l -panel key sequencesthat def ine each parameter (you migh t need to refer to the user ' s manual fo r yourpr in ter to determ ine ho w each param eter is se t ) . At the com put er end , you can usual ly se t comm unicat ion para me ters d i rec t ly th rough the app l ica t ion so f tware that isdo ing the p r in t ing . Chan ge param eters i f necess ary to se t bo th p r in ter and com pute rto the same parameters . Reboo t the p r in ter and computer.

Check to m ake su re that bo th the p r in ter and com pu ter are us ing the sam e seria l f low con tro l . F low con tro l i s impor tan t because the host computer o f ten mustw a i t fo r t h e p r in t e r t o ca tch u p . X O N /X O F F ( so f tw are ) an d D T R /D S R (h a r d w are )f low con tro l are typ ical ly used . Adjust the p r in ter o r computer so that bo th use thesame t ransfer p ro toco l . Reboo t the p r in ter and computer.

If the printer st i l l fai ls to operate (and you are certain that the computer is communicat ing p ro pe r ly ) , the comm unicat ions por t is p robab ly defe ct ive . Try rep lacingthe ECP or in ter face module .

Laser-delivery problemsOn ce the pho tose nsi t ive d rum gets a un i fo rm elect r ica l charge f rom the p r im ary

corona, a la ten t ima ge is wr i t te n to the d ru m surface . Wri t ing i s accom pl ished by d ischarging desired points along the drum surface with directed l ight . The classicalmethod of writ ing is to scan a laser beam across the drum surface as shown in Fig .7 -3 . Th is p roce ss i s wh ere the ter m laser printer com es f rom. Th e laser beam or iginates at a single stat ionary point in the printer, and is d irected at a hexagonal mirro r that i s ro ta ted a t h igh spee d . As a mirro r ro ta tes , the laser beam is d i rected (o rscanned) th rough a com pensa t ing lens and across the d rum surface . By tu rn ing thelaser beam on and o ff co rrespond ing to the p resence o r absence o f do ts a long anyone scan l ine, the desired bit image is writ ten one l ine at a t ime. Typical laser printers can turn the laser on or off 300 t im es in any inch of scan l ine. W he n a scan l ine iscom plete d , the d rum ro ta tes 4o inch , and the ne x t scan l ine beg ins . Th is p rocess d etermin es the p r in ter overal l reso lu t ion (30 0 x 300 dp i in th is ex am pl e) .



120 Image-formation system

Laser diode

Sweep pattern-

Hexagonalmirror

Light-to voltagesignal converter

i -Photosensitivedrum

Beam-detectsignal ECP(main logic)

Laser controlsignal

7-3 Scanning a laser beam across the drum su rface.

Remember that the fo l lowing symptoms and so lu t ions are in tended fo r laserp r in ters because the laser and op t ica l assembl ies needed to implement a wri t ingmechan ism are par t icu lar ly del ica te . Remember that the er ro r messages you see on

your par t icu lar p r in ter migh t no t appear exact ly as shown below—refer to youruser 's manual to com pare you r er ro r code to the p rob lem typ es show n.

S y m p t o m 1 Y ou see a BEAM DETECTION error. A t the beg inn ing o f ever yscan l ine, the laser beam strikes an optical beam sensor. This registers the start of anew scan l ine and ensures that the data composing the new scan l ine is synchronizedwith the beam. Usually, the beam sensor is the only feedback that synchronizes thescan l ine and tel ls the laser printer that i ts laser is working. From time to t ime, unexpected variat ions in mirror rotation speed, age of the laser source, or the eventualbuildup of dust or debris on laser optics might cause the beam to miss i ts sensor.The se p rob lems wi l l cause the p r in ter to reg is ter an er ro r— tha t scan line wi l l p robably be missing on the printed page, but the error is usually recoverable.

I f you f ind that the p r in ter is reg is ter ing random and occasional beam dete ct ionerro rs , check the p r in ter op t ics . P r in ters wi th long serv ice l i fe t imes migh t have accumulated en ough dust o r debr is on the op t ics o r bea m sensor face to reduce beampo we r jus t enough to p rod uce in term i t ten t p rob lem s. Use a can o f ph o tograp hy-grade compressed air and gently try blowing the dust away. If you cannot clear thecon taminat ion , use l in t - f ree , pho tography-grade wipes l igh t ly dampened wi th h igh-qual i ty, pho tography-grade lens c leaners to wipe the lenses . Remember to be verygen t le— tak e your t ime and le t the wip e do the work . I f you knock op t ics ou t o f a l ignment, i t wil l be virtually impossible to realign them again without factory service. If



Laser-delivery problems 121

you m ark the lens , i t wi l l be perm anen t , and you wi l l nee d to have th e lens rep lacedand real igned .

If the o ptics look good , the laser source m ight be fai l ing. Olde r laser printers use asmall , gas-fi l led helium-neon laser to produce the beam. With t ime, the helium-neongas will escape and laser power will fall off. If the gas has escaped, the laser will have

to be replace d and realigned (wh ich requires factory s er vic e). Most current laser printers use solid-state laser d iodes as the laser source. The semiconductor laser is combined with som e switching and control circuitry and incorp orate d in to a laser assem blywith the scanning mirror and scanner motor as shown in Fig . 7-4 . This assembly iscalled the L/ S laser scanning assembly. Because the L/S assembly is completely prefabricated and designed as a replaceable component, i t can be replaced outright withlittle risk of alignment problems.

7-4 An installed laser/scanning assembly. Hewlett-Packard co.

U s e E X T R E M E c a u t i o n w h e n e v e r w o r k i n g w i t h l a s e r b e a m s . A l th o u g hthe beam is inv is ib le to the human eye and con ta ins on ly a few mil l iwat ts (m W ) ofpower, look ing d i rect ly a t the beam (o r ref lec t ions o f the beam from o ther ob jects)can cause eye in jury. Re fer t o wa rning s l isted on the laser/scanning assem bly for specific instructions.

S y m p t o m 2 You see a BEAM LOST error. A l th o u g h in t e rm i t t en t b ea m lo ssm ig h t r e su l t in a r eco v era b le b eam d e tec t io n e r ro r, a p ro lo n g ed loss o f t h e l a se rb eam (m o r e th an tw o sec o n d s ) w i l l r e su l t in a m o re se v e re p r in t e r fau l t . T h e reare many pos s ib le causes o f laser loss . Be g in by che ck in g ea ch vo l ta ge a t the d cp o w e r su p p ly. Mo s t su p p l i e s m ark th e i r o u tp u t l ev e l s , so y o u can ch ec k each o u tpu t wi th a mu l t imet er. I f any dc level i s low or abse n t , you can t roub lesh oo t thesupp ly as shown in chap ter 6 , o r rep lace the dc supp ly ou t r igh t . Check a l l conn ec to r s b e tw ee n th e l ase r /scan n in g assem b ly an d th e p r in t e r t o b e su re th a t eac hi s sea t ed p ro p er ly. P ay p a r t i cu la r a t t en t io n to th e f ib e ro p t i c cab le ru n n in g f ro mthe L /S assem bly to the E C P Th is cab le i s the o ne that carr ies laser l igh t to the



Fusing-assembly problems 123

Paper passes between the two ro l lers . The bo t tom ro l ler s imply p rov ides p ressure against the top roller. The top roller is heated from the inside by a long, th inquar tz lamp p ow ere d by your p r in ter ac supp ly. Al tho ugh the top ro l ler is ma de wi tha mater ia l that p reven ts toner f rom s t ick ing , some toner par t ic les wi l l inev i tab lystick. If those part icles are not cleaned away, they could st ick on later parts of thepage and cause p rob lems. A c lean ing pad is included in the fus ing a ssemb ly to w ipeoff any toner part icles from the heating roller. Many manufacturers provide an easilyrep laceab le c lean ing pad that can be ch anged w he n a n ew ton er car t r idge i s ins ta lled . F inally, a tempe ratu r e sensor (usual ly a the rm is to r ) i s included wi th the assemb ly to regu la te the heat being app l ied to the p age.

S y m p t o m You see a SERVICE error indicating a fusing malfunction. F u sing is imp ortan t to the successful ope ratio n of any EP printer. To ne r that is not fusedsuccessfully rem ains a po w de r or crust that can flake or rub off on you r hands oro ther pages . Main log ic in terp re ts the temperatu re s ignal developed by the thermisto r and mod ulates ac po w er to the quar tz lamp . Th ree cond i t ions wi l l gen erat e a fusing malfunct ion er ro r : (1 ) fus ing-ro ller temp eratu re fa ll s be low abou t 140 °C, ( 2 )fus ing-ro l ler temperatu re c l imbs above 230°C, o r (3 ) fus ing-ro l ler temperatu re doesno t reach 165°C in 90 seconds af ter the p r in ter i s powered up . Your par t icu larp r in ter migh t use s l igh t ly d i fferen t temperatu re and t iming parameters . When suchan er ro r occurs , your f i rs t ac t ion shou ld be to power down the p r in ter and reboo t .No te that a fusing error will often re m ain with a prin ter for 10 min utes or so after i ti s po we red dow n, so be su re to a l low p len ty o f t ime fo r the sys tem to coo l befo re reboo ting after a fusing error.

I f the er ro r pers is ts , po w er d ow n the p r in ter and exa min e the ins ta lla t ion o f you rfusing assembly. Check to see that al l wiring and connectors are t ight and seatedproper ly. An ac po we r supp ly is o f ten e qu ip ped w i th a fuse o r c ircu i t b reak er that

protects the printer. If th is fuse or circuit breaker is open, replace your fuse or resetyour c i rcu i t b reaker, then re- tes t the p r in ter. Remember to c lear the er ro r, o r a l lowenough t ime fo r the er ro r to c lear by itself. If the fuse or breaker tr ips again duringre-test , you have a serious short circuit in your fusing assembly or ac power supply.You can a t tempt to i so la te the shor t c i rcu i t , o r s imply rep lace your suspected assem bl ies— ac p ow er su pp ly fi rst , then the fus ing assembly.

U n p lu g th e p r in t e r an d ch eck y o u r t em p era tu re sen so r th e rm is to r b y m easu r in g i t s r e s i s t an ce w i th a m u l t im e te r. A t ro o m t em p er a tu re , t h e th e rm is to rsh o u ld r ead ab o u t 1 k £ i (d ep en d in g o n th e p a r t i cu la r t h e rm is to r ) . I f t h e p r in t e rh as b een a t ru n n in g t em p era tu re , t h e rm is to r r e s i s t an ce m ig h t b e m u ch lo w er. I ft h e t h e r m i s t o r a p p e a r s o p e n o r s h o r t e d , r e p l a c e i t w i t h a n E X A C T r e p l a c e m e n tpar t and re- tes t the p r in ter.

A th e rm al sw i t ch ( so m et im es ca l l ed a thermoprotector) i s ad de d in ser ieswi th the fus ing lamp . I f a the rm is to r o r main - log ic fa i lu re shou ld a l low tem per atu re to c l imb ou t o f con tro l , the thermal swi tch wi l l open and b reak the c i rcu i to n ce i t s en ses t em p e ra tu r es o v e r it s p re se t t h resh o ld . T h i s sw i t ch p ro te c t s t h ep r in t e r f ro m sev e re d am ag e — an d p o ss ib ly a f i r e h aza rd . U n p lu g th e p r in t e r, d i sco n n ec t t h e th e rm al sw i t ch f ro m th e fu s in g l am p c i r cu i t , an d m easu re i t s co n t inu i ty wi t h a m ul t im eter. Th e swi tch sho u ld norm al ly be c losed . I f you f ind an op ensw i tch , r ep lace i t. C h eck th e q u a r t z l am p n ex t b y m easu r in g co n t in u i ty ac ro ss th e



124 Image-formation system

bulb itself. I f you read an open c i rcu i t , rep lace the quar tz lamp (o r the en t i re fusin g as sem b ly ) . B e su re to secu re an y d i sco n n ec ted w i res .

If the printer st i l l does not reach i ts desired temperature, or continuously opensthe therm al switch, there is prob ably a fault in the EC P. Try replac ing the EC P. If youwish to troubleshoot the ECP, refer to chapter 8 for a d iscussion of sensors, or chap

ter 9 for detailed inform ation about the E CP.

Image-format ion problemsT h ere a re m an y v a r i ab les a t w o rk in th e fo rm at io n o f an e l ec t ro p h o to g rap h ic

im ag e , so ev en th o u g h th e p r in t e r m ig h t b e o p e ra t in g w i th in sa fe l im i t s w h ere n oe r ro r m essag es a re g en era t ed , t h e p r in t ed im ag e m ig h t n o t b e fo rm ed p ro p er ly.Al though p r in t qual i ty i s a lways a sub ject ive decis ion , there are cer ta in physicalchara cter is t ics in E P p r in t ing that s ignal t rou b le in ima ge fo rm at ion . I t i s v i r tual lyimpossib le to def ine every poss ib le image fo rmat ion p rob lem, bu t th is par t o f the

cha p ter i l lus t ra tes a b ro ad range o f bas ic sym pto ms that can te l l yo u whe re tolook fo r t roub le .

S y m p t o m 1 Pages are completely blacked out, and might a ppear blotchedwith an undefined border (Fig. 7-6). U n p lu g th e p r in t e r, r em o v e th e E P ca rt r id g e , an d ex am in e i ts p r im ary co ro n a w i re . R em em b e r f ro m ch ap te r 5 th a t a p r imary co rona app l ies an ev en cha rge across a d ru m surface . Th is charge read i lyrep e l s t o n e r— ex ce p t a t t h o se p o in t s ex p o se d to l ig h t b y th e w r i t in g m ec h an i smthat a t t rac t toner. A fa i lu re in the p r imary corona wi l l p reven t charge developm ent on the d rum. As a resu l t , the en t i re d rum surface wi l l a t t rac t ton er ( ev en i fy o u r w r i t in g m ech an i sm w o rk s p e r fec t ly ) . I f t h e en t i r e su r face a t t r ac t s t o n e r, t h e

image w i l l be to ta l ly b lack . I f you f ind a b ro ke n o r fou led coro na wi re , c lean thew i re o r r ep lace th e E P ca r t r id g e .

7-6Printed page is blacked out.



Image-formation problems 125

If your b lacked-ou t page shows p r in t wi th sharp , c lear ly def ined borders , yourwrit ing mechanism might be running out of control . LEDs in a solid-state print bar orlaser beam migh t be shor ted in an ON cond i t ion , o r receiv ing er roneous data b i t sf rom i t s con tro l c i rcu i t ry (a l l log ic I s ) . In th is exam ple , the p r imary c orona is working just f ine, but a writ in g mec han ism that is always on will effec tive ly ex po se th e ent i re d rum and d ischarge whatever charge was app l ied by the p r imary corona. Thenet resu l t o f a t t rac t ing toner would be the same, bu t whatever image is fo rmedw o u ld p ro b ab ly ap p ea r c r i sp e r —m o re d e l ib e ra t e .

Your bes t course h ere i s s imply to rep lace the E CP. If you choos e to t roub lesh oo tthe ECP, use your osci l loscope to measure the data s ignals reach ing your wri t ingmech an ism du r ing a p r in t cycle . You shou ld f ind a sem irandom square w av e r epr esenting the Is and Os com pos ing th e image . If you find only on e logic state, troubleshoot your main logic and driving circuits handling the data. If data entering thewri t ing mechan ism appears normal , rep lace your wri t ing mechan ism.

S y m p t o m 2 Print is very faint (Fig. 7-7). Befo re a t tem pt ing any th ing e lse ,try adjusting the printer contrast control . If that fai ls to help , unplug the printer, re

m ove the EP car t r idge, and try red is t r ibu t ing to ner in the c ar t r idge. Your user 's manual prob ably offers pre ferr ed instru ctions for redistr ibuting ton er. K ee p in mind thattoner is largely organic—as such, i t has only a l imited shelf and useful l ife. If redist r ibu t ion temp orar i ly o r par t ia l ly impr oves th e imag e, o r i f the E P car t r idge has b eenin serv ice fo r mo re than s ix months , rep lace th e EP car t r idge . If you are us ing a paper with a moisture content, f in ish , or conductivity that is not acceptable, image format ion migh t no t take p lace p roper ly.

7-7Printed page is very faint.

Check your t ransfer co rona. The t ransfer co rona app l ies a charge to paper thatpu l ls toner o ff the d rum. A weak t ransfer co rona migh t no t app ly enough charge toattract al l the ton er in a drum ima ge. This low cha rge can resu lt in ve ry faint ima ges.Unplug the p r in ter, a l low ample t ime fo r the h igh-vo l tage p ow er su pp ly to d ischarge




completely, then inspect al l wiring and connections at the transfer corona. If themonof i lament l ine enci rc l ing the t ransfer co rona is damaged , rep lace the t ransfercorona assembly, or at tempt to rethread the monofilament l ine. If faint images persist , repa ir or rep lace the high-vo ltage po w er sup ply assemb ly.

Finally, chec k the dru m groun d conta cts to be sure that they are secu re. Dirty or

dam aged g roun d con tacts wi ll no t read i ly a l low exp ose d d rum areas to d ischarge. A sa result , very l i t t le toner will be at tracted and only faint images will result .

S y m p t o m 3 Print appears speckled (Fig. 7-8). Your first ste p shou ld b e toturn off printer power and check the cleaning pad on the fusing roller. A pad that iso ld o r worn wi l l no t wipe the ro l ler p roper ly and shou ld be rep laced . Rep lace thecleaning pad if necessary and re-test the printer.

'vfiafroB print appears: marked5?TPS speckled. This- I j S -oftenS§£$t** to ji.;problem in the

ttsjfjwieck the clearing pad•iiSfcpid the heating roller.

replacing the cleaningi i ^ ^ y cleaning the hea'yng:';./ pller assembly (wait for

unit to cool.

7-8The print has a speckled or dirtyappearance.

If the cleaning pad checks out, speckled print is probably the result of a fault inyour p r imary corona g r id . A g r id is essen t ia lly a f ine wire m esh be tw ee n the p r imarycorona and d rum surface . A constan t vo l tage ap p l ied across the g r id serves to regulate the charge applied to the drum to establish a more consistent charge distr ibution. Grid fai lure will al low much higher charge levels to be applied unevenly. Thehigher charge results in dark splotches in the print . Because the primary grid assembly is part of the EP cartr idge, replace the EP cartr idge and re-test the printer.

If speckled print persists, repair or replace the high-voltage power supply assembly.S y m p t o m 4 There are one or more vertical white streaks in the print (Fig.

7-9). Beg in by check ing your toner level . Ton er migh t be d is t r ibu ted uneve n ly a longthe car t r idge leng th . Unplug the p r in ter, remove the EP car t r idge, and red is t r ibu tethe toner. Fo l low your manufactu rer ' s recommendat ions when hand l ing the EP cart r idge. I f th is improves your p r in t qual i ty (a t leas t temporar i ly ) , you know that theEP car t r idge must be rep laced soon .

Nex t , examine your t ransfer co rona fo r areas o f b lockage o r ex t reme con taminat ion . Such fau l ts would p reven t the t ransfer co rona f rom generat ing an even




O n e o ir e v e r t i c ; w h i t e

s t r e a k p e a r i n t l 5

p r i n t e g e . T h i s y p e

o f s y r n i s u s u e y d u e

t o a 1 r u n e v e n ; o n e rl e v e l .

7-9T r y n i n g t h e E The print contains one or morec a r t r i i r s t I f t l i t f a i l s vertical white streaks.t h e r e • b e a b i o c i n

t h e o j o r l a s e r / ; a n n i n g

a s s e m T r y c le g t i n g

t h e a ; o l y

charge a long i ts leng t h— corros ion acts as an insu la to r that reduce s the corona e lectric f ield . Uncharged page areas will not at tract toner from the drum, so those pageareas will remain white. Clean the transfer corona very carefully with a clean cottonswab. If your printer comes with a corona cleaning tool, use that instead. Whencleaning, be sure to avoid the monofilament l ine wrapped around the transfer coronaassembly. If the l ine breaks, you will need to rewrap i t or replace the entire transfercorona assembly.

Check the optics for any accumulated dust or debris that could block out sec

tions of l ight . Because ES drums are only scanned as f ine horizontal l ines, i t wouldtake l i t t le mo re than a fragm ent of debris to block l ight throug h a focusing lens . Gently blow off any dust or debris with a can of high-quali ty, photography-grade compressed air available from any photography store. For stains or stubborn debris,clean the afflicted lens gen tly with high-quality, lin t-free w ipe s and lens cleane r fromany photography store. Be very careful not to dislodge the lens from its mounting.Ne ver b low on a lens o r mirro r yoursel f Breath vapor and par t ic les can con dens eand dry on a lens to cause even more problems in the future.

S y m p t o m 5 Right-hand text appears missing or distorted (Fig. 7-10). Usually, th is is simply a result of low ton er in you r E P cart r idge . If any area of the de velopment roller receives insufficient toner, i t wil l result in very l ight or missing image

areas. Unplug the printer, remove the EP cartr idge, and redistr ibute the toner. Follow your manufactu rer' s recom me ndat ions fo r toner red is t r ibu t ion . I f you see an improv em ent in image qual i ty (a t leas t tem po rar i ly ) , rep lace the E P car t r idge.

Examine the mount ings that suppor t your wri t ing mechan ism. The mechan ism,along with i ts associated optics, is usually mounted to a removable cover. Make surethat the wri t ing mechan ism ( laser, LED, o r LCS) i s mounted correct ly, and that i t scover is closed completely. If the writ ing mechanism is not mounted correctly, scanl ines migh t no t be d el ivered to the p rope r d rum locat ions . Try rep lacing the w ri t ingmechan ism.




T h e r i g h t h a n d t e x t s

t o b e m i s s i n g o r d i s t c ~

T h i s t y p e of p r o b l e m ':

a l m o s t a l w a y s a m a r

a t i o n o f a n E P c a r t r i c:

;p r o b l e m .

T r y r e p l a c i n g t h e E P

c a r t r i d g e . I f t h e p r o

p e r s i s t s , t h e p r o b l e m

b e i n t h e l a s e r / s c a n n

a s s e m b l y

7-10Right-hand print appears missingor distorted.

If you are using a laser writ ing m echan ism, pay special at ten tion to the instal lat ion and alignment of the laser/scanning assembly. If the assembly is not instal ledwith the corr ect al ignm ent, the scanning bea m might start or stop at d ifferent pointsalong the drum. An end portion of the image might be distorted or missing. Reseator replace any incorrectly posit ioned laser/scanning assembly. If you are workingwith an older laser printer, check the al ignment of the scanning mirror.

S y m p t o m 6 You consistently encounter faulty image registration (F ig .7-11 ). Pa pe r shee ts are draw n into the printer by a picku p roller, and held by a set ofregistrat ion rollers until a drum im age is read y to be tran sferred to paper. Und er n ormal c i rcumstances , the lead ing edge o f paper wi l l be matched (o r registered) p r ecisely with the beginning of a drum image. Poor paper quali ty, mechanical wear, andpaper path obstructions can all contribute to registrat ion problems.

JeWrMfrtrF t h l ^ i n t e d

i m a g e . T h e p a p e r m a y b e

o f t h e w r o n g w e i g h t , f i n i s h ,

o r m o i s t u r e c o n t e n t . T r y

n e w p a p e r i n t h e p r i n t e r .

I f t h e p r o b l e m p e r s i s t s ,

c h e c k t h e r e g i s t r a t i o n

a s s e m b l y f o r d a m a g e o r

w e a r . T r y r e p l a c i n g t h e

r e g i s t r a t i o n a s s e m b l y

7-11The image does not registerproperly.




Begin by inspecting your paper and paper tray assembly. Unusual or specializedpaper m igh t no t work p rop er ly in your pap er t ranspor t sys tem ( th is a lso can lead toPAPER JAM erro rs as d iscussed in chap ter 8 ) . Check the paper speci f ica t ions fo ryou r printer l isted in you r user 's manual. If you find that your pa per is nonstand ard,try about 50 sheets of standard-bond xerographic paper and re-test the printer. Be

cause paper is fed from a central paper tray, any obstructions or damage to the traycan adversely affect page reg is t ra t ion (o r eve n cause pap er jam s) . Exam ine you rtray carefully. Correct any damage or restr ict ions that you might f ind, or replace theentire tray outright.

If registrat ion is sti ll inco rrec t , it usually suggests m echa nical w ea r in you r p ap erfeed assembly. Check the pickup roller assembly first . Look for signs of excessivero l ler wear. Re m ove your p r in ter housings to expo se the pap er t ranspor t sys tem . Youwill have to defeat any housing interlock switches, and perhaps the EP cartr idge sensit iv ity switches. Perform a self-test and watch paper as i t moves through theprinter. The paper pickup roller should grab a page and move i t about 3 inches ormore in to the printer before registrat ion rollers activate. If the pickup roller clutchsolenoid turns on, but the pickup roller fai ls to turn immediately, your pickup assembly is wor n ou t . Th e reco mm end ed p roced ure i s s imply to rep lace the p ickup assembly, but you migh t be able to adjust the picku p roller or clutch tens ion to im pro vesomewhat p r in ter perfo rmance.

An oth er co m m on pr ob lem is we ar in the reg is t r a t ion ro l ler assembly. I f th isse t o f ro l lers does no t g rab the wai t ing page and pu l l i t th rough even ly a t theproper t ime, the image on the paper migh t no t be correct . As you in i t ia te a p r in terse l f - tes t , watch the act ion o f your reg is t ra t ion ro l lers . The ro l lers shou ld engageimmediate ly af ter the p ickup ro l ler s tops tu rn ing . I f the reg is t ra t ion c lu tch so len o id ac t iv a t es , b u t p ap er d o es n o t m o v e im m ed ia t e ly, y o u r r eg i s t r a t io n ro l le r a s

sem b ly i s w o rn o u t . T h e r eco m m en d e d p ro ced u re s im p ly is t o r ep lace th ereg is t ra t ion assembly, bu t you migh t be ab le to ad jus t to rs ion sp r ing tens ions tos o m e w h a t i m p r o v e p r i n t e r p e r f o r m a n c e .

Pay part icular at tention to the components in your drive train assembly. Dirty ordam aged gears can jam or s lip , which leads to er ra t ic paper m ove m ent and fau ltyregistrat ion. Clean your drive train gears with a clean, soft cloth . Use a cotton swabto clean gear teeth and t ight spaces. Remove any objects or debris that might b lockthe drive train , and replace any gears that are damaged.

S y m p t o m 7 You encounter horizontal black lines spaced randomly throughthe print (Fig. 7-12). Remember that b lack areas are ult imately the result of l ightstr ik ing the drum. If your printer uses a laser writ ing mechanism, a defective or improperly seated beam detector could send false scan t iming signals to the main logic.The laser would make i ts scan l ine while main logic waits to send i ts data. At the beginning of each scan cycle, the laser beam strikes a detector. The detector carrieslaser l ight through an optical f iber to a circuit that converts l ight in to an electroniclogic signal that is comp atible w ith main logic. Main logic in terpre ts th is beam-detectsignal and kno ws the scanner mirro r is pr ope rly al igned to beg in a ne w scan. Mainlog ic then modu lates the laser bea m on and o ff co rresp ond ing to the p resen ce o r absen ce of do ts in the scan l ine.

Posit ioning and alignment are cri t ical here. If the beam detector is misaligned orloose, the printer motor vibrations might cause the detector to occasionally miss the




I P I - P are Vinrii?nnta1 hlarVl i n e s s p a c e d r a n d o m l y

t h r o u g h t h e p r i n t e d p a g e .

T h i s K i n d u f i n l m m i l L m i l

p r o b l e m i s u s u a l l y d u e t oa p r o b l e m w i t h t h e l a s e r

b e a m s e n s o r o r o p t i c a l

c a b l e . T r y r e p l a c i n g t h e7-12Th ere are horizontal black lines inthe print.

beam . Main logic resp ond s to th is by activating th e laser full- t ime in an effort to synchronize i tself again . Reseat or replace the beam detector and optical f iber.

A loose or misaligned laser/scanning assembly also can cause th is type of problem. Vibrat ions in the mirro r migh t occas ional ly de f lec t the be am around th e dete ctor. Realign, reseat, or replace the laser/scanning assembly.

S y m p t o m 8 Print is slightly faint. Prin t that is only slightly faint do es notnecessari ly suggest a serious problem. There are a series of simple checks that cannarrow down the problem. Check the print contrast control d ial . Turn the dial to a

lower sett ing to increase contrast (or whatever darker sett ing there is for your part icular prin te r) . If the contrast co ntrol has l i t t le or no effect , you r high-voltag e po we rsupply is probably fai l ing. Replace your high-voltage power supply.

Check your paper supp ly. Unusual o r specia l ly coated paper migh t causefused toner images to appear fa in t . I f you are unsure abou t the paper cu rren t ly inthe p r in ter, inser t a good -qual i ty, s tand ard-w eigh t xe rog rap h ic pa per and tes t th epr in ter again .

Check your toner level . Unplug the p r in ter, rem ove the EP car t r idge, and red ist r ibu te toner. Fo l low al l manufactu rer ' s recommendat ions when i t comes to red istr ibuting toner. Th e tone r supply migh t just be sl ightly low at the de velo pin g roller.

Unplug your printer and examine the EP cartr idge sensit iv ity switch sett ings.

These microswi tches are ac tuated by molded tabs a t tached to your EP car t r idge.This tab configuration repr esen ts the relat ive sen sit iv ity of the drum . Main logic usesthis code to set the power level of i ts writ ing mechanism to ensure optimum printquality. Th es e sw itches also tel l main logic wh eth er an E P cartr idge is instal led at al l.If one of these tabs are broken, or if a switch has fai led, the drum might not be receiving enough l ight energy to achieve proper contrast . Check your sensit iv ityswi tches as ou t l ined fo r a NO EP CA RT RID GE erro r shown in chap ter 8 .

Over t ime, natural dust part icles in the air wil l be at tracted to the transfercoron a and accum ulate the re. The dust eventu ally causes a layer of debris to form on




the wire . Th is type of accumulat ion cu ts down on t ransfer co rona effect iveness ,which places less of a ch a rg e on paper. Less ton er is pu l led f rom th e d r u m , so the r e

sult ing image appears fainter. Unplug the printer, al low t ime for the h igh-vo l tagep o w er su p p ly t o d ischarge, then gen t ly c lean the t ransfer c orona w ith a clean c o t tonswab or corona cleaning tool. Be very carefu l not to b reak th e mon of i lament l inew rap p ed ab o u t th e t ransfer c orona assem bly. If th is l ine does break, th e t ransfercorona assembly will have t o be r e w r a p p e d or rep laced .

S y m p t o m 9 Print has a rough or suede appearance Fig. 7-13). Usually,suede p r in t is th e result o f a serious failure in the m a in lo g ic sy s t em ( E C P ) . The wr i ting mechan ism is be ing a l low ed to turn on and off randomly dur ing its scanning cy

cles . Th is type of s y m p t o m is dominan t in laser p r in ters wh ere a faulty laser d rive rcan allow t he b e a m t o act errat ically. Your best cour se is usually t o r ep lace the ECP

outright. If you wish t o t roub leshoo t th e E C P, use your osci l loscope t o t r ace t he printdata signal from th e writ ing m echa nism into main logic circuitry. You wi l l nee d a

schemat ic d iagram of your p r in ter fo r th is troubleshooting.

The- p;:: it :»'.*•: a i>:uyh Mf|ji;ll;i•ahi.o-.H a l w a y s t he fault o j

t h e ECP v m >i?~ t h e i- i

• w v*« i t cn 'and s l i jarnJojxily•a> the printer r . m >

1 re-place the EC? outright/^ |

ffilj^vN-^;lii iiiiii m

S y m p t o m 10 Print appears smeared or improperly fused Fig. 7-14). Te mp era tu re and p ressure are two key variables of the EP p r in t ing p rocess . Ton er m ustb e m e l t ed and b o n d e d to a p a g e to fix an image perman en t ly. If fusing tem peratu reor roller pressure is too low during th e fusing op erat ion, toner m ight rema in in its

powder fo rm. Resu l t ing images can be sm eared or sm u d g ed w i th a touch .P e r f o r m a s imple fusing check by running several continuous self-tests ( the

pr in ter does not h av e to be d isassembled fo r t h i s ) . Af ter abou t te n printo uts, p lacethe first and last printout on a firm surface and rub bo th su rfaces wi th you r f ingert ips . N o smearing should occur. If your fus ing leve l var ies be tw ee n page s (one pagemigh t smear, and ano ther m igh t no t ) , c lean th e therm is to r tempe ratu re sensor and

repeat th is tes t . Remember to w a i t 10 minu tes or so and unplug th e p r in ter befo rework ing on the fusing as sem bly. If fusing perfo rman ce d oes no t im p ro v e , r ep lace the

7-13The print has a suedeappearance.




The p r in t appears s me aredor fused improperly.

The fusing assembly ispro bab ly defective. Tr yreplacing the fusinga s s e m b l y 7-14

Print smears or appearsimproperly fused.

thermistor and troubleshoot i ts signal condit ioning circuit . If smearing persists, replace the fusing assembly and cleaning pads.

Static tee th just be yon d your transfer corona are used to discharg e the pa peronc e toner has be en attra cted awa y from the drum . This helps pap er to clear thedrum withou t being a t t rac ted to i t . An even charge i s needed to d ischarge paperevenly, o therwise, some portions of the page might keep a local charge. As papermoves toward the fusing assembly, remaining charge forces might shift some tonerresult ing in an image that does not smear to the touch, but has a smeared or pulledappearance. Examine the stat ic d ischarge comb once the printer is unplugged and

discharged. If any of i ts teeth are bent or missing, replace the comb.A cleaning pad rubs against the fusing roller to wipe away any accumulations of

toner particles or dust. If this pad is worn out or missing, contamination of the fusingroller can be transferred to the page , result ing in sm eared p rint . Check y our cleaningpad in the fusing assembly. Worn out or missing pads should be re place d imm ediately.

Inspect your drive train for any gears that show signs of damage or excessivewear. Slipping gears could allow the EP dru m and pap er to m ove at d ifferent spe eds .Th e d i fferen t speeds can eas ily cause por t ions o f an image to appear sm ud ged —such areas would appear bolder or darker than other portions of the image. Replaceany gears that you find to be defective. If you do not f ind any defective drive traincomp onen ts , t ry rep lacing the EP car t r idge.

Finally, a foreign object in the pape r path can rub against a tone r po w de r imag eand smudge i t before fusing. Check the paper path and remove any debris or paperfragments that might be in terfering with the image.

S y m p t o m 11 Printed images appear to be distorted (Fig. 7-15). Distortionis at best a vagu e term w he n app lied to printed im ages , but for the purp ose of th issymptom, you might see one of two types of d istort ion: image size distort ion, andscanning distort ion.

Image size d istort ion is indicate d when c haracters app ear too large or too sm allin the vert ical d irection. Large (or stretched') characte rs sugge st that som e por tionof the pickup or registrat ion assem blies is fai ling, or that there is some obstru ction in




If the printer cannot detect the laser beam, the laser will f ire full-duty as the printertr ies to re-establish synchronization. This will expose the drum and result in blackprint . The white l ines indicate that synchronization is briefly restored. Your first act ion shou ld be to check the f iberop t ic cab le between the laser /scann ing assemblyand the ECP. I f the cab le appears to be connected p roper ly, i t shou ld be rep laced .

N o te th a t y o u a lso m ig h t see a B E A M D E T E C T I O N e r ro r w i th th i s t y p e o f p ro b lem .If the problem persists, the trouble is probably due to a defect in the laser/scannerassembly. Try replacing the laser/scanner assembly outright.

S y m p t o m 1 4 The image appears skewed (Fig. 7-18). Image skew is usuallythe resu l t o f a p rob lem in the paper path—someth ing is happen ing as paper t ravelsthrough the printer. Begin by checking the paper tray. Make sure that the right typeand weigh t o f paper i s ins ta l led p roper ly in the t ray ( the t ray migh t have too muchpaper) . Remove and reseat the paper t ray to be su re that i t i s inser ted even ly andcom plete ly. Try swi tch ing t rays wi th ano ther com pat ib le p r in ter.

If the paper and tray both appear in tact , unplug the printer, and open the outerco v er s . Inspect the paper pickup roller for signs of unusual or uneven wear. If thep ickup ro l ler is uneven , pape r wi l l walk (o r skew) befo re it reaches the reg is t ra t ionro l lers . W he n you f ind a wo rn o r dam aged p aper p icku p ro l ler, you m igh t be ab le toad jus t the ro l ler tens ion mechan ism to com pen sate , bu t you are usually bes t adv isedsimply to rep lace the pa per p ickup assembly.

I f the paper p ickup assembly looks good , check the reg is t ra t ion ro l ler assemblynext. Registrat ion rollers hold the page in place unti l the latent drum image is posit ioned p rop er ly fo r the pag e. As wi th the paper p ickup , a wor n o r dama ged reg is t rat ion assembly will al low the page to skip or walk as i t is being passed through theprinter. Note that the registrat ion assembly uses torsion springs to ensure even pressure across the rollers. A m issing or de fec tive spring can result in un eve n press ure. Ifyou find uneven pressure, you can readjust the torsion springs, but your best course(especially for o lder printers) is to replace the registrat ion roller assembly outright.

The image seems to beskewed at one or morepoints along the page.

7-18The image appears skewed at oneor more p oints.

Check the paper pathand clear any obstructionsthat you may find.




I f the p rob lem pers is ts , there i s p robab ly an obst ruct ion som ew her e in the paperpath . You will need to examine the paper path very carefully to look for obstructions.Stuck labels and paper fragments are typical causes. Gently clear any obstructionsthat you find—be very careful not to damage any rollers or mechanisms in theprocess of clearing an obstruction.

S y m p t o m 1 5 The image is sized improp erly. Th e EP p r in ter con tro l c i rcu it rysizes an imag e based on the pap er in the tray. Print ers use a series of mic rosw itche sthat are actuated by a specially shaped tab attached to each tray, and each tray usesa differently shaped tab to actuate these tray-detect swi tches in a un ique seque nce.Make sure that each tray is f i t ted with the proper tab for the tray being used.

I f the t ray is fi t ted wi th the p rope r tab , there migh t be a p rob lem w i th the p r in tert ray -dete ct m icroswi tches . E i ther the s wi tches o r EC P m igh t be a t fau lt . If you wishto t roub leshoo t the paper s ize microswi tche s , refer to the p rocedu res in chap ter 8 .

S y m p t o m 1 6 There are vertical black streaks in the image (Fig. 7-19). Th issymptom is indicated by one or more dark vert ical l ines. Each l ine might be a different width , but the width of each l ine will remain constant throughout the length ofthe page. Usually, you will f ind that the primary corona is d irty. Remember fromchap ter 5 that any gunk or debris that accum ulates on the primary c oron a will pr event an even charge distr ibution. Fortunately, th is problem can be rectif ied simplyby cleaning the primary corona wire. Use care when cleaning the primary corona. Ifi t breaks , the E P c artr idg e will have to be rep lace d. If the prim ary co rona is clean bu tthe p rob lem p ers is ts , there m igh t be a manufactu r ing p ro b lem w i th the E P car t r idgeitself. Nicks o r manufactu r ing defects around the develop m ent ro l ler can a l low mu chmo re toner in p rox im i ty o f the d rum. Try a ne w E P car t r idge.

Ver-.i ;al blackin the

of sympti• s indicates

prim iry corona

a p p e a r

t y p e

alwa

Cleancare f

the prima. i l l y.

s1 - ?aks

or.ge. Th .salmost

'ha t thi;: dirty.

corora 7-19Vertical black streaks appear inthe image.

For L ED p r in ters , there m igh t be a p rob lem wi th the scan l ine buffer mem ory. Ascan l ine buffer holds the individual p ixels that will u l t imately app ear in the l ine. If ane w EP c ar t r idge does no t so lve the p rob lem, t ry rep lacing the EC P to excha nge thescan l ine memory. If you wish to try troubleshooting the ECP in detail , refer to theprocedures o f chap ter 9 .



C H A P T E R

Mechanical systems

M ore is ne ed ed than just goo d elec tron ics and a fancy ca se to ma ke a f irst-classpr in ter. A ser ies o f t igh t ly in tegrated mechan ical sys tems are need ed to perfo rm thevariety of physical tasks that every laser printer (Fig . 8-1) must do. The most obvious mech an ism is the paper t ranspor t sys tem res ponsib le fo r moving p aper th roughthe printer, but the image formation system also needs mechanical force to function.Laser p r in ters need a wel l - regu la ted scanner mo tor assem bly that can d i rect a laserbeam across a l ight-sensit ive drum surface. You have already read about a selectionof imp ortant m echan ical parts in cha pter 2 , but th is chap ter sho ws you ho w to dealwi th mechan ical p rob lems.

8-1 A Tandy LP800 laser printer.

137



Paper problems 139

code that is unique to that part icular paper size. Microswitches are activated by thepresence o r tabs . Main log ic in terp re ts th is paper type code, and knows au tomat ically what k ind of media (paper, envelopes, etc.) that i t is working with . Table 8-1shows a typ ical paper c ode .

Table 8-1.Typical tray switch configurations

Tray type S W l SW2 SW3Executive 1 1 1A4 1 1 0Legal 0 0 1Envelope 0 1 1Letter 1 0 0No tray 0 0 01 = on (engaged) .

0 = off (diseng aged ).

Th e p res enc e o f paper i s de tec ted by a mech an ical sensing lever as shown in F ig .8-3. W he n p ape r is available, a leve r rests on the paper. A me tal or p last ic shaft l inksthis lever to a thin plastic flag. While paper is available, this flag is clear of the paper-out sensor. If the tray be co m es e mp ty, th is leve r falls through a slot in the tray, which

8-3 Operation of a paper-sensing arm.



14 0 Mechanical systems

rotates i ts f lag in to the paper-out sensor, which shows that paper is exhausted. Thepaper-ou t sensor is usually mo unt ed on an aux i liary PC boa rd (kn ow n as the paper-

control b o a r d ) .Beg in your check by removing the paper t ray. Be su re that there i s paper in

the t ray, and that any ID ( iden t i f ica t ion) tabs on the t ray are in tact—especia l ly i f

you have jus t rec en t ly d r op pe d the t ray. Re- inser t the f i lled pap er t ray carefu l lyan d co m p le te ly. I f t h e PA P E R O U T m essag e co n t in u es , t h en th e re i s e i th e r ap ro b lem w i th y o u r p ap er t r ay ID m ic ro sw i t ch es , p ap e r sen s in g l ev e r, o r t h e p ap er-ou t op to iso la to r.

You can check the paper ID microsw i tches by rem oving th e paper t ray and actuating the paper sensing lever by hand (so the printer th inks that paper is available).Refer to Table 8-1 and actuate each switch in turn using an eraser of a pencil . Actuate one swi tch a t a t ime and observe the p r in ter d isp lay. Th e P A PE R O UT erro rshou ld go away whenever a t leas t one microswi tch i s p ressed . I f the er ro r remainswhen a swi tch i s p ressed , that swi tch i s p robab ly defect ive . Unplug the p r in ter anduse your mul t ime ter to check con t inu i ty across the suspect swi tch as you actua te i t .Rep lace any defect iv e sw i tch o r swi tch assembly. Inspect the paper-ou t leve r and optoisolator next.

When paper is available, the paper-out lever should move i ts p last ic f lag clear ofthe optoisolator. When paper is empty, the lever should place i ts f lag in to the opto iso la to r s lo t. No te : th is log ic migh t b e reve rsed dep end ing on the par t icu lar log ic o fthe p r in ter. Actuate the paper-ou t lever by hand and see that i t moves f reely andcomplete ly. Th is check conf i rms that the paper sensing arm works p roper ly. I f yousee the lever mechan ism jammed or ben t , repai r o r rep lace the mechan ism. I f theprob lem p ers is ts , rep lace the paper-ou t op to iso la to r ( o r rep lac e the paper h and l ingPC b oa rd ) . I f the p rob le m s t il l con t inues , rep lace the defec t ive ECP.

S y m p t o m 2 You see a PAPER JAM message. Th e EP p r in ter must det ect andrepor t a paper- jam cond i t ion . For most p r in ters , a jam occurs wh en any one o f thefour following si tuations do not take place . First , a she et of pape r must rea ch the fusing assembly within some amount of t ime after the printing cycle starts (usually under 30 sec on ds ) . Pap er reach ing the fus ing assem bly actuates the paper e x i t sensor.Second, paper that reaches the fusing assembly, must clear the fusing assemblywith in some amount o f t ime (a lso abou t 30 seconds , bu t i t depends on the papers i z e ) . The actuated paper sensor re tu rns to i t s o r ig inal s ta te when paper passes .Th ird , pape r that i s p resen t in the fus ing assem bly must be p rese n t w he n fus ing temperatu re i s above some min imum temperatu re (abou t 150°C) . I f any one o f these

three condit ions is false, a paper jam will be registered.As sho wn in F ig . 8 -4 , the e lect ros ta t ic pap er t ranspor t sy s tem is mu ch mo re so

phist icated than those used in more conventional serial or l ine printers. As a resulto f th is add i t ional comp lex i ty, main - log ic c i rcu i try must dete ct w het her o r no t pap eren ters and ex i ts the pap er path as expe cte d . F or th is d iscuss ion , assume that a jamcan occur in th ree g eneral areas : the paper- feed area , the reg is t ra t ion and t ransferarea, and the fusing area.

Beg in b y check ing pap er in the pa per t ray. I f a jam cond i t ion i s show n, bu t the reis no paper, i t shows that your paper sensing lever is not functioning properly. I tmigh t be b roke n , ben t , o r jam m ed. W he n there i s am ple pap er avai lab le , take a mo-



Paper problems 141

8-4 A cross-sectional diagram showing the paper path. Hewlett-Packard Co.

merit to be sure that paper is the right size, tex tur e, and we igh t for your printer. U nusual or special paper might n ot be p icked up reliably. If you are uncerta in as to t hecorrect p aper typ e, remo ve i t and inser t a quan t i ty o f good-qual i ty x erograp h ic paper. This type of paper usually has the weight and texture characterist ics ideal forES printing. If the error continues, look at potential jam locations—paper-feed area,registrat ion/transfer area, and exit area—as described below.

Paper-feed area T h e p ap er- f eed a rea co n s is t s o f t h e p ap er t r ay ( an d p a p e r ) ,p ickup ro l ler mechan ical assembly, and e lect romechan ical c lu tch as shown in F ig .8 -5 . I f paper i s no t reach ing your reg is t ra t ion ro l lers , the t roub le i s p robab ly inthis area.

Mechanical

Registrationrollers

Paper tray'Paper

8-5 Simplified diagram of a paper pickup/feed m echanism.



142 Mechanical systems

Inspect your paper tray carefully. Although the tray might seem foolproof, i t actually plays an important role in paper feed. If the plast ic tray housing is cracked ordam aged , rep lace it wi th a ne w t ray and re- tes t the p r in ter. No te th e mova b le metalp la te in the t ray bo t tom. Th is l i f t mechan ism keeps paper posi t ioned agains t thepickup roller at al l t imes. Remove the tray and paper. Make sure that th is p late can

m ove f r eely— rep lace your p aper t ray i f i t does no t . Obs erve th is li ft p la te as you insert i t in to the p rinter. Th e p rinter should l ift th is p late up as the tray is inser ted. Ifth is does no t happen , repai r o r rep lace the p r in ter l i f t mechan ism assembly. Addsome fresh paper (50 sheets or so) and gently insert the paper tray. Be sure to insertthe tray fully and squarely. If there is any obstruction (or the tray does not seatsq u are ly ) , f ind and rem ove th e obst ruct ion .

Ne xt, mak e sure that you r main moto r is functioning. K ee p in mind that the mainmo tor d rives al l ro llers in the printer, as we ll as the p hotos ensit ive drum . If th is m otorhas fai led, paper will not be drawn into the printer at al l . You can observe the mainm otor and i ts gear-train assem bly by turning the printer on, opening an acces s co ver,defeating the associated in terlock switch (if an y) , defea ting E P cartr idge sensit iv ityswitche s, and init iat ing a printer self-test . If the fusing roller tem per atu re is above i tslower tem peratu re l imi t , you shou ld see m otor operat ion imm ediate ly. I f the main m otor fai ls to operate, replace the main motor driver circuit or ECP as required. If themain motor continues to malfunction, replace the main motor assembly.

Th e main mo tor turns wh en a print signal is f irst gen era te d in main logic. I t cont inues tu rn ing as long as the re i s pape r in the feed path . Ho we ver, eve n though themain motor sup p l ies the fo rce to op erate ever y ro l ler, p ickup and reg is t ra t ion ro l lersare ope rate d on ly b r ief ly dur ing each p r in t cycle . An e lec t rom agn et ic c lu tch (a lsoshown in Fig . 8-6) is used to switch the pickup roller on and off at desired t imes.Main logic generates a clutch control signal that is amplified by driver circuits be

fo re being fed to the e lect romagnet . When deact ivated , the p lunger d isengages the

TransferTransfer corona

roller

8-6 Simp lified diagram of a registration/transfer area.



Paper problems 143

pickup roller from the drive train . When activated, the plunger engages the pickuproller, wh ich causes the pickup roller assembly to turn and grab the ne xt available pie ceof paper. A separation pad benea th the pickup roller prevents m ore than one sheet frombeing taken at any one time. Paper stops when it reaches the idle registration rollers.Not ice h ow pap er will bow — this is a normal and harmless function in the pape r path .

If the main motor operates, but the pickup roller does not turn (you can see th isdur ing an open-cov er tes t p r in t ) , inspect the p ickup ro l ler c lu tch so leno id . No tethere are p robab ly two major so leno ids—one fo r the p ickup ro l ler, and one fo r theregistrat ion rollers. When printing starts, one of the two solenoids ( the pickup soleno id ) shou ld eng age imm ediate ly. If no so leno id eng ages , there i s an e lect ron ic p roblem. If the pickup solenoid engages, but your pickup roller does not turn (or does nottu rn p r op er l y ) , repai r o r rep lace the p ickup mecha n ical assembly. Ther e migh t be afaulty clutch or other mechanical defect .

W hen th e p ickup so leno id fai ls to ac tuate , use your m ul t imeter t o measure vo l tage across the solenoid. You should see voltage toggle on and off as the solenoid isswitched. If voltage changes, but the solenoid does not function, replace the sole

no id . I f vo l tage remains a t zero (o r does no t swi tch f rom some o ther vo l tage) , thereis probably a fault in the solenoid driver circuit . Troubleshoot the solenoid signalback into main logic. You also mig ht simp ly repla ce the p ickup clutch PC boar d. If theprob lem pers is ts , rep lace the ECP.

Finally, you can check for feed roller wear by measuring the distance betweenthe trai ling ed ge of the pap er and the end of the pa pe r tray just as the she et stop s.Th is t ime occurs betw een the po in t whe re the p ickup ro l ler s tops , and the reg is t rat ion rollers start . Norm ally, th is trai ling ed ge should adva nce about 3 inches or m ore .If i t does not advance this far, your pickup roller is probably worn out. Replace thepickup roller assembly and separation pad.

Registrat ion/transfer area Reg istration rollers hold on to the page until its leading edg e is al igned with the drum image . Force is supplied by th e main motor, but another electromagnetic clutch switches the registrat ion rollers on and off at theappropriate t ime. Once paper and the drum image are properly al igned, main logicsends a clutch control signal that is amplified by driver circuits to operate the registrat ion clutch solenoid. After the clutch is eng aged , registrat ion rollers will carry the pageforward to receive the dev elop ed ton er image. The registrat ion/transfer assem bly usually consists of registration rollers, the drive train, a registration clutch solenoid, atransfer guide, and the transfer corona assembly as shown in Fig. 8-6. If paper entersthe prin ter but doe s not reach th e fusing rollers, your fault is probab ly in this area.

You can see registrat ion roller opera tion by open ing a housing, defeat ing any

correspond ing in ter locks , and defeat ing any EP car t r idge sensi t iv i ty swi tches , theninitiating a self-test. U s e e x t r e m e c a u t i o n t o p r e v e n t in j u r y f r o m h i g h - v o l t a g eo r o p t i c a l r a d i a t i o n f ro m t h e w r i t i n g m e c h a n i s m — e s p e c i a l l y fro m l a s e r s .Watch the paper path and drive train very carefully. If you see an obstruction in thepap er path or in the drive train , unplug the p rinter and allow 10 min utes for the fusing assembly to coo l befo re a t tem pt ing to c lear the obst ruct ion Rep lace any gears o rbushings that appear damaged or worn. Pay close at tention to any tension equalizingsprings (called torsion sp r ings) a t tached to the reg is t ra t ion ro l lers . Reseat o r replace any torsion springs that might be damaged or out of posit ion.



144 Mechan ical systems

Carefully inspect the monofilament l ine encircling the transfer corona. Makesure that the l ine is in tact , and not in terfering with yo ur pap er path . Do not app roac hthe corona wit h you r hands or any meta l tool If yo u see signs of phys ical in t erference, unp lug the p r in ter and a l low h igh-vo l tage to d ischarge befo re rep lacing themonofilament l ine or transfer corona assembly.

If your main motor and drive train operate, but registrat ion rollers do not turn(or turn properly), inspect the registrat ion solenoid clutch. I t wil l usually be adjacen t to the p ickup so leno id c lu tch . The so leno id shou ld engage moments af ter thepickup roller d isengages. If the registrat ion solenoid does not engage, there is anelectrical problem. If the solenoid does engage, but registrat ion rollers do not turn ,your mechan ical c lu tch o r reg is t ra t ion ro l lers are p robab ly worn ou t . Rep lace themechanical registrat ion assembly.

When your registrat ion solenoid fai ls to actuate, measure the signal driving thesolenoid. You should see the signal toggle on and off with the solenoid. If the signalchanges , bu t the so leno id does no t f ire , rep lace the jam m ed or d efect ive s o leno id . I fvoltage does not change, there is probably a fault in the solenoid driver circuit or

main logic. Troubleshoot the solenoid signal back into your main logic. You might rep lace the reg is t ra t ion c lu tch PC board . I f the p rob lem pers is ts , rep lace th e E C P

Exi t area At the ex i t area , a pag e has bee n com plete ly dev elop ed wi th a tonerp o w d er im ag e . T h e p ag e m u s t n o w b e c o m p ressed b e t w e en a se t o f fus in g ro l l e r s—one p rov ide s heat , and the o ther app l ies p ressure . Heat me l ts the toner p owd er, andro l ler p ressure fo rces mol ten toner permanen t ly in to the paper f ibers to f ix the image. As a f ixed page leaves the rollers, i t might st ick to the fusion roller. A set ofeven ly spaced separat ion pawls p ry away the f in ished pa ge, which is del ive red to theoutput tray. This step completes the paper feed process as shown in Fig . 8-7 . Mainm oto r force is de liver ed to the fusion rollers by a ge ar ed dr ive train . Th er e are no

clu tches invo lved in ex i t area operat ions , so the d r ive t ra in moves th roughou t theentire printing cycle.

I t i s impor tan t that the p r in ter detect when paper en ters and leaves the ex i tarea. Based on paper size and fusion roller speed, a page has only a set amount of

8-7 Simp lified diagram of the fusing/exit area.




8-8Smudged horizontal band andoverprint.

an overpr in ted sm udge. Th e so lu t ion to th is p rob lem is to rep lace th e en t i re p ickupassembly. You also might wish to replace the pickup roller clutch. Check the associa ted gear t rain to make su re there are no obst ruct ions o r damag ed com pone n ts .

Sensors play a major role in EP printer operation. With so many variables in theoutside w orld influencing the ult im ate print quali ty that yo u see in a f in ished p ag e, i tis v ital that an EP printer detect the physical condit ions within the printer. For thepurpose s o f th is book , the p r imary in teres t i s in mon i to r ing te mp eratu re , as we l l asthe s ta te o f physical con tacts . There are th ree types o f sensors used fo r these purposes: resist ive sensors, mechanical sensors, and optical sensors.

Resistive sensorsElect rosta t ic printer s use a fusion-roller assem bly to app ly hea t and pressu re to

f ix a tone r image on paper. Fusion temp eratu re m ust be carefu lly ma in ta ined a tabou t 180°C to ach ieve an op t imum toner mel t . To con tro l temperatu re , i t i s necessary to dete ct tem pera tu re , wh ich is the job o f a thermistor. Therm istors ( t h e rm alres is to rs) are res is to rs whose values change in p ropor t ion to thei r temperatu re . De

pend ing on thei r fo rmulat ion , thermis to rs can be const ructed to increase o r decrease wi th temperatu re . For the purpose o f th is book , a thermis to r i s assumed toincrease i ts value wi th tem pera tu re .

A s imp le tempe ratu re a larm is show n in F ig . 8 -9 . Th e ad jus tab le res is to r ( R a d j )se ts the a larm t r ip po in t , and the therm is to r ( T ) fo rm s a vo l tage d iv ider wi th th ef ixed res is to r (R ) . Wh i le tem pera tu re i s be low the a larm set t ing , V sens e x c e e d s V r e {, sothe ou tpu t i s abou t +V as shown. When temperatu re r i ses above the a larm set t ing ,Xsens u r o P s b e l o w V e f , so the ou tpu t d rops to abou t g round (0 V ) .

Because a thermis to r value i s rough ly p ropor t ional to temperatu re , i t can beused in a pro por tionin g circuit to regu late such things as fusion roller tem pe rat ur e.

Sensor and interlock problems



Sensor and interlock problem s 147

Thermistor

Over temp

Under temp

8-9 Schem atic of a simple temp erature-alarm circuit.

A p ropor t ion ing c i rcu it i s show n in F ig . 8 -10 . As t em pera tu re increases , the oscil lato r wi l l p rod uce shor ter pu lses , and v ice versa . Th e ou tpu t can be d ete cte d by a micropro cesso r o r ASIC , o r used to d r ive a heater con tro l c i rcu i t d i rect ly.

Thermistor

Proportionedoutput

8-10 Schematic of a simple temperature-proportioning circuit.

Mechanical sensorsW h en p o s i t io n o r p re sen ce m u s t b e d e t ec t ed , a s im p le m ech an ica l sw i t ch can

b e u sed . A se t o f m ech an ica l co n tac t s m ig h t b e n o rm al ly o p en o r c lo sed , t h en actu a ted b y th e p resen ce o f p ap er, t h e p ap er t r ay, c lo sed h o u s in g s , e t c . T h e co n d it io n o f each sw i t ch (w h e th e r u sed in d iv id u a l ly o r i n se t s ) i s o f t en d e tec t ed an dact ed upon d i rect ly by a log ic c i rcu i t . The sw i tche s used in a p r in ter ' s con tro l




p an e l o f t en a re co n s id e red to b e m ech an ica l sen so r s b ecau se th ey a re d e tec t in gy o u r in p u t f ro m th e o u t s id e w o r ld .

Optical sensorsMechanical sensors generally lack rel iabil i ty over long-term use. Electrical con

tacts wear ou t th rough use and env ironmenta l co rros ion . Con tacts a lso are sub jectto electrical ring —an outpu t that migh t vary on and off for seve ral mill isecond s befo re reach ing a s tab le cond i t ion . Opt ical sensors are imm une to the se p rob lem s.

A basic optical sensor (called an optoisolator') is show n in Fig . 8-11. A n optoisolator consist of tw o parts: (1 ) a transm itter, and ( 2 ) a rece iver. Both are separated by a physical gap. The transmitter is usually an IR ( in f ra red ) LE D kep t on a tal l t imes . Th e rec eive r is typica lly a ph otos ens it ive transistor that is mo st sens it ive tol igh t waveleng ths g ene rated by the L ED . W hen i ts gap is c lear, l igh t passes th roughunobstructed and saturates the phototransistor, in turn producing a logic 0 outputsignal. If an object (such as pap er or the ca rria ge) in te rrup ts the l ight path , the p ho

totransistor will turn off and allow a logic 1 outpu t signal. Th er e are no mo ving partsin an optoisolator, so i t can operate at h igh speeds, and i t wil l never wear out mechanically. Your printer paper supply and paper exit sensors are typically optical .

Transmitter Paper

+ V d c c - AAA/Pullup

resistor300 kH ( R l)

Receiver

Optoisolatorassembly

Pullupresistor4.7 k i l (R2 )

+Vdc

£>°—P>° o Output

L E D o ff — ^

LED on

Blocked (paper IN)

C lea r (p ap er O U T )Output waveform

CD4049(IC1)

8-11 Simplified d iagram of an optical sensor.

Troubleshooting sensors and interlocksBefore perfo rm ing sensor checks , make i t a po in t to exam ine any conne cto rs o r

interconnecting wiring that t ie the sensor in to i ts condit ioning circuit . Be certainthat a ll cab l ing is ins ta lled correct ly and com plete ly befo r e p rocee d ing . Also rem em -



Troubleshooting sensors and interlocks 149

ber that most sensors are mou nted on small PC board assem bl ies that migh t con ta inother sensors or electronics. If you do not have the schematics or test instruments tocheck a sensor as discussed below, i t is acceptable simply to replace that sensor PCboard ou t r igh t . If you do ch eck the sensor and it checks p rop er ly (and i t s cond i t ioning circuit appears to be functional) , there is l ikely a problem with the ECP.

S y m p t o m 1 You see a PAPER OUT m essage even though paper is available. Also, the error might not appear when paper is exhausted. I f your pape rsensor is a mechanical switch, p lace your multimeter across i ts leads and try actuating it by hand. You should see the voltag e read ing shift be tw ee n a logic 1 and logic 0as you t r igg er the sw i tch . I f you m easure som e vo l tag e across the swi tch bu t i t doesno t respond (o r responds on ly in termi t ten t ly ) when actuated , rep lace the defect iveswitch. If i t responds as expected, check i ts contact with paper to be sure that i t isac tuated when paper i s p resen t . You migh t have to ad jus t the swi tch posi t ion o rth read pap er th rough again to ach ieve bet ter con tact .

Check an optical paper sensor by placing your multimeter across the photosensit ive output; then, try to actuate the sensor by hand. You might need to place a pieceof paper o r cardboard in the gap betw een t ransmit ter and receiver. You shou ld see thephototran sistor output shift be tw ee n logic 1 and logic 0 as yo u tr igge r the optoisolator. If i t does not respond, check for the presence of dust or debris that might b lockthe l ight path . If excitat ion voltage is present, but the phototransistor does not respond, i t is probably defective. Replace the optoisolator or sensor PC board assembly.When a sensor responds correctly, the trouble is probably in your ECP. If you do nothave sch ema tics or test instruments to troub leshoot in detail, simply rep lace the EC P.

S y m p t o m 2 Fusing temperature control is ineffective. Temp erature neverclimbs, or climbs out of control. Temp erature fluctuations affect print quality orinitialization for EP printers. Unplug the printer and allow at least 10 minute s for

the printer to cool and discharg e. Disco nne ct the therm istor at i ts connector . Use yourmu ltimeter to me asure i ts resistance. A short or open circuit reading migh t indicate afaulty thermistor, so replace any suspect part . If you get some resistance reading,warm the thermistor with your f ingers and see that the reading changes (even a l i t t leb i t ) . A rea ding that does not cha nge at al l suggests a faulty thermistor. N ev er tou ch ahot thermistor with your f ingers You migh t have to replace the thermisto r PC boa rd.

If the thermistor is in tact , the problem is l ikely in the ECP. Reconnect the thermis to r, res to re p r in ter power, and use your osci l loscope to check the ou tpu t o f thethermistor condit ioning circuit on the ECP. If you do not have schematics or test ins t ruments to check the EC P in deta i l, rep lace the E CP ou tr igh t .

S y m p t o m 3 You see a PRINTER OPEN message. P r in ters can be ope ned toperfo rm rou t ine c lean ing and EP car t r idge rep lacement . The cover(s) that can beopened to access your p r in ter are usual ly in ter locked wi th the wri t ing mechan ismand high-voltage power supply to prevent possible in jury from laser l ight , fusingheat, or h igh-voltages while the printer is opened. A typical in terlock assembly isshown in F ig . 8 -12 . The top cover (o r some o ther cover assembly) uses a s implepushrod to ac tuate an e lect r ica l swi tch . W he n the top co ver i s ope ned , the in ter lockswi tch opens , and the p r in ter d r iver vo l tage (+24 Vdc is shown) i s cu t o ff f rom al lother circuits. In some printers, h igh-voltage is cut off d irectly, which effectively disables printer operation. When the top cover is closed again , the in terlock switch isreact ivated , and p r in ter operat ion i s res to red .




zTop cover

Pushrod

V

Interlock switch +2 4 Vdc dc powersupply

\

Switch shownnormally open

(N O )

8-12 Simplified diagram of a cove r interlock assembly.

F i rs t , make su re that your cover(s) are a l l shu t securely ( t ry open ing and re-c los ing each cover f i rmly) . Inspect any actuat ing levers o r pushrods carefu l ly. Rep lace any ben t , b roken , o r miss ing mechan ical levers . Unplug the p r in ter andobserve how each in ter lock is ac tuated ( i t migh t be necessary to d isassemble o thercovers to obser ve in ter lock op era t ion ) . Adjust the pushrods o r swi tch posi t ions i fnecessary to ensure f irm con tact .

Unplug the p r in ter and use your mul t imeter to measure con t inu i ty across anyquest ionab le in ter lock swi tches . I t migh t be necessary to remove a t leas t one wire

from the sw i tch to p rev en t fa lse read ings . Actu ate the s wi tch by hand to be su re thati t works p roper ly. Rep lace any defect ive in ter lock swi tch , re-a t tach a l l connecto rsand in terconnec t ing w ir ing , and re- tes t the p r in ter.

If a switch i tself works correctly, check the signals feeding the switch. Check thedc vo l tage a t the swi tch . I f the vo l tag e i s low or absen t , t race the vo l tag e back to thepower supply or o ther signal source at the ECP. If signals are not behaving as expec ted , o r PR IN TE R O P E N messag e remains , t race the in ter lock s ignal in to the mainlog ic board and t roub leshoo t your e lec t ron ics . I f you do no t have schemat ics o r tes tins t ruments , you shou ld rep lace the EC P ou tr igh t .

Del iver ing an accurate , wel l - t imed , modulated laser beam to a d rum surface i sno easy feat . Laser p r in ters have evo lved over the las t decade f rom large , heavy,c lunky (an d del ic a te) m echan isms wi th d iscre te lasers , mir ro rs , and op t ics , in tosvel te , rugged workhorses that in tegrate the laser, scanner, and some op t ics andelectronics r ight in to a single, easily replaceable laser/scanner assembly. You learnedho w to deal wi th laser del ivery p rob lem s in chap ter 7 . He re , you w i l l see ho w to d ealwi th scanner and main m otor p r ob lem s. Ke ep in mind that a laser /scanner assem blyis only ne ed ed fo r ES p r in ters us ing laser wri t ing mech an isms. LE D or LC S wri t ingmech an isms d o no t use scann ing m irro rs .

Scanner-motor/main-motor problems



EP cartridge problems 151

S y m p t o m 1 You see a general SCANN ER ERRO R message. Th e scanner is anoptical-grade h exago nal mirror d riven by a small , brushless dc mot or that opera tes independently of the main motor. Printing will only be enabled after the scanner hasreach ed i ts pro pe r operating sp eed . Th e scanner is eng age d at the beginning of a printing cycle. For ma ny laser printers, you will rec ogn ize the scanner m otor by a som ewh atdist inctive sound. Hewlett-Packard calls i t a variable pitch wh irring noise. Mo to rspeed is constantly monitored and controlled by main logic. If the motor fai ls to turnwhe n pow er i s app l ied , o r at any po in t during the p r in t ing p rocess , a SCA NN ER ERRO R is gen erat ed. Th er e are tw o sources of pro blem s here. Either the laser/scannerassembly has fai led, or the E C P m otor co ntrol circuit has deve lop ed a fault.

Unplug the printer, open i ts housings, and carefully inspect connectors and interconnecting wiring between the laser/scanning unit and main logic circuits. Reseatany connecto rs o r wir ing that appears to be loose . I f the p rob lem seem s in termi t ten t ,try a new cable assembly. The scanner is usually tested briefly during printer in it ialization. If you cannot hear the scanner motor, use your multimeter to measure the dcexci ta t ion vo l tage across the motor. A correct vo l tage read ing suggests a defect ivescanner motor, so replace the entire laser/scanning assembly. A low or missing excitat ion voltage indicates a defect in your main logic or driving circuitry that switchesits motor voltage on and off . Troubleshoot the excitat ion voltage and switching circu i try back in to the main log ic board . Rem em be r that you migh t have to defeat cove rinterlocks to enable the printer low-voltage power supply. If your readings are inconclusive or you can ' t t roubleshoot the main logic circuits, replace the ECP.

S y m p t o m 2 The main motor does not turn, or turns intermittently. T h emain motor is responsible for providing all of the mechanical force that drives an EPprinter. If the main motor fai ls to work, or works only in termittently, al l printer operat ion wi l l cease . Open the p r in ter and defeat the cov er in te r loc k(s ) as requ ired to

al low the p r in ter to work . U s e e x t r e m e c a u t i o n t o p r e v e n t s h o c k s o r b u r n sd u r i n g p r i n t e r o p e r a t i o n . Make su re that the motor ' s cab les are a t tached complete ly and correct ly.

I f the motor i s p roper ly connected , check motor operat ion dur ing a se l f - tes t .W hen the m otor tu rns bu t the gears and o ther mec han ics d o no t , there i s a p rob lemwith the mechan ical assemb l ies . Som eth ing is loose , jam m ed , o r dam aged . F ind andclear any jam that you migh t f ind . I f you locate a dam aged com pon en t , rep la ce thedefect ive mechan ical assembly. I f the motor i t se l f does no t tu rn , check each ou tpu tf rom the dc po w er supp ly. A lo w supp ly vo l tage can resu l t in mo tor p rob lems . I f youf ind a low or absen t ou tpu t , rep lace the dc power supp ly. You a lso migh t t roubleshoot the supply as in chapter 6 . If al l supply voltages appear correct , replace themain motor assembly ou t r igh t . Use care when reassembl ing the mechan ical components. If the trouble persists, the fault is in the ECP. Try replacing the ECP.

EP cartridge problemsTh e e lect ropho tograph ic (E P ) car t r idge p lays an impor tan t ro le in the operat ion

of your printer. An EP cartr idge contains the photosensit ive drum, primary corona,dev elop m ent roller, and toner supply. By placing these vital comp one nts in to an easilyreplaceab le assembly, i t is a simp le matter to maintain the print er— all the major componen ts that develop the image are excha nged every t ime the EP car t r idge is rep laced .




One attr ibute of the photosensit ive drum is that not al l drums have the samesensit iv ity. When a drum is manufactured, i ts sensit iv ity is tested, and tabs areplaced on the EP cartr idge. When the cartr idge is instal led, i ts tabs actuate a set ofmicroswi tches in the p r in ter. The ECP reads the se t t ings o f these microswi tches toset laser pow er. Low er-sensi t iv i ty car t r idges use h igher laser pow er, and h igher-sen

sit iv ity cartr idges use lower laser power.S y m p t o m 1 You see a NO EP CAR TRIDGE message. A n e l ec t ro p h o to g rap h ic

(E P ) car t r idge assembly uses several tabs (kn ow n as sensitivity tabs) to reg ister itspre sen ce, as we ll as inform th e printer about the rela t ive sensit iv ity leve l of thedrum. Main log ic regu la tes the ou tpu t power o f i t s wr i t ing mechan ism based onthese tab ar rangem ents ( that is , h igh-power, med ium -pow er, low-pow er, o r no-p o w er—N O C A RT R ID G E ) . S en s i t iv i ty t ab s a re u sed to ac tu a te m ic ro sw i t ch es located on a secondary PC board . The sequence o f swi tch con tacts fo rms a code thatis in terpreted by main logic.

Begin by checking the instal lat ion of your current EP cartr idge. Make sure thati t is in place and seated properly. Check to be sure that at least one tab is actuatinga sensor switch. If the re are no tabs on the E P c artr id ge, replac e i t wit h a ne w or co rrect -model EP car t r idge hav ing a t leas t one tab . Re- tes t the p r in ter. I f your NO EPCA RT RID GE erro r pers is ts , check a l l sensi t iv i ty swi tches .

Unplug the p r in ter and use your mul t imeter to measure con t inu i ty across eachsensi t iv i ty swi tch . I t migh t be necessa ry to rem ove a t leas t one w ire f rom each swi tchto p reven t fa lse con t inu i ty read ings . Actu ate each swi tch by hand and see that eac hone works p roper ly. Rep lace any micros wi tch that appears de fect ive o r in termi t ten t .Rep lace any connecto rs o r in terconn ect ing w ir ing , and re- tes t the p r in ter.

If you st i l l receive an error message, troubleshoot each switch signal in to themain log ic board . Th ere migh t be a p rob lem w i th your s ignal cond i t ion ing c i rcu its o r

main log ic comp onen ts . You migh t s imply rep lace the E CP ou tr igh t .S y m p t o m 2 You see a TONER LOW message constantly, or the error never

appears. A toner sensor i s located wi th in the EP car t r idge itself. Functionally, thesensor is l i t t le more than an antenna receiving a signal from the high-voltage ac developer bias as shown in Fig . 8-13. When toner is p lentiful , much of the electromagnetic f ield generated by the presence of high-voltage ac is b locked. As a result , thetoner sensor on ly gener ates a small vo l tage . Th is weak s ignal i s o f ten c ond i t ione d byan am plifier using som e ty pe of oper ationa l amplifier circuit that com pa res sen sedvo l tage to a p reset reference vo l tage . For the comparato r o f F ig . 8 -13 , sensed vo l tage is normally below the reference voltage, i ts output is a logic 0 . Main logic wouldinterpret th is signal as a satisfactory toner supply. As toner volume decreases, moreh igh-vo l tage energy is p icked up by the toner sensor, in tu rn develop ing a h ighervo l tage s ignal . When toner i s too low, sensed vo l tage wi l l exceed the reference, andthe comparator 's output will switch to a logic 1 . This operation is handled in mainlo g ic , an d a TO N E R L O W w arn in g is p ro d u ced .

Unfortunately, there is no good way to test the toner sensor. High voltage is verydangerous to measure d i rect ly wi thou t the appropr ia te tes t p robes , and the s ignalsp icked up a t the receiv ing w ire are too smal l to measure w i thou t a sensi t ive me ter o rosci l loscope. Beg in your check by shak ing the toner to red is t r ibu te the toner supp ly(or insert a fresh EP cartr idge). Refer to the user 's manual for your part icular printer



EP cartridge problems 153

Toner supply

Tone r blocks signalproduced by high-

voltage ac bias

Toner LOW

Toner OKac developing

voltage(high-voltage)

Alarm reference

voltage

8-13 Simplified schematic of a toner sensor.

to find the reco m m end ed p roc edu re fo r red is tr ibu t ing toner, then re- tes t the p r in ter.I f the er ro r mes sage pers is ts , t ry rep lacing the E P car t r idge.

Your next option is to repair or replace the high-voltage power supply. If youhave the p roper tes t ins t ruments to measure h igh-vo l tage safe ly, check the h igh-volta ge lev el at your de velop ing-bias conn ector. If th is volt ag e is absent or low, repairor replace the high-voltage supply. U s e e x t r e m e c a u t i o n w h e n a t t e m p t i n g ah i g h - v o l t a g e r e p a i r A l l o w p l e n t y o f t i m e f o r t h e s u p p l y t o d i s c h a r g e b e f o r e

d i s a s s e m b l i n g t h e p r i n t e r. I f the power supp ly appears defect ive , you shou ld rep lace the h igh-vo l tage supp ly ou t r igh t . If your h igh-vo l tage sys tem ch ecks correct ly,there is probably a fault in the ECP. Replace the ECP.



C H A P T E R

The electronic

control packageR eg ard less o f h o w s im p le o r h o w so p h i s t i ca t ed y o u r e l ec t ro p h o to g rap h ic p r in t e rmigh t be (F ig . 9 -1 ) , ev ery o pera t ion i s con tro l led by a se t o f e lec t ron ic c i rcu i tsthat i s commonly cal led the controller o r ECP ( e l e c t r o n i c c o n t r o l p a c k a g e ) . T h esp ec i f i c a rch i t ec tu re an d co m p o n en t s u sed in E C P s v a r i e s b e tw een p r in t e r m o dels and manufactu rers , bu t the bas ic se t o f operat ions to be perfo rmed are remarkably similar.

9-1 An Epson ActionLa ser 1500 laser printer.

154



156 The electronic control package

cation l inks. Th e com pu ter also rec eive s comm and s and status information backfrom the printer. Th ese ret urn signals are used to regu late the flow of data. Althou ghthere are many variat ions of communication l inks, data is transferred using either aserial or parallel tech niqu e. Data is sent ove r a parallel l ink as wh ole c ha rac ter s—that is , al l the binary digits (or b its) that compose a character are sent at the same

time over multip le signal wires. A serial l ink transfers data one bit at a t ime over asingle wire. Before you see the specifics of parallel or serial communication, somebackground in fo rmat ion is in o rder. Exp er ien ced t roub leshoo ters can sk ip the nex tfew sections.

Printer communicat ion backgroundA com pu ter can send three type s of information to a printer: ( 1 ) text ch aracte rs,

(2) control codes, and (3) graphics data. Keep in mind that not al l printers will accept character or graphic data, or in terpret them, in the same way. Character datarepresen ts tex t—let ters ( in any language) , numbers , punctuat ion , o r o ther tex tsymbols. Control codes are used to send commands to the printer. Control codes canset gene ral operatin g m od es such as font sty le, enhan cem ents , or p itch, but they alsocan direct immediate operations such as form feed or l ine feed. Using control codeseliminates the need to operate a control panel manually while a document prints.Other control codes provide the printer with graphic data.

ASCII exp la inedBefore any commu nicat ion can take p lace , bo th the p r in ter and comp uter must

speak the same language—when a computer sends ou t the character H , its printermust recognize that character as an H. Oth erw ise, it wil l just print unintell ig iblegarbage. Because each character and control instruction is represented by i ts ownunique numerical code, both printers and computers must use a common set ofcodes that descr ibe some min imum num ber o f codes . In the ear ly days o f comp uters ,each manufacturer had i ts own code set . You can probably imagine how difficult i twas to combine equ ipment made by d i fferen t manufactu rers . As the e lect ron ics indust ry matured and p r in ters became more commonplace , the demand fo r equ ipme nt comp at ib i li ty fo rced manufactu rers to accep t a s tandard character code se t .

Th e America n S tandard Code fo r In fo rmat ion In terchange (kno wn as AS CI I) hascome to represent a single, standard code set for computer/printer communication.The s tandard ASCII code covers le t ters (upper- and lowercase) , numbers , s imple

symbols for punctuation and math functions, and a few basic control codes. For exam ple, if you want to print an up perc ase D, your comp uter must send the number 68to the printer. Th e printer wo uld th en translate 68 into a dot pat tern that reflects theselected fon t , character p i tch , and enhance ments to fo rm the le t ter D. To print thewo rd "H ello , " a comp ute r must se nd a series of num bers: 72, 101, 108, 108, and 111.Pu re (or igin al) AS CII uses cod es 0 to 127.

Because o f the way character co des are ac tually sen t , how ever, most comp utersalso can use cod es ranging from 128 to 255, but kee p in min d that any code ov er 127is not pur e ASC II. Instead , codes from 127 to 255 are som etim es called an alternate



Printer comm unication background 157

character set. Such an alternate character set can contain single-block graphic characters , Gre ek sym bols, or o ther languag e charac ters. In some cas es, cod es 128 to 255just duplicate codes 0 to 127. If your computer sends a code from 128 to 255, youmigh t be p r in t ing characters that are d i fferen t f rom thos e on your com pute r screen .

Control codesNo t only must a com pu ter spec ify wh at to print , i t also must spec ify ho w to print .

For m feeds , fon t s ty les , and enhancem ents are som e of the con tro ls that a com putermust exercise to automate the printing process. Just imagine the confusion if youstand by your p r in ter to change these m odes manual ly whi le a docum ent w as p r in ting Unfortunately, control codes are often the cause of some incompatibil i t ies between computers and p r in ters .

When ASCII was f i rs t developed , p r in ters were ex t remely p r imi t ive by today ' sstandards. Multip le fonts and type sizes, graphics, and let ter-quali ty print had noteven been considered . Few con tro ls were needed to operate these ear ly p r in ters , soon ly those few cr i t ica l con tro ls were incorporated in to ASCII . You migh t recogn izesuch con tro ls as fo rm feed (F F ) , l ine feed (L F ) , o r carr iage re tu rn ( C R ).

With the inclusion of advanced electronic circuitry, a greater amount of in tel l igence became avai lab le in p r in ters (especia l ly EP p r in ters) . Th is in te l l igence hasmade so many current printer features possible. ASCII codes are st i l l standard, butthere s imply are no t enough unused codes to hand le the wide var ie ty o f commandsthat are needed . Manufactu rers faced the cho ice o f rep lacing ASCII (and obso le t inga long-es tab lished and g rowing custom er b a s e ) , o r develop ing a new scheme to dealwi th advan ced con tro l functions . Ul t imate ly, manufactu rers resp onde d to th is by d ev is ing a ser ies o f mul t i -code con tro l sequences . These were known as escape sequences because the AS CII code 27 (esc ap e) i s used as a p ref ix . EP p r in ters make

ex tensive use o f escape sequences .Pr in ter capab i l i t ies can vary g reat ly between models and manufactu rers . As a

result , escape codes are not standard. If computer software is not writ ten or configured properly for i ts part icular printer, control codes sent by the computer mightcause er ra t ic o r unw anted p r in ter operat ion .

Escape sequences are typ ical ly two o r th ree ASCII codes long , and each beg inswi th ASCII co de 27 . Th e escape cha racter te ll s the p r in ter to accep t subsequen tcharacte rs as part of a con trol cod e. For ex am ple , to set a laser print er to S EL F-TEST, a computer migh t have to send an ASCII code 27 , fo l lowed by an ASCII code122 (« ) . Sof tware in the p r in ter main log ic would in terp re t th is code seq uen ce andal ter the appropr ia te mo des o f operat ion accord ing ly. Mul t i -code sequenc es wi l l certa in ly become more common as p r in ters get even more soph is t ica ted .

Nu mb er sy s temsNo t on ly must a com puter and p r in ter exchan ge c odes that they unders tand , bu t

every code must be sent using a number system that is compatible with electronic(digital) circuitry. You already know the decimal (or base 10) number system. Thesymbols 0 through 9 are used in combinations that can express any quanti ty. Thesymbols themselves are i r re levan t—t en o ther symbols cou ld as eas ily have been used ,



15 8 The electronic control package

but 0 through 9 are the ones acc ept ed d ow n through the centur ies. Wh at is impo rtantis the quanti ty of characters in a number system. In decimal, one character can express 10 unique levels or magn itudes ( 0 - 9 ) . Wh en the magni tude to be express ed exceeds the capacity of a single character, the number carries over in to a higherrepresentative place, which is equal to the base of the system raised to the power of

that p lace. For ex am ple, the nu mb er 276 has a 2 in the hund reds pla ce, a 7 in the tensplace, and a 6 in the one s place. You have wo rk ed w ith th is since gramm ar school.

If elect ronic circuits could rec og nize te n different levels for a single digit, then d igital electronics would be directly compatible with the human decimal system, andASCII codes would be exchanged directly in their decimal form. However, d igital electronics can only recogn ize tw o signal levels. Th ese condit ions are O N or OF F (tru e orfa l se ) . This system is the binary (or base 2 ) num ber system . Becau se only tw o condit ions can be expressed, only two symbols are needed to represent them. The symbols0 ( O F F ) and 1 (O N ) have com e to represen t the two poss ib le cond it ions fo r a b inarydigit (commonly called a bit). AS CII cod es must be sent as sets of binary digits.

As wi th the decimal number sys tem, when a quan t i ty to be expressed exceedsthe capacity of a character (here the character is a 1) the number carries over in to ahigher place, which is equal to the base of the system (bas e 2 ) raised to the po w er ofthe place. You have probably seen binary signals expressed as 2", where n is the bitp lace posit ion. As Fig . 9-3 shows, the decimal number 20 equals the binary number10110. A lower-case u w ith an AS CII co de of 117 wo uld be exp res sed as 1110101 ina digital system. Keep in mind that seven bits can express numbers from 0 to 127.Eight b its can express numbers up to 255.

7 6 5 4 3 2 1 02 2 2 2 2 2 2 2

1 2 8 6 4 3 2 1 6 8 4 2 1

1 O 1 B B = = 0 x 1 = 0a X 2 = a1 X 4 = 4a X 8 = a1 X 1 6 = 1 6

2 8 D E C I H A L

1 X 6 4 E 6 41 X 3 2 A 3 21 X 1 6 1 60 X 8 a1 X 4 40 X 2 — 01 X 1 1

1 1 7 D E C i N A L

9-3 Converting binary numbers into decimal numbers.

Binary digitsTo ha ve any meaning at all in electron ic circuits, the re must be a clearly defined

relationship be tw ee n a binary digit and a voltag e level . Beca use a binary 1 is considered to be an ON cond it ion, i t usually indicates the pre sen ce of a voltag e. A binary 0is cons idered OFF, so i t denotes the abse nce of a voltage . In reali ty, the actual am ount



Printer commu nication background 159

of voltage that describes a 1 or 0 dep end s upon the logic family in use. Co mm on digital circuits using conventional TT L (transistor-tra nsistor lo gi c) ICs classify a logic 0 as0.0 to +0 .8 Vdc , and a logic 1 as +2.4 to + V c c ( the vo l tage powering the IC) .

Communica t ion l i nks

Informa tion, in the form of binary digits , must trave rse the ph ysical d istance b etween a computer and p r in ter by a communicat ion l ink—a wire cab le . The construction and characterist ics of th is cable will depend upon which method ofcomm unicat ion is in use . Th ere are two dom inan t m ethods o f send ing p r in ter in fo rmation: serial and parallel. A parallel l ink is easiest to unde rstand becaus e of i tsstraightforward operation. Notice that eight b its of an ASCII code are transferred simultaneously (DO through D7). Data l ines alone, however, are not enough to transfer information successfully. Both the com pu ter and printer must be syn chro nized sothat the printer will acc ept d ata whe n i t is offered , or ask the com pu ter to wait unti li t is ready. Synchronization of a parallel l ink is accomplished using several controlwires , in addit ion to data l ines. Som e co ntrol l ines signal the printer, and o thers willsignal the computer. This mutual coordination is known as handshaking.

Parallel operation is reasonably fast . The printer will accept information as fastas the computer can send i t , often operating at speeds exceeding 1 ,000 CPS (characters per second). At eight b its per character, that amounts to more than 8 ,000 bitsper se cond . Th e main disadva ntage to parallel l inks is i ts l imited cable distanc e. Withso many high-speed data signals running together in the same cable, i ts effectivelength is a few meters. Beyond that , electrical noise and losses can cause distort ionand loss of parallel data.

A serial l ink might appear simp ler beca use of the simp lified wiring requ irem ent,but i ts actual operation is somewhat more involved. Two wires are used to transfer

information. One of these wires carries data from computer to printer, and the othercarries data from printer to computer. Because data can travel in both directions,this is known as a bidirectional data l ink. Only one wir e is available to send (or rec e i v e ) , so a character must be sent one bit at a t ime. Serial data also must be synchronized between the computer and printer. To accomplish th is over a single wire,synchronization bits are added at the beginning and end of each character. An extrabit (known as a parity bit) also might be include d to al low error check ing.

Serial handshaking can be provided either through hardware or software. Software handshaking takes advantage of the bidirectional nature of serial communication by allowing the printer to transmit control codes back to the computer. Twocodes used commonly fo r so f tware handshak ing are XON and XOFF. Older ser ia lhandshaking might use the codes ETX and ACK, but EP printers rarely use that conven t ion any more .

Hardware handshaking does not support data transfer from printer to computer.Instead, a serial handshake l ine signals the computer that the printer is busy. Someinterfaces carry mo re than one handshak ing l ine. Usually, you can ex pe ct to se e a variety of handshaking schemes between printer generations, so pay part icular at tention to the wiring in your serial printer cable. In spite of their added operatingcomplexity, serial communication is extremely popular because of i ts b idirectionalnature, f lexibili ty, and i ts abil i ty to wo rk w ell ov er long distance s.



16 0 The electronic control packag e

Communica t ion s tandardsThere a re l i t e ra l ly hundreds of ways tha t you can implement a communica t ion

in te r face . You can be l iev e tha t a g rea t m any vers ions have bee n t r ied and aban doneds ince the ear ly days of commerc ia l p r in te rs . The evolu t ion of t echnology favors thebes t methods and techniques , so those tha t work wel l and grow wi th advances intechn ology can some t imes de ve lo p in to s tandards tha t o ther manufac turers ad opt inthe future. Standards are basical ly a deta i led set of rules and perfo rm an ce chara cter is t ics that c learly define the construct ion, connect ion, and operat ion of a circui tor sys tem — in th i s case , a comm unica t ion in te r face . By ado pt ing an es tab l ished s tandard , manu fac turers can be sure tha t p r in te r b rand Y wi l l opera te f ine w i th com pute rbrand X, and vice versa.

Para l le l communica t ionA paral lel communicat ion l ink requires two sets of s ignal wires as shown in Fig.

9-4. One data l ine is needed for each bi t of paral lel data (usual ly eight l ines) . Aground wire is of ten supplied as a separate return path for each data l ine. The f lowof da ta a l so mus t be coord ina ted wi th the computer. The coord ina t ion i s accomplished through a ser ies of handshaking l ines. Bo th handsh aking and data wir in g isrou ted to a female conn ec tor m oun ted on the pr in te r. This conn ec tor i s a t tached tothe com puter o ver a mu l t iconductor cab le . Th e d iagram o f F ig . 9-5 shows para l le l inte r face hardware in more de ta i l .

Inn ialize

StrobeW

Auto Feed

Errorw

Select

Acknowledgew

Busy

DATA (8 bits)

Ground

l

9-4 Data and handshaking lines of a parallel port.

W he n the com pu ter sen ds ou t a chara c te r, it p lace s a l l e igh t b i t s o f the charac te r on da ta l ine DO throu gh D7. Data f rom the com pu ter fo l lo ws th e s tan dardc onve n t i o ns e s t a b l i she d f o r T T L d e v i c e s . A l og i c 0 i s r e p r e se n t e d by a vo l t a gel e ve l be tw e e n 0 .0 a nd + 0 .8 V d c , a n d a l og i c 1 i s r e p r e se n t e d b y a vo l t a ge be tw e e n+2.6 and +4 .9 Vdc . You can eas i ly mea sure th ese vo l ta ges wi th a log ic pro be or osc i l los cop e . The t im ing d iagra m in F ig . 9 -6 provid es a go od i l lus t ra t ion of para l le lpo r t ope r a t i on .



Printer comm unication background

Connector

D 0 -D l -D 2 -D 3 -D 4 -D 5 -D 6 -D 7 -

A U TO —P O - * -

INIT —

STROBE —A C K + -

B U S Y- * -

RST 'S E L "

G R O U N D •

+ 15 VdcDatabus

8-bitlatch

Latch

Auto LFPaper outInitializeAcknowledge

Outputenable

Supportinglogic

Papersensor

Databus

MemoryRAM/ROM

Connector15 Vdcdatacontrol

ResetSelect Secondary CPU

or ASIC

9-5 Simplified diagram of a parallel comm unication circuit.

Initializesignals

S e l e c t

B u s y

A c k n o w l e d g e

D ATA ( D 0 - D 8 )

Datatransfersignals

P t r s e l ec t ed • N e w d a t a r e q u e s t

A c k n o w l e d g e

P a p e r o u t p a p e r o u t /

O l d d a t a p r o c e s s e dN e w d a t a r e q u e s t

Erro rsignals S e lec t

B u sy

P r o c e s s i n g d a t aN e w d a t a l o a d e d

N e w d a t a av a i l ab l e

P a p e r r e p l a c e d

O n - l in e o r• s e l e c t k e y

p r e s s e d

9-6 Parallel-port timing signals.



162 The electronic control packa ge

When al l data bi ts are avai lable at the interface, a Strobe Signal from the computer causes support ing logic to generate a pulse. This pulse la tches al l data bi ts intothe printer. A t this point , the com pu ter can send an othe r character, or go on to othe rwork . Af te r the pr in te r rece iv es a va l id s t robe s igna l , i ts micropro cessor c i rcu i t readsda ta on to the pr in te r da ta bus wh ere i t i s s to red in a segmen t of t empo rary me m ory

cal led the data buffer. Fo r an E P printer, the data buffer can be up to sev eralm egaby tes depe ndin g on how mu ch mem ory i s f i t t ed to the pr in ter. Dur ing the pr in ting process , charac te rs a re removed f rom the da ta buffe r, p rocessed by the microprocessor, then output to the wr i t ing m echanism .

Although the printer is accepting and storing data , i t sets i ts Busy Signal to alogic 1 level . A co m pu ter wil l not s t rob e any mor e char acters wh ile the busy l ine islogic 1. Af ter data is s tor ed su ccessful ly in i ts data buffer, th e busy signal wil l be r eleased to a log ic 0 , and it s Ac kn ow led ge S igna l wi l l s t robe br ie f ly to requ es t anoth ercharac te r. Onc e an ackn ow ledge i s gen era te d , the comp uter can then send out a ne wcharacter. This cycle is rep ea ted un ti l eve ry ch arac ter is sent successful ly.

Rea l ize tha t the t ransfe r o f da ta be tween computer and pr in te r occurs independent ly of i t s in te rpre ta t ion and pr in t ing . In prac t ice , com mu nica t ion can take p lacemuch fas te r than any pr in te r can pr in t—tha t i s why a da ta buffe r i s p rovided . Apr in te r can accept charac te rs in la rge groups , then opera te on those charac te rs unt i l the buffer is empty. I f the buffer is not large en oug h to hold al l data , the com pu terwil l s tand by unti l the buffer is empty, then send another group of characters . Thet iming of data t ransfer is control led ent irely by handshaking l ines.

Ser ia l communica t ionA serial-communicat ion l ink exhibi ts s l ight ly different character is t ics . In the se

r ial c ircui t of Fig. 9-7, the re are only two da ta-carrying l ines: Trans mit Data ( T x ) and

9-7 Simplified diagram of a serial comm unication circuit.



Printer commu nication background 163

Receive Data (Rx) . Any o ther remain ing s ignal l ines are fo r handshak ing purposes .Notice that a serial in terface is b idirectional . Data sent from the computer is receiv ed by the printer Rx l ine, wh ile data ( if an y) sent back from the printe r leav es i tsTx l ine. Because a serial in terface al lows a printer to talk back to the computer,handshaking can be accomplished by transferring control characters back and forth .The serial communication bus is usually called RS-232. All signal and data l ines arerouted to a female connector mounted to the printer. Figure 9-8 shows the pinout ofa serial communication port .

Frame ground

9-8Wiring diagram of a serialconnection.

Signal ground

Notice that the signal levels of serial data and control l ines also are differentfrom thos e of a parallel in terfac e. Serial signals are bipo lar— tha t is , one logic level isreprese n ted by a posi t ive vo l tage , and the opposing level is rep r esen ted b y a negative voltage. This kind of bipolar operation allows serial in terfaces to carry data overgreater d istances with less noise than parallel in terfaces. Although Fig. 9-7 showsbipolar ( loosely referred to as analog) signals ran ging from + 9 to - 9 V, a serial interface can use voltage leve ls from ±5 to ±1 5 V. You can meas ure the se serial levelswith an oscil loscope.

Unfortunately, b ipolar voltages are not comparable with the digital logic devicesat work in the printer, so data and handshaking signals must be translated betweenbipolar and TTL levels as required. This transit ion is accomplished by a set of dev ices known as line transceivers. For exam ple , b ipo lar b i t s rece ived f rom a computer are conver ted to TTL levels us ing a line receiver, and T T L b its must be

converted to bipolar levels using a line driver.The problem with serial data is that i t can only be transferred one bit at a t ime.

Each bit must be assembled into a complete character before i t can be stored inmemory. Just the reverse is true for control codes sent back to the printer. A character (usually eight b its long) must be disassembled and sent as a series of individual b its . This task of data manipulation is handled by a specialized IC that canperfo rm these convers ions . Th e bas ic name fo r such a dev ice i s a U A RT (un iversa lasynchronous receiver / t ransmit ter ) . Not on ly does a UART perfo rm data conversions, but i t also can attach or remove the overhead bits of a serial character. Overhea d bits include such things as the Start Bit , Stop B i t ( s ) , and Pari ty Bit .

2 ^

Rx3<—

^ 2

Rx— • 3

CTS CTS

RTS4 4 —

RTS— • 4

DSR6 *

DTR— • 6

Optionalsignals



164 The electronic control package

When a bipolar bi t is received by the printer on i ts Rx l ine, a l ine receiver t ransla tes i t in to a cor respon ding T TL leve l . Each b i t en te rs the U A R T wh ere overh eadbi t s a re s t r ipped away. Af te r an en t i re charac te r has been assembled , the microprocesso r c i rcu i t accepts tha t charac te r o n to the pr in te r da ta bus , wh ere i t is s to redin the data buffer to await fur ther processing.

Most se r ia l p r in te r in te r faces op era te in e i ther a hardw are or sof tware handshaking mode , and communica t ion can be coord ina ted wi th d i sc re te s igna l l ines(such as DTR in Fig. 9-7) or control characters sent back from the printer. A charac te r to be sen t mus t be loaded in to the UART. The microprocessor de te rmineswhich charac te r to send , then wr i tes tha t charac te r o ver the da ta bus . On ce lo aded ,the charac te r i s d i sassembled , overhead b i t s a re added , and each TTL b i t i s sen tthroug h a l ine driver IC. Th e l ine driver t ranslates each TT L bi t into a corres pon din gbipolar bi t pr ior to leaving the printer.

Isolat ing the communicat ion interfaceA communica t ion in te r face involves much more than pr in te r c i rcu i t s . The suc

cessful t ransfer of data requires pro per ope rat ion of a com pu ter and inter con nec t ingcable as well . Trouble in any one of these three areas can interrupt the f low of data .Before you disassemble your EP printer, you should isolate the problem to the printeritself. Th e quickest and mo st cer tain way to do this is to test a wor kin g printer (o nethat you kn ow is wor king w el l) on your exist ing co m pu ter using the same paral lel orser ial interface. I f a wo rking p rinter works prop erly, then you hav e ruled out the co mputer, cable, or software progra m. If a work ing prin ter also fails to op erat e , you migh thave a pro blem in you r compu ter, its softwa re con figurat ion, o r the ca ble.

On ce you have i so la ted the prob lem to you r pr in te r, run a pr in te r se l f - tes t . T hese l f- tes t wi ll t es t the pr in te r motors , mem ory, wr i t ing m echanism , pow er supply, and

most of the ECP. If a self- test pat tern looks good, you can be pret ty certain that theprinter interface circui t is defect ive. I f the self- test pat tern is faul ty, then yourprinter is suffer ing from a de fect e lse wh ere .

Troubleshooting a parallel interfaceS y m p t o m Printer does not print at all. There m ight be a PRINTER NOT

READ Y error displayed on the control panel. The printer self-test looks correct. B egin by examining your interface cable. If it is loose at either end, data and handshakingsignals might not reach the printer. If you have run another printer successfully usingyou r current cable, then the cable is almost ce rtainly good . If you are in doubt, try a newcable. If you wish to check the original cable, disconnect the cable, and use your multimeter to measure continuity across each wire. Wiggle the cable to stimulate any possibleintermit tent con nect ions. Replace any defect ive interface cable. I f you do n ot have thetools or inclination to perform detailed procedures, replace the printer 's ECP outright.

Disassemble your pr in te r and expose i t s communica t ion c i rcu i t ry. Wi th thepr in te r connec ted and running , use your log ic probe or osc i l loscope (as shown inFig. 9-9) to exa min e each handsha king l ine. Table 9-1 is a l is t ing of typical pin assignments , but refer to your user 's manual for the specif ic pin designat ions used withyour para l le l in te r face . Con nec t you r tes t ins t rument to the s igna l g rou nd (S G ) l ine ,then measure each of the handshaking signals in turn.




There are four key status signals that communicate the current operating condit ions of you r printer : (1 ) Busy, (2 ) Select , ( 3 ) Pap er Out, and (4 ) Error. Tab le 9-2shows the in teraction of these status l ines versus the printer on-l ine or off-l ine condit ion. If you find thes e four signals at their app ropr iate leve ls wh en th e printer is onl ine, the printer should be ready to accept data. If on-l ine condit ions are incorrect , a

pro blem exists in you r in terface or main-logic circuitry. Chec k the supp orting log icthat provides your handshaking signals, or replace the ASIC or microprocessor thatdirects handshaking signals. If you do not have schematics or instruments to tracethes e signals, you migh t simply replace the E CP entirely.

Table 9-2.Handshaking condit ions

in a parallel interface

Condi t ion BUSY SEL PO ERROROff-line 0 1 0 1

On-line 1 0 0 0Paper out 1 0 1 0

Place your test instrument on any of the data l ines (DO through D7) and tryprinting under computer control . If your printer is correctly on-line, charactersshould begin arriv ing in rapid succession. This transmission will appear as a pulsesignal on a logic prob e, or as a random s quare wa ve on an oscil loscop e. Ke ep in mindthat you should see som e indication of data as long as handshaking signals are set upfor on-line status.

If you do not f ind a regular f low of data, watch the Acknowledge signal. I t nor

mally rests at logic 1, but i t wil l pulse low br iefly w he ne ve r th e Busy l ine is retu rnedto logic 0 . It should ap pear as a pulse signal on a logic pro be, or as a random squarewa ve on your osci lloscope. An absen t Ackn ow ledg e s ignal suggests a p rob lem in supporting logic that handles handshaking. Check your supporting logic, or replace theASIC or microprocessor that d irects handshaking operation. If no technical data orinstruments are available, you can replace the ECP.

Check parallel data in to i ts latch shown in Fig . 9-4 . As data enters the printer,you should be able to monitor the pr ese nc e of any data bit . An active d ata line shouldappear as a pulse signal on your logic probe, or a random square wave on an oscil loscope. Repeat your check at each latch input. Refer to your printer schematic or

manufacturer 's data for the latch IC to determine input and output p ins.Monitor the latching pulse and output enable signal. These signals control datacaptur e and transfer to the data bus. A L atch pulse is ge ne rat ed w he ne ve r a Strob eis received. The pulse loads data in to the latch, but does not al low it on the output.W he n the microp roce ssor or AS IC is read y to store the ch aracter in the data buffer,an Output Enable signal p laces latched data onto the data bus. During normal operation, both control signals should appear as pulse signals on a logic probe, or as arandom square wave on your oscil loscope. If both of these signals are present, thelatch might be defective. Replace the latch and retest the printer. If the problem persists, replace the printe r data buffer m em ory ICs. If ei the r or both c ontro l signals are



Troubleshooting a serial interface 167

miss ing , there is a p rob lem in your suppor t ing log ic o r ASIC . Check yo ur suppor t inglogic or replace the ASIC. If you do not have the schematics or test instruments toperfo rm th is typ e o f t roub leshoo t ing , rep lace the E CP ou tr igh t .

Troubleshooting a serial interfaceS y m p t o m Printer does not operate at all. A PRINTER NOT R EADY error

might occur at the printer. The printer self-test looks correct. Your first step shouldbe to check the communication interface cable. If i t is loose at ei ther the printer orcomputer end, data and handshaking signals might not be able to reach the printer. Ifyou have run another printer successfully using your exist ing c om put er and cable, thenthe cable itself is almost certainly good. To verify cable wiring, try a new cable. If youwish to test the cable, use your multimeter and measure continuity across each cablewire . Be careful here — m an y serial cables f lip (or rev ers e) the Tx and Rx l ines be tw ee ncomputer and printer, so you must consider that when making measurements.

Serial com mu nication requ ires a fair ly large num ber of param eters to specify thestructure and speed of each serial character. Word length, stop b i t ( s ) , parity, baudrate, and handshaking method, are some of the more common options that can be selecte d wh en sett ing up a serial comm unication l ink. Ho we ver, each o ption must be setexactly the same wa y at both th e com pu ter and printer. If not , a printer ca nnot in terpret just wh ere data starts and ends. Th e result ing confusion w ill cause an erratic jumble of unintell ig ible print ( if i t prints at al l) . Com mu nication p aram eters are usually setby softw are, or a series of jum per s or d ip switches within the printer. Check thes e settings against those listed in your user's manual. If you have run another printer, checkits configuration and compare sett ings. If you do not have the tools or inclination toperform detailed procedures, replace the printer 's ECP outright.

13

• • • •25 14

SubminiatureD-type

connector

SubminiatureD-type

connector

9 -10 Testing a serial-port connector.




Disassemble your p r in ter and expos e i t s comm unicat ion c i rcu it ry. With the c omputer runn ing and conn ected , p lace your p r in ter on- l ine , and use your osci l loscope(as shown in Fig . 9-10) to measure the activ ity on each handshaking l ine. Handshaking l ines of primary interest to serial printers are DTR (Data Terminal Ready)and RTS (Req ues t To Se nd ) . No te that RTS is s imply ignore d in man y ser ia l in ter

faces , so concern yoursel f p r imar i ly wi th DTR at the p r in ter. DTR connects to DSR(Data Set Re ad y) at the printer. Figu re 9-11 show s a typical serial l ink. Re m em be r, ifyour printer is set up to use software handshaking, i t might be unnecessary to inspect hardware handshaking l ines.

Computer Printer

T x

Rx

DSR

3 2 Rx

Tx

DTR

T x

Rx

DSR

2 w 3Rx

Tx

DTR

T x

Rx

DSR 6 20

Rx

Tx

DTR

T x

Rx

DSR7 Ground 7

Rx

Tx

DTR

9-11 Diagram of a typical serial connec tion.

Serial signals at the in terfac e con nec tor are bipolar. If the p rinter is rea dy to r eceive data , i ts DT R l ine shou ld be a t a posi t ive vo l tage (+ 5 to +1 5 V ) . A neg at ive v o l tage (-5 to -15 V ) indicat es that the printer is not ready, so no data will be sen t. If youhave a schematic diagram for your printer, check the l ine driver circuit , and the signal from your UART (serial data functions can be in tegrated in to an ASIC). You willhave to refer to your printer schematic, or manufacturer 's data for your specific ICsto determine which IC pins serve input and output functions. If you find a faultyhandshak ing l ine , rep lace the defect ive l ine d r iver o r UART (o r ASIC) . I f the ser ia lport appears faulty, you also might replace the ECP outright

When your handshaking signal is correct (or your printer is configured for software handshak ing) , p lace your osci l loscope a t the p r in ter Rx l ine and t ry send ingdata from the computer. You should see characters begin to arrive immediately, so arandom square wa ve vary ing f rom a posi t ive to a nega t ive vo l tage shou ld appear onthe os cil loscop e. If th is signal is absent, data is not reac hing y our printer. C heck thecondit ions of any other handshaking l ines that might be incorporated in to your part icular serial in terfac e, and rete st the printer.

Use your oscil loscope to measure data in to and out of the l ine receiver IC. Youshould find bipolar data signals entering the l ine receiver, and a corresponding TTL(0 to 5 V ) output from the receiver. Refe r to your printer schem atic or m anufacturer 'sdata for your particular line IC to find specific pin functions. If data does not leave theline receiver, replace the defective IC or replace the ECP entirely. TTL data signalsalso should enter the UART. If signals enter the UART (or ASIC) and the printer doesnot function, ther e might be a fault in the U AR T (or A SI C ) or data buffer me mor y. Tr yrep lacing the UA RT (o r AS IC ) , o r rep lace the data buffer mem ory ICs. I f you do no thave sch ema tics or technical data available for the printer, or you cannot test or troubleshoot the serial communication circuit , simply replace the ECP outright.



Memory 169

If your printer is operating in a software handshaking mode, then i ts DTR signalwill usually be ignored. Control characters known as XON and XOFF are sent serial lyto the computer th rough the p r in ter TX l ine . Older p r in ters migh t use o ther codes .Und er normal operat ion , the p r in ter shou ld send an X O N ch aracter to the com pute ronce an in it ial ization has taken place. The computer can then begin sending data at

will . W he n the printer data buffer is full , i t sends an X O FF chara cter that im m edia telystops al l data transmission. An XON is sent along when the printer is ready to acceptmore data. Place your oscil loscope at the printer TX line and try sending data to theprinter. You should find an occasional random square wave representing XON andX O FF handshaking cod es as we ll as o the r data. This signal should be bipolar signal. Ifcontrol codes are missing, handshaking data is not being sent to the computer. I t ispossible that the printer never sent a preliminary XON code when i t was in it ial ized.

I f handshak ing cod es are absen t , use you r osci l loscope o r log ic p rob e to m easureTTL data en ter ing the l ine d r iver IC . Remember that the p r in ter must be t ry ing topr in t in o rder fo r any data to be sen t . You shou ld see a random T T L square wa ve represen t ing X O N and XO FF characters as we l l as o ther data . I f th is s ignal appears p resent, but there is no bipolar Tx output, replace your defective l ine driver IC. If thereare no T T L s ignals reach ing your line d r iver, your U A R T (o r A S IC ) i s p robab ly fau l ty.If you do not have the technical information or instruments to trace these signals,you migh t s imply rep lace the ECP ou tr igh t .

M e m o r yA laser o r LED pr in ter accep ts data and con tro l codes f rom i t s host computer,

p roce sses and in terp re ts that in fo rmat ion , then ope rates i t s pap er t ranspor t and image fo rmat ion m echan isms to t ranscr ibe that in fo rmat ion in to a perm anen t fo rm.So l id -sta te me m ory p lays an impor tan t ro le in th is operat ion . An E P pr in ter opera teson a f ixed set of instructions that tel ls the printer how to operate. This program res ides perm anen t ly ins ide the p r in ter, so i t is s to red in a perm anen t m em ory dev ice(a ROM—read-on ly m em or y) . Oth er data , such as fon t s ty les and enhancem ents ,a lso can be s to red in perm anen t mem ory, o r loade d in to tempo rary m em ory RAM—

r an d o m -access m em o ry ) a s n eed ed .

Most in fo rmat ion c hanges con stan t ly as the p r in ter work s . Characters , g raph ics ,and con tro l codes received f rom the computer are on ly s to red un t i l they areproce ssed . M ain log ic a lso requ ires locat ions to s to re c on tro l panel var iab les and resu l ts f rom calcu la t ions . Temporary memory dev ices are used to ho ld rap id ly chang

ing in fo rmat ion dur ing p r in ter operat ion .

Permanen t memoryAs the nam e suggests , in fo rmat ion in perm anen t m em ory is re ta ined a t a ll times ,

even whi le power i s removed f rom i t s c i rcu i t . You migh t hear permanen t memory refer red to as nonvolatile o r read-only mem ory. Th er e are th re e bas ic c lasses o f nonvo la t i le m em ory that you shou ld be familiar wi th : R OM ( r e a d -o n l y m e m o r y ) , PROM(p ro g ram m ab le r ead -o n ly m em o ry ) , an d EPROM(erasab le p rogr am m able read-on lymemory) . Your p r in ter migh t use any o f these c lasses , a l though the s impler PROMdevices are encoun tered most f requen t ly.




A R OM is the o ldes t and most s t ra igh t fo rward c lass o f perm anen t mem ory. I t s information is specified by the purchaser, but the actual IC must be fabricated alreadyp ro g ram m ed b y th e IC m an u fac tu re r. R O Ms a re ru g g ed d ev ices . B ecau se th e i rp ro gra m is ac tual ly a phy sical par t o f the d ev ice , i t can wi th s tan d a lo t o f e lec t r ica l and physical abuse , yet s t i l l main ta in i t s con ten ts . However, once a ROM is

p ro g ram m ed , i t s co n ten t s can n ev er b e a l t e red . I f a p ro g ram ch an g e i s n eed ed , anen t i r e ly n ew d ev ice m u s t b e m an u fac tu red w i th an y d es i r ed ch an g es , t h en ins ta l led in the c i rcu i t . ROMs are o f ten used in fon t car t r idges that are manufactu red in h igh vo lumes.

T h e P R O M can b e p ro g ra m m ed b y a p r in t e r m an u fac tu re r i n s t ead o f r e ly in go n a R O M m a n u f a c t u r e r t o s u p p ly t h e p r o g r a m m e d d e v i c e s . A P R O M c a n b e p r og r a m m e d ( o r burned) o n ce , b u t i t can n ev er b e a l t e red . F ac to ry - f r e sh P R O M Sare buil t as a matrix of fusible l inks. An intact l ink produces a binary 0 , and abur ned l ink p rodu ces a b inary 1 . On e link is avai lab le fo r eac h b i t in the d ev ice . Asp ec ia l p i ec e o f eq u ip m en t ca l l ed a P R O M P ro g ram m er i s f ed th e d es i r ed d a ta fo reach P R O M ad d ress . I t t h en s t ep s th ro u g h each P R O M ad d ress an d b u rn s o u t an yl in k s w h ere a b in a ry 1 is d es i r ed . Wh e n th e P R O M i s fu lly p ro g ra m m ed , i t co n ta in sth e d es i r ed d a ta o r p ro g ram . O n e o r m o re P R O Ms a re u sed to h o ld th e p r in t e r i nt e rn a l p ro g ram .

A n E P R O M can b e e rased an d r ep ro g ram m ed m an y t im es . B in a ry in fo rm at io n i ss to red as e lec t r ica l charges p laced across MOS (m e ta l -o x id e sem ico n d u c to r ) t r ansistors. One transistor is provided for each bit in the device. An absence of charge isa binary 0 , and the presence of charge is a b inary 1 . Programming is very similar tothat o f a PR O M . An E PR O M prog ram me r is loaded wi th the des i red in fo rmat ion fo reach locat ion . I t then s teps th rou gh ea ch locat ion and locks charges in to the a ppropr ia te b i t locat ions . To erase an EP RO M , you must rem ov e charge s f rom ev ery b i t locat ion . That i s accompl ished by exposing the memory dev ice ( the d ie i t se l f ) to asource o f shor t -w aveleng th u l t rav io le t l igh t fo r a p res cr ibed per iod o f t ime. L igh t isin t roduce d th rough the t ransparen t quar tz win do w on top o f the IC package. You cante l l an EP RO M b y the c lear quar tz win do w in the IC .

Tempora ry memoryDigi ta l in fo rmat ion con ta ined in tem pora ry m em ory can be a l tered o r up dated

frequently, but i t wil l only be retained as long as power is applied to the device. Ifpow er fa il s , a ll m em ory con ten ts w i l l be los t . Th is k ind o f m em ory dev ice i s refe r redto as volatile, read-write, o r RAM ( r an d o m -access m em o ry. T h e t e rm random access m eans the dev ice can be accessed fo r read ing o r wri t ing opera t ions as ne ed ed .The two basic types o f RAM that you shou ld know are static and dynamic.

A s tat ic RAM (S R A M ) u ses co n v e n t io n a l l o g ic f li p - flo p s ( ca l l ed cells) to s to rein fo rm at ion . O ne cel l i s p r ov ide d fo r each b i t . A read / wri te co n tro l line is ad de d tose l ec t b e t w ee n a r ead o r w r i t e o p e ra t io n . D u r in g a w r i t e o p e ra t io n , an y d a ta b i tsex is t ing on the data bus are loaded in to the cel l s a t the address speci f ied on thead d ress b u s . I f a r ead o p era t io n i s se l ec t ed , d a t a co n ta in ed a t t h e se l ec t ed address i s made avai lab le to the data bus . Once data i s loaded in to a s ta t ic RAM, i twi l l remain un t i l i t i s changed , o r un t i l power i s removed . SRAMs are used heav i lyin p r in ters .



Control panel 171

DRAM ( d y n am ic R A M ) d ev ice s u se sm a l l MO S ce ll s t o s to re d a ta in th e fo rmo f e lec t r i ca l ch a rg es . A l th o u g h read in g an d w r i t in g o p era t io n s r em ain v i r tu a l lyid en t i ca l t o th o se o f S R A Ms, D R A Ms m u s t b e r e f r e sh ed ev e ry f ew m i l l i seco n d s , o rthei r data wi l l be los t . Refr esh i s p r ov ide d by a com bina t ion o f ex ter na l c i rc u i t ryan d c i r cu i t s w i th in th e D R A M ch ip itself. A l th o u g h th e n e ed fo r r e f r e sh in c reases

th e co m p lex i ty o f a m em o ry c i r cu i t , MO S t ech n o lo g y o ffe r s v e r y lo w p o w erco n su m p t io n an d a l a rg e am o u n t o f s to rag e sp ace a s co m p a red to m o s t S R A Md ev ices .

Troubleshoot ing memoryMemory is usually one of the most rel iable sections of an ECP, but when a fai lure

does occur, the resu lts can manifes t th em selves as los t characters , occas ional operating hang-ups for no app aren t reaso n, or up front in it ial ization failure. T he difficultyin test ing m em ory is that i t is v irtually imp ossible to tel l for su re just wh at location

or b i t the p rob lem is coming f rom. To tes t a RAM dev ice p roper ly, a known pat ternof data would have to be wri t ten to each locat ion , then read back and compared towhat was writ ten. If there is a match, that location is assumed to be good. If there isno match , that locat ion (and the en t i re IC) i s defect ive . Most p r in ters perfo rm am em ory check on init ial ization. Unfortu nately, the re is no way of per form ing thissort of test in the prin ter during norm al op erat ion.

ROM dev ices are a lmost as d i ff icu l t to tes t . Each locat ion would have to beread and a checksum value would have to be calcu la ted based on the con ten ts o feach R O M ad d ress . T h e ca l cu la t ed ch eck su m i s co m p ared ag a in s t a ch eck su mvalue on the ROM. I f bo th checksum values match , the ROM is assumed to be val id .I f t h e re i s n o m a tch , t h e R O M i s d e fec t iv e . R O Ms a re t e s t ed w i th R A M w h en th eprinter is in i t ial ized.

As a result of thes e test ing difficult ies, the re are no specific tes t pr oce du res form em ory c i rcu i ts in par t icu lar. Th is book suggests the rep lacem ent o f mem ory dev ices on a symptomat ic bas is on ly—when o ther symptoms po in t to the poss ib i l i ty o fa m e m o r y f a ilu re . A l th o u g h m o s t m em o ry p ro b lem s w i ll b e d e t ec t e d an d r ep o r t eddur ing the p r in ter se l f - tes t p rocess , memory can fa i l spon taneously dur ing p r in teroperat ion jus t l ike any o ther ac t ive com pon en t . Troub leshoo t ing p roc edu res in o thersect ions o f th is book suggest where and when to suspect memory p rob lems. When am em ory p rob lem is susp ected , mem ory d ev ices can be rep laced sys temat ical ly, bu ti t also is acceptable to replace the ECP entirely.

Control panelThe p r in ter con tro l panel i s the user in ter face between you and the p r in ter.

F i rs t , i t a l lows you to operate cer ta in imm ediate funct ions such as fo rm feed , pap ertray select , reset , or on/off l ine. Certain key combinations let you alter options andrunning modes (your user 's manual will specify the exact key strokes and their eff e c t ) . Indicators and LCD-alphanumeric displays also are included to display variouspr in ter s ta tus cond i t ions o r er ro r codes . F igure 9 -12 shows a s imple b lock d iagramfor an EP printer control panel.



174 The electronic control packag e

Microprocessor operat ionsI f your EP p r in ter cou ld be compared to a symphony o rchest ra , the main micro

proces sor wou ld be the conducto r. Techn ical ly speak ing , a microprocessor is a programmable log ic dev ice that can perfo rm mathemat ical and log ical man ipu la t ion o fdata , then p ro du ce de s i red ou tpu t s ignals . Al l microp roces sors a re gu ided b y a f ixedseries of instructions (called a program) tha t is s to red in the p r in ter perma nen tm e m o r y ( R O M ) .

Al thou gh the micropro cessor s found in your p r in ter are o f ten less com plex thanthose found in ma ny com put ers , you can exp ect to f ind many o f the sam e s ignals asshown in Fig . 9-14. Notice that a set of related signal wires (known as a bus) is ofte nrepr esen ted as a s ing le wide l ine . Th is p ract ice s impl if ies d iagrams and schem at ics .Arrows are used to depict the possible f low of data. You will f ind three major busses:(1 ) the Addre ss bus , (2 ) the Data bus , and (3 ) the Contr o l bus .

Address lines speci fy the p recise locat ion o f ins t ruct ions o r data any wh ere inthe p r in ter. Ad dre ss locat ions can refer to mem ory locat ions , operat ing addre sses o f

s lave microprocessors , ASICs, o r o ther c i rcu i t -speci f ic p laces . Because a microprocessor on ly generates address s ignals , i t i s a lways the con tro l l ing e lement in amain- log ic c i rcu i t. Th e exa mp le microp roces sor show n in F ig . 9-14 p rov ide s 16 address l ines (AO th rough A15) that can speci fy 2 1 6 or 65536 unique locations. Yourpar t icu lar microprocessor p robab ly o ffers more .

+V d c

Address bus

(AO to A1 5)

Stabilizationcapacitors

jH r— ^ = c XTAL

Piezoelectriccrystal

Microprocessor(ixP or CP U)

Data bus(DO to D 7)

— Interrupts> ReadAVrite

— Reset

9 -14 Major microprocessor signals.

A microprocessor can read data f rom or wri te data to any locat ion speci f ied bythe address l ine. Data is sent over the data bus. For example , when the p r in ter i sf irs t tu rned on , i ts microp rocess or wi l l au tomat ical ly ge ner ate an address (usual ly0000 hex idecimal) and a t tempts to read whatever character i s avai lab le f rom that



Main logic 175

address. The microprocessor automatically assumes this to be i ts f irst instruction. Ifmemory is defect ive , o r the character a t that address i s somehow incorrect , the microproc essor wi l l be com e hopeless ly confused . Th is confusion leads to er ra t ic o r unpred ic tab le operat ion .

During a wri te op erat ion , the micr oproc essor w i l l gen erate an address , thenplace valid data to that address. Any device that is active at that address (such as aRAM IC) wi l l accep t th is data . Al though a microprocessor can wri te to any address ,no t a ll addresses can accep t data (suc h as RO M loc at ion s) .

F inal ly, a microprocessor i s equ ipped wi th a number o f control lines, Aread /wri te (R/W) s ignal in fo rms the sys tem whether the microprocessor i s perfo rming a read o r a wri te operat ion a t it s se lecte d add ress . An in ter rup t reque st ( I R Q ) issen t to the micropro cessor w hen eve r the p r in ter must deal wi th an imm ediate p roblem or cond i t ion . There migh t be several levels o r in ter rup ts depend ing on the part icular IC in use. Oth er contr ol l ines that you m ight enc ou nte r are the halt (H L T ) andreset ( R S T ) s ignals . Con tro l s ignals are g rou ped toge ther in to the con tro l bus .

The system clockA microprocessor i s a sequential d ev ice—in s t ru c t io n s a re ex ec u ted o n e s t ep a t

a t ime. As a resu l t, t iming beco m es a very cr i t ica l aspect o f the micro proce ssor operation. Timing signals are provided by a precision oscil lator buil t in to the microp ro cesso r itself. This oscil lator is known as the master clock or system clock. T oach ieve a pre cise and stable clock, a p ie zoe lect ric crystal is ad de d extern ally, and th eoscil lator will run at the resonant frequency of the crystal . For example, if an 8 MHzcrysta l were used , the c lock would run a t 8 MHz. Crysta l f requencies are markedright on the part . You can read these clock pulses with your oscil loscope.

ASIC opera t ionsA sing le microprocessor canno t poss ib ly hand le the wide var ie ty o f operat ions

required of a typical printer. Older printer designs use a secondary (or slave) m icroprocessor to perfo rm such tasks . The secondary microprocessor reduces p rocess ing t ime (o r overhead) f rom the main micropro cessor. S lave microp rocessor shave largely been rep laced by Appl icat ion Speci f ic In tegrated Circu i ts (ASICs) .W he re a s lave microp rocess or i s som ewh at gen er ic in i ts s t ructu re and operat ion , anASIC is a semi-custom or fu l ly custom IC designed especia l ly to serve specia l izedfunctions in a part icular printer. An ASIC can generate al l motor and laser/scanningunit control signals, operate the control panel, handle the printer serial and parallelin ter face , in terp re t sensor s ignals , and mo re . The re m igh t be one o r m ore AS ICs d epending on the features and sophist ication of your part icular printer.

A key advantage to ASICs is that they eliminate a large portion of glue logic thatresults in smaller, simpler, and less power-demanding circuits. ASICs also work withrninimum control from the main microprocessor, and are optimized for their use in aprinter. ASICs can vary greatly in complexity depending on the in tended capabil i t iesof your printer, but most require large pin counts to provide enough signal l ines. Thedisadvantage of ASICs is their potential d ifficulty to acquire as replacement parts.Manufacturers t ightly control the prod uction an d distr ibution of thes e proprieta ry ICs.




Troubleshooting main logicMost p r in ter t roub les do no t invo lve a ca tas t roph ic fa i lu re o f the main micro

processor o r ASIC. Ins tead , typ ical ECP prob lems invo lve o ther componen ts that d irectly in terface with other parts of the printer such as the main motor, the laser/

scann ing assembly, the paper p ickup and reg is t ra t ion c lu tches , the con tro l panel ,and so on . However, there are some occasions when the microprocessor, sys temclock , o r o ther h igh ly in tegrated log ic componen t wi l l fa i l and cause ser ious p roblems for your printer.

When you first turn an EP printer on, main logic performs an init ial ization procedure that checks and c lears a l l RAM locat ions , es tab l ishes a communicat ion l inkwith the host computer, and brings the high-voltage and fusing unit up to workinglevels . Th is p rocedure typ ical ly requ ires less than two minu tes . I f the p r in ter doesnot in it ial ize, or must be reinit ial ized during normal operation, the problem is almostcer ta in ly in the micropr ocess or o r o ther main log ic .

S y m p t o m 1 The printer does not initialize from a cold start. N o v is ib le act iv i ty shou ld take p lace in the p r in ter af ter p ow er i s tu rned on , bu t po we r ind icato rsare l i t . Self-test does not work. If a printer encounters an error condit ion during i tsin it ialization, the re will usually be s om e visible or audible ind ication of a fault co ndit ion . Aud ib le tones , flash ing l igh t sequen ces , o r an a lphanum eric er r o r cod es are jus tsome typical fai lure indicators. The user 's manual for your part icular printer will l istany er ro r codes and thei r mean ings .

I f the er ro r i s ex pe cte d— th at i s , an er ro r that is che cke d and hand led by thepr in ter so f tw are — the p r in ter wi ll s imply wai t un t il the er ro r is co rre cted . Pape r ou te r ro rs a re a co m m o n ly ex p ec te d e r ro r. H o w ev e r, u n ex p ec ted e r ro r s can cau se th epr in ter to f reeze o r behave er ra t ica l ly fo r no apparen t reason . ROM or microproces

sor defects are considered unexpected fa i lu res—main log ic has no way o f deal ingwith such p rob lems—so there i s no way o f knowing jus t how your p r in ter wi l l respond (if i t works at al l) .

Use your mul t imeter to measure the log ic supp ly vo l tage levels in your p r in ter(usual ly +5 V d c ) . If th is vo l tag e i s low o r absen t , log ic dev ices wi l l no t funct ion p ro per ly ( i f a t a l l ) . A lo w or miss ing vo l tage ou tpu t sugg ests a defe ct ive po we r supp ly. Youcan t roub lesh oo t the dc sup p ly us ing the p roc edu res d iscussed in chap ter 6 , o r s imply replace the dc supply outright. If you do not have the tools or inclination to perform detailed procedures, replace the printer 's ECP outright.

The nex t area to check is your sys tem clock . I f you have schemat ics o r tes t in

s t ruments avai lab le , use your log ic p robe o r osci l loscope to measure the osci l la to rsignals on both sides of the piezoelectric crystal (make sure that your logic probecan measure frequencies as high as the rat ing of the crystal) . If you are using an osci l loscope, there shou ld be a rough ly square wa ve a t the f requ ency ma rked on th ecrystal . If the clock signal is low or missing, replace the crystal and stabil ization capaci to rs . I f th is does no t res to r e your c lock source , rep lace the main micr oproc essorand retest the printer. If you do replace the microprocessor, be sure to instal l an approp r ia te IC socket fi rs t ( i f po ss ib le) .

An init ial ization process can stal l i f the microprocessor detects a faulty RAM orRO M locat ion . Unfor tunate ly, there is no wa y to check a me m ory IC com plete ly w i th -



Troubleshooting main logic 177

out using specialized test equipment. Some printers will d isplay an error code indicat ing a memory er ro r. I f the er ro r suggests a RAM fau l t , remove any op t ion RAMboards o r cards and re tes t the p r in ter. I f the p rob lem d isappears , that op t ion mo duleis defec t ive . I f the p rob lem pers is ts , rep lace the R A M c h ip (s) and re tes t the p r in ter.I f normal operat ion re tu rns , you have iso la ted the defe ct ive co m po ne n t (s) . Be su re

to ins tal l approp r ia te IC ho lders ( i f poss ib le) wh en rep lacing R AM ch ips . I f a RO M erro r is ind icated , rep lace the R OM IC (s ) and re tes t th e p r in ter. If you do no t have theschemat ics o r tes t equ ipm ent fo r such deta i led wo rk , t ry rep lacing the EC P en t i re ly.

I f symptoms con t inue, rep lace the s lave microprocessor o r ASIC. A fa i lu re herecan p reven t m otor op erat ion , d isab le sensor s ignals , and cu t o ff com mu nicat ion . Th emain microproc essor m ust in teract c losely wi th these com pon en ts , so any fau l t herecan hang up main micro proces sor operat ion . As befo re , you migh t ch oose to s implyreplace the ECP outright if you do not have the technical data or test instruments toperfo rm deta i led checks .

S y m p t o m 2 Printer operation freezes or becomes highly erratic du ring operation. You mu st activat e the printe r from a cold start to res tore ope ration . Self-

tes t migh t work un t i l the p r in ter f reezes . Check the log ic supp ly vo l tage wi th yourmultimeter. You should find about +5 Vdc. If th is voltage is absent, low or in termitten t ly low, log ic dev ices wi l l beh ave er ra tica lly. Troub leshoo t the dc po we r supp ly using the p rocedu res o f chap ter 6 , o r rep lace the dc sup p ly ou t r igh t .

A microprocessor requ ires constan t access to i t s p rogram ROM to operate p roperly. Each instruction and data location must be correct , or the main microprocessorwill become hopelessly misdirected. If you find that the printer only operates to someconsistent point where i t freezes or acts strangely, the ROM might be defective. Rep lace the p rogram ROM and re tes t the p r in ter thorough ly. I f normal operat ion returns, you have probably isolated the problem. Instal l an appropriate IC socket ( if

possible) before insert ing a new ROM IC. If you do not have the schematics or testequ ipment t o perfo rm th is typ e o f p rocedur e , try rep lacing the EC P ou tr igh t.

Inspect al l of your main-logic ICs for any devices that might appear excessivelyho t— espe cia l ly i f the p r in ter has on ly bee n on fo r a shor t t ime. NEV ER touch po tentially hot com pon ent s in a l ive circuit . Instead, sm ell around the circuit for unusuallyheated air, or hold the palm of your hand over the circuit . If a part seems unusuallyhot, spray i t with l iquid refrigerant. If normal operation returns temporari ly (youmight have to reactivate the printer or press a reset or on-l ine button), replace thethermally in termittent component. You also might replace the ECP entirely.



AA P P E N D I X

Typical cleaning and

maintenanceW heth er your p r in ter nee ds to be repai red o r no t , check and c lean it per iod ical ly toensure that no unexpected p rob lems develop . Most laser p r in ters have two rep laceab le componen ts : the toner car t r idge, and the development "eng ine" con ta in ing theEP drum and p r imary corona. These e lements must be located and removed befo reclean ing can take p lace . On c lose inspect ion , you can p robab ly f ind where thesecomponen ts are la tched in to a swing ing deck . Release the la tches and swing thetoner car t r idge and EP eng ine up and away f rom you .

Ordinary maintenancePr op er c lean ing can be don e in und er 15 minu tes , bu t you wi l l ne ed a supp ly o f

clean, l in t-free cloths (pr efe rab ly a ntis tat ic) , a supp ly of clean, l in t-free sw abs, afew ounces o f f resh denatu red a lcoho l , and a supp ly o f f resh , demineral ized water.Avoid us ing o rd inary tap water—minerals and con taminan ts can leave res idue andcause corros ion .

Transfer corona

Use a clean, l in t-free swab dipped l ightly in fresh denatured alcohol to clean inand around the t ransfer co rona ra i l. High-vo l tag e po ten t ia ls de velo pe d a long thewir e at tracts dust and debris in the air nearby. Fore ign m atter c ollects on the coro nawire and can eventually affect the image quali ty that is transferred to paper. Cleanthe length of wire gently but f irmly. Be extremely careful no t to b reak the coronaw i r e . If i t breaks, you must replace i t . Also clean inside of the metal track surrounding the corona and be sure that al l residue is removed.

C a u t i o n : N ot ice th e th in monof i lament l ine that i s wr app ed around the t ransfercorona case . Th is l ine p reve n ts the a t t rac ted p aper f rom b eing d raw n in to the corona

178



Typical cleaning and maintenan ce 179

case and jamming . You must be extremely careful to avoid brea king the l ine. If i tb reaks , the t ransfer co rona wi l l have to be re-wr app ed o r rep la ced .

Transfer guideN e x t , u se a c l ean , l i n t -f r ee w ip e d am p en ed l ig h t ly w i th co ld , d em in era l i zed

water to c lean the t ransfer gu ide area that i s located jus t befo re the t ransfercorona assem bly. Th e pap er passes th rough th is gu ide to be ch arg ed and get i t simage f rom the EP d rum. Be su re to c lean up any paper dust , debr is , o r res idualtoner that migh t be in the area . Do no t use ho t water to c lean because ho t watermigh t mel t the res idual tone r par t ic les and cause a pe rm an en t me ss . If i t i s poss ib le to open the t ransfer gu ide lock t ray, do so and c lean ins ide the lock t ray area .A l so w ip e th e ad jo in in g t r an s fe r g u id e ro l l e r. U se cau t io n w h en c l ean in g —rem em be r that the assem bl ies ins ide a laser p r in ter are del ica te and unforg iv ing .Excess ive fo rce and care lessness can eas i ly resu l t in co l la tera l damage to thepr in ter.

Static eliminator teethI f y o u lo o k b e t w e en th e t r an s fe r co ro n a an d th e p ap er- f eed g u id e , y o u w i l l s ee

a ro w o f m e ta l l i c t ee th , w h ich a re th e an t is t a t ic t ee th . O n ce th e p ap er ( ch a rg e db y th e t r an s fe r co ro n a) r ece iv es i t s l a t en t t o n e r im ag e f ro m th e E P d ru m , th e p aper must be d ischarged . I f d ischarge does no t occur, the paper wi l l re ta in a s ta t iccharge, and each sheet wi l l c l ing to one ano ther. Worse , the charged sheet migh tjam in the p r in ter paper path . Use a f ine b rush (such as a so f t too thbrush o r thebrush end o f the corona c lean ing b rush) to sweep away any accumulat ions o f paper dust o r debr is .

Paper-feed guideYou shou ld c lean the paper feed gu ide nex t . Use a c lean , l in t - f ree wipe damp

ened l igh t ly wi th co ld , demineral ized w ater to wip e away any dust o r res idual tonerin the fee d gu ide area. This is just straigh tforwa rd cleaning, but be careful not towipe debris in to more cri t ical areas of the printer.

Primary coronaLoc ate the p r imary corona in tegrated in to the EP assem bly that has bee n sw ung

up and away from the rest of the printer. Like the transfer corona, the primary

corona high-voltage operation attracts dust and debris from the air. As debris coatsthe wire , the corona su rface charge becomes uneven and can affect the image quali ty on the EP drum. If you have not already found the primary corona cleaning brushthat is usually kept near the paper-feed guide, look for i t now. Gently ease the fel t-t ipped brush over the primary corona wire and sl ide the brush back and forth a fewtimes to clean away any residue that might have accumulated. If you cannot f ind abrush, use a clean, l in t-free swab dipped l ightly in fresh denatured alcohol. Use caution when cleaning the primary corona. If the wire breaks, the entire EP cartr idgewill have to be replaced.



180 Appendix A

Separation pawls and cleaning padClean the fuser separation pawls next. To find the pawls, open the printer fuser

area located in the paper-e ject ion area (usual ly near the rear beh ind the E P d r um ).You will see several large plast ic pawls (claws) leading to the fusing roller assembly.Use a c lean , l in t - f ree wipe dampened l igh t ly wi th c lean water to wipe the lead ingedge of each pawl. Be careful not to touch the fusing roller assembly.

Although the fusing rollers should not retain any toner, long-term use can wearthe roller lubricant and allow spots of tone r to rem ain on the roller. This residual ton ercan then appear on s ubsequ ent sh eets of pap er as spots or stains. A cleaning pa d instal led against the heated fusing roller wipes away any residual toner that might adhere during toner fusing, and helps to keep the fusing roller lubricated so toner willnot st ick . New cleaning pads are often included with new toner cartr idges, so younee d not usually replac e the cleaning p ad unless it is t ime to repla ce a toner cartr idge .

Ozone filterDuring normal operation, the coronas in your printer generate ozone from the

high-energy ionization in surrounding air. Because ozone can be an irritant if there isenough exposure t ime, an ozone fi l ter is often added to laser printers to reduce theamou nt of ozone gas release d into the air. Typica l ozon e filters sh ould be rep lace d afterabout 40,000 to 50,000 pages. Consider whe ther a new ozon e fi lter is warran ted. M ostozone filters are readily accessible while the printer is open for standard cleaning.

Reass em ble the prin terSwing down the frame with the toner cartr idge and EP assembly and relatch i t

in to place. Unlatch and remove the toner cartr idge i tself and rock i t back and forth

along its long axis. Althou gh m any ton er cartr idg e designs n ow allow for an agitator tokee p toner even ly distr ibuted, heavy or irregular use might al low the toner to b ec om ethin in one or mo re areas. Agitat ing the toner c artr idge ens ures that remaining ton erwill be eve nly d istr ibuted and helps m axim ize tone r cartr idge l ife. Re-install the to nercartr idge and secure al l outer door s or panels. Cleaning should now be com plet e.

Restart the printer and init iate one or more self-tests. Compare the new and oldself-tests. Under routine condit ions, you should see no substantial d ifference between the tests. If the original self-test suggested maintenance neglect , you shouldsee a no t iceab le imp rovem ent in the new er se l f- tes t .

Spott ing maintenance-related problemsInciden ta l o r in f requen t users migh t over look o r neg lect rou t ine main tenance.

When main tenance i s neg lected fo r too long , p rob lems can occur in the f inalp r in ted p roduct . Th is par t o f the append ix shows you s ix typ ical main tenance-rela ted p rob lems.

Vertical fadeVert ica l - fade p rob lems are character ized by one o r more faded s t reaks in the

ver t ica l d i rect ion . Under m ost c i rcumstances , the toner level i s get t ing low. R em ove



Typical cleaning and maintenan ce 181

the toner cartr idge and rock i t back and forth to redistr ibute the toner evenly in i tscontainer. Replace the toner cartr idge and adjust the print density.

Another cause of vert ical fade is a dust- or debris-fouled transfer corona. Uneven charge distr ibution will al low light streaks to form. Check the transfer coronaand clean i t with a l in t-free swab dipped l ightly in fresh isopropyl alcohol. After

c lean ing the wire , swab ou t any res idue wi th in the corona case . Be ex t remely careful not to break the monofilament l ine wrapped around the corona. If the corona orwrap is b roken , the t ransfer co rona assem bly wi l l have to be rep lac ed .

Dro p o u t sDrop outs genera l ly appear as faded areas that are typ ical ly round in shape. T he

most common source o f d ropou ts i s due to p rob lems wi th the paper itself. P ap erwi th uneve n mois tu re con te n t o r mois t spo ts can resu lt in d ropou ts . Even manufactu r ing defects in the paper can resu l t in paper that p roduces d ropou t spo ts . Tryfresh, dry paper from a different sour ce. It also is rem ote ly pos sible that the transfer

corona is sl ightly dirty. If new paper fai ls to correct the problem, try cleaning thetransfer corona as described earl ier in th is appendix.

Vertical linesVertical l ines appear as one or more black streaks or smears directly from top to

bottom of the page. When vert ical l ines appear, i t usually indicates that one or moreareas of the primary corona wire are fouled with accumulations of dust or debris.Clean the primary corona as described earl ier in th is appendix.

If the dark vert ical marks appear smeared, i t is possible that the fuser rollerc lean ing pad migh t be d i r ty and nee d rep la cem ent . I f you d o no t have ano ther c lean

ing pad on hand , you can gen t ly c lean the pad by re mo ving i t f rom the fuser assembly and gen t ly b rush ing away any obv ious accumulat ions o f toner. A new clean ingpad shou ld be instal led as soon as poss ible. If a new cleaning p ad is available, re plac eit according to the instructions for the part icular printer.

Finally, i t is possible that the EP drum has been scratched—this problem is rare,but can occu r in printe rs serving a hea vy wo rk load. Unfortu nately, th e EP drumwou ld have to be rep laced , bu t the d rum is typ ical ly par t o f the imag e fo rmat ion eng ine , which can eas i ly be rep lace d as an en t i re modu le . Rep lace the EP assembly andtry the printer again .

StainingA stain is typically a dark horizontal patch that reoccurs to lesser degrees down

the page. Usually, the transport rollers that handle the paper are dirty. Clean thetransport rollers and transport feed guide as described earl ier in th is appendix.

I t a lso i s poss ib le that the fuser ro l ler c lean ing pad m igh t be d i r ty and ne edrep lacem en t . I f y o u d o n o t h av e an o th e r c l ean in g p ad o n h an d , y o u can g en t lyclean the pad by removing i t f rom the fuser assembly and gen t ly b rush ing awayany obv ious accumulat ions o f toner. Ins ta l l a new clean ing pad as soon as poss ib l e . I f a ne w clean ing pad is avai lab le , rep lace i t acc ord ing to the ins t ru ct ions fo rthe par t icu lar p r in ter.



182 Appendix A

Repetitive defectsRepeti t ive defects occur at regular in tervals along the page. I t is the spacing of

the defects along the page that can really cue you to the actual problem. When thedefects are spaced c losely together (under 2 inches) , there i s poss ib ly a p rob lemwith your paper t ranspor t ro l le r (s ) . Check and c lean each ro l ler to rem ove a ll debr is .

If the defects occur at in tervals of 2 inches or greater, than there is a defect onei ther the development ro l ler o r the EP d rum. Because bo th e lements are incorporated in to the image -fo rmat ion en g ine ( th e EP car t r id ge) , the en t i re assembly wi l lhave to be rep laced in e i ther case . Try rep lacing the EP car t r idge.

Badly formed charactersCharacters that are bad ly fo rme d ge neral ly appea r wavy o r unsteady. Th is p r ob

lem is almost always caused by paper stock that is too sl ick or sl ippery—the rollershave a difficult t ime handling the paper. Try the printer again with standard 20-pound xerography-grade paper. I f the p rob lem s t i l l occurs on s tandard paper, thepr in ter scanner assembly is defec t ive and the p r in ter wi l l requ ire mu ch mor e e x tensive bench repair.

Spotting user-related problemsA number of laser printer problems are caused by oversights or errors in judg

men t . Th is sect ion o f the appen d ix show s you some of the typ ical user- re la ted p roblems found with laser printers.

1. You are troubled with frequent jams. A laser prin ter is only as go od as themedia that is used with i t . Paper, envelopes, and labels must be chosencarefu l ly to ensure p roper p r in ter operat ion . Remember that the p r in termechan ics are des igned to work w i th media o f cer ta in th icknesses , tex tu re s ,finishes, and we igh ts. Me dia that is to o hea vy or flimsy o r has an unusual orchemical ly t rea ted f in ish migh t no t be hand led p r oper ly by the p r in terpaper handling mechanism. Refer to the printer manual to f ind what isa l lowable , bu t s tandard whi te 20-pound bond xerography-grade papershould always work prope rly. The e asiest solution her e is to try differenttypes of media.

2. You find unusually light or dark images. Adjust the print den sity con trolto achieve an optimum print density. Casual users often forget that they

must adjust the print density wheel throughout the l ife of the tonercartr idge. When a new toner cartr idge is instal led, the print density must beredu ced o r the resu l t ing image m igh t be too dark . As the tone r i s consum ed ,print will become l ighter, so print density must be increased. Very l ight printmigh t ind icate that the toner car t r idge must be rep laced .

3. The printer runs out of mem ory. Th e m em ory in your p r in ter serves as abuffer for data being sent from the host computer. For single-l ine printers,the buffer need no t be very large . For laser p r in ters , how ever, th ere must beenough m em ory to ho ld comp lex , h igh-reso lu t ion ima ges (usual ly abou t 1 o r2 M B ) . T he en t i re imag e must be loaded in to the laser p r in ter buffer befo r e



Typical cleaning and maintenance 183

the page is printed. If the image to be printed is too large to f i t in to exist ingmemory, the p r in ter wi l l reg is ter an OUT OF MEMORY erro r. Scale downthe image o r add me m ory to the p r in ter.

4 . Paper holds too much static when ejected—could cause paper jams.Th e pap er i t se lf is o f ten to b lame. Try a s tandard 20-pound xerog raph ic-

g rad e paper wi th averag e mois tu re con te n t . Pape r that i s too d ry re ta insstat ic charges . It also is possible that the stat ic el im inator t eet h in th eprinter are not working properly. Try cleaning the stat ic teeth as describedearl ier in th is appendix.



6A P P E N D I X

Comprehensive

troubleshooting guidesChapter 6: Troubleshooting acand dc linear supplies

1. Po we r supp ly i s com plete ly de ad . Laser p r in ter does no t opera te and nopower indicators are l i t .a. Chec k ac l ine volta ge powe ring the printerb. Check/replace the main ac fuse

c . Check the ou t pu ts(s) f rom the supp lyd . Check the regu la to r(s)e. Che ck th e filter(s)f. Check the rect i f ie r (s)g . Chec k the transform erh . Check the supp ly PC board fo r d amag ei. Rep lace the pow er supp ly

2. Supp ly operation is in term ittent . Print er oper ation cuts in and out alongwith the supply.a . Check ac l ine vo l tag e powerin g the p r in terb. Check a ll pow er connecto rs and wir ingc. Che ck the supply PC board for dam aged . Check power componen ts fo r thermal in termi t ten tse . Rep lace the po we r supp ly

3. Laser printer is not operating properly. I t might be functioning erraticallynot at al l . Power indicators might or might not be l i t .a. Che ck ac l ine volta ge po we ring the printerb. Check a ll pow er connecto rs and wir ingc. Che ck the outpu t (s ) from the supplyd . Ch eck the reg u la to r (s)e . Check the f i l ter (s )

184



Comprehensive troubleshooting guides 185

f. Check the rec t i f ier (s)g . C heck the transform erh . C heck the supp ly PC board fo r dam agei. Rep lace the po we r supp ly

4. Fusing quali ty is in term ittent or poor ( t on er sm udg es ea sily ), or fuser fai ls

to reach operat ing tem peratu re wi th in 60 to 120 seconds .a . Check a ll po we r connecto rs and wir ingb. Check ac vo l tag e pow ering the quar tz heat ing lampc. Check/ replace the therm al cutou t switc hd . Check /rep lace the therm is to r tem pera tu re sensore. Re pla ce the quartz heating lampf. Re pla ce the fusing assemb ly

Ch apter 6: Sw itching -sup ply dc)

troubleshooting1. Po we r supp ly i s com plete ly d ead . Laser p r in ter does no t operate , and no

power indicators are l i t .a. Check ac l ine vo l tag e powe ring the p r in terb. Check/ replace the main ac fusec . Check a ll po we r connecto rs and wir ingd . Check the ou tpu t(s) f rom the supp lye . Check EM I sh ie ld ing cover in g the supp lyf. Check primary and seco nda ry ac at the tra nsform erg . Check p res wi tch ed dc vo l tag e levels

h. Ch eck ch op pe d dc and filter(s)i. Che ck operatio n of the switching circuit or IC

Replace the power supp ly2. Sup ply operatio n is in term ittent . Laser printe r operatio n cuts in and out

with the supply.a. Check ac l ine vo l tag e powe ring the p r in terb. Check a ll po we r connecto rs and wir ingc. Check the supp ly PC board fo r dam aged . Check power componen ts fo r thermal in termi t ten tse . Rep lace the po we r supp ly

3. Lase r printe r is not opera ting prop erly. It mig ht be functioning erratically ornot at al l . Power indicators might or might not be l i t .a . Check ac l ine vo l tag e pow ering the p r in terb. Check a ll po we r connecto rs and wir ingc. Che ck the outpu t (s ) from the supplyd . Check EM I sh ie ld ing cover ing the supp lye . Check p r imary and second ary ac a t the t ransfo rmerf. Check p res wi tch ed dc vo l tage levelsg . Check cho ppe d dc and f i l ter (s )h . Che ck opera tion of the switching circuit or ICi. Rep lace the po we r supp ly



18 6 Appendix B

Chapter 6: High-voltage supplytroubleshooting

1. Lase r printing is too l ight or too dark.

a. Check/adjust the con tras t sett ingb . Rep lace the h igh-vo l tage supp ly2. Cannot control laser printer contrast

a. Check/adjust the contr ast settingb. Rep lace the h igh-vo l tage supp ly

Chapter 7: System start-up problems1. Noth ing h appens when p ow er i s tu rned on .

a. Check a ll po we r connecto rs and wir ingb. Check /rep lace the po we r supp lyc. Check/replace control panel cabled . R ep lace the ECPe. Rep lace the contro l pane l

2. Your p r in ter never leaves i ts warm -up mo de. T he re i s a con t inuousW A R M IN G U P s t a tu s co d e o r m essag e .a . Check the comm unicat ion in ter face cab le and host com puterb. Check the con tro l panel connecto rs and wir ingc. Rep air or rep lace the control paneld . R ep lace the ECP

3. You see a CH EC KS UM ER RO R message ind icat ing a fau lt has bee n det ecte d

in th e E C P p ro g ram R O M .a. Try a cold restart of the p rinterb. Rep lac e the EC P

4. You see an er ro r ind icat ing commun icat ion p rob lems be tw een th e p r in terand computer.a . Check comm unicat ion param eters (ser ia l con nec t ion)b. Check cable and cable conn ection sc. Che ck flow control sett ingsd . Rep lac e the ECP

Chapter 7: Laser delivery problems1. You see a B E A M D E T E C T IO N e r ro r.

a. Check/clean/replace the print er opticsb. Check /rep lace the f iberop t ic bea m-d etecto r cab lec. Che ck/r eplace the laser or laser/scanning assem bly

2. You see a BE AM L OS T erro r.a. Check/rep lace the dc po we r supplyb. Check /rep lace the f iberop t ic beam de tect o r cab lec. Che ck me chan ical safety in terlockd. Replace the laser/scanning assembly




Chapter 7: Fusing assembly problems1. You see a SE RV ICE error indicating a fusing malfunction.

a. Che ck all con nec tors and wiring pow erin g the fusing ass em blyb. Ch eck all fusing circuit fuses or circuit b rea ke rs

c. Che ck or repla ce the therm istord . Check o r rep lace the therm opro tecto re. Che ck or rep lace the quartz lam p (or entire fusing ass em bly )f. Rep lace the EC P

Cha pter 7: Im age-form ation problems1. Pag es are com plete ly b lacked ou t and migh t appear b lo tched wi th an

undef ined border.a. Che ck the primary corona in the EP cartr idge/rep lace the E P c artr id ge

b. Check /rep lace the f iberop t ic bea m-d etec to r cab leb. Check/replace the laser/scanning assemblyc. Rep lac e the ECP

2. Prin t is ve ry faint.a. Che ck ton er level/replace the EP cartr idgeb. Check pape r quali tyc. Check/rep lace the transfer coron a assemb lyd . Check /rep lace the h igh-vo l tage pow er supp lye . Check /res tore the d rum groun d in tegr i ty

3. P r in t appears speck led .a. Check/rep lace the fusing roller cleaning p ada. Check/rep lace the prima ry corona control grid or E P ca rtr idg eb. Rep lace the h igh-vo l tage power supp ly

4. There are one or more vert ical white streaks in the print .a. Che ck tone r level/replace the E P cartr idgeb. Check/clean the transfer corona assem blyc. Check /rep lace the f iberop t ic bea m -dete cto r cab led. Replace the laser/scanning assembly

5. Right-hand text appears missing or distorted.a. Che ck ton er level/replace the E P car tr idg eb. Check /correct laser/scanning unit al ignm ent and mou nting

c. Re plac e the laser/scanning assem bly6. You consisten tly enc oun ter faulty ima ge registr at ion.

a. Che ck pap er qualityb. Check the pape r trayc. Check/ replace the pap er picku p assemblyd. Check/ replace the registrat ion/transfer ass em blye. Check/repair faulty drive train assem bly

7. You encounter horizontal b lack l ines spaced randomly through the print .a. Check/replace the laser bea m de tec t sensorb. Check /rep lace the f iberop t ic beam detec to r cab le



18 8 Appendix B

c. Check/replace the laser/scanner assem blyd. Chec k all con nec tors and wiring from the scanner

8. Prin t is just slight ly faint.a. Check/adjust the cont rast con trol settingb. Chec k pap er quali ty

c. Check toner level/replace the E P ca rtr idged. Check/replace the drum sensit iv ity sensor sw itchese. Check/clean/replace the transfer corona assem blyf. Chec k or rep lace the high-volta ge po w er supply

9. Print has rough or suede appearance,a . Rep lace the EC P

10 . Print appears smeared or fused improperly.a. Check/replace fusing roller cleaning p ad (s )b. Clean/replace the fusing assem blyc. Check/replace the stat ic d ischarg e com bd. Check the drive traine. Check and clear obstructions in the pape r path

11 . P r in ted images appear to be d is to r ted .a. Check the pap er path and mech anical assem bliesb. Check or rep lace the scanning assem bly

12 . Print shows regular or repeti t ive defects.a . 3 .75 inch defe cts— che ck o r rep lace the EP car t r idgeb. 2 inch defec ts— che ck o r rep lace the EP car t r idgec. 3 .060 inch de fec ts— ch ec k or repla ce the fusing rollersd. Che ck and clean all oth er rollers or belt s

13 . Th e page appears complet e ly b lack excep t fo r hor izon ta l whi te s t r ipes .

a. Check/replace the fiberop tic be am de tec tor cableb. Check/replace the laser/scanning assem bly

14. The image appears skewed .a. Check the pap er trayb. Check the pape rc. Check/replace the pap er pickup assem blyd. Check/replace the registrat ion assem blye. Check/clear any obstructions

15 . Th e imag e is sized impro perly.a. Check the pap er tray tabsb. Check/replace the pap er tray switchesc . Rep lac e the ECP

16 . There are vert ical b lack streaks in the image.a. Check/clean the prima ry coronab. Rep lac e the EP car t r idgec. Rep lace the ECP

Chapter 8: Paper problems1. You find a P A P E R OU T me ssage ,

a. Chec k the pap er supply



19 0 Appendix B

Chapter 8: EP cartridge problems1. You see a NO EP C AR TR ID GE messa ge.

a. Check/replace the EP cartr id geb. Check/replace drum sensit iv ity sensor switch es

c . Rep lace the ECP2. You see a TO N ER LO W m essage constan tly, o r the er ro r nev er appears .

a. Check/replace the EP c artr idg eb. Check /rep lace the h igh-vo l tage pow er supp ly assemblyc. Rep lac e the ECP

Chapter 9: Troubleshootinga parallel interface

1. Pr in ter does no t p r in t a t a ll . A P RI NT ER NO T R EA D Y erro r migh t occur a tthe computer. The printer self-test looks correct .a. Check/replace the com mu nication interfac e cableb. Chec k print er DI P switch sett ingsc. Chec k the handsha ke status l inesd. Check data linese . Check the Ac kn ow led ge pu lsef. Chec k or repla ce the data latch IC or AS ICg . Rep lac e the EC P

Chapter 9: Troubleshootinga serial interface1. Pr in ter does no t opera te at a ll . A PR IN TE R N O T R EA D Y erro r migh t occu r

at the printer. The printer self-test looks correct .a. Check/replace the com mu nication interface ca bleb. Check p r in ter DIP swi tch se t tings and parame tersc. Chec k the hard war e handshaking status l ines (s )d . Chec k the flow co ntrole. Chec k data and circuitry on the rec eiv e (R x ) l inef. Chec k data and circuitry on the transmit ( T x ) l ine

g . Rep lac e the ECP

Chapter 9: Troubleshooting a control panel1. Th e contro l panel does not function at al l. N o keys or indicators resp ond .

Pr in ter appears to operate norm al ly under com puter con tro l .a. Chec k all con nec tors or in terc onn ecting w iringb. Check con tro l panel supp ly vo l tage (s )c. Rep lace the contr ol pane l assem blyd . R ep lac e the EC P




2. One o r mo re keys is in termi t ten t o r defect ive . Ex cess iv e fo rce o r m ul t ip leat tempts migh t be needed to operate the key(s) . P r in ter appears to operatenormal ly o therwise .a. Check/replace all quest ionable key sb. Rep lac e the con tro l panel assembly

3. One or more indicators fai l to function, or the LCD alphanumeric displayappears er ra t ic . P r in ter appears to opera te normal ly o therw ise .a. Check all conn ector s and interc onn ecting w iringb. Check con tro l panel supp ly vo l ta ge (s)c. Rep lace the contro l pane l assem blyd . R ep lace the ECP

Ch apter 9: Trou bleshoo ting m ain logic1. Prin ter do es not in it ial ize from a cold-start turn-on. Th er e is no visible

activity in the printer after power is turned on, but power indicators are l i t .Self-test does not work.a. Check/replace the dc po w er supplyb. Check/replace the syste m clockc. Rep lac e p r in ter RA M ch ips to check fo r defec t ive m em oryd . Rep lac e p r in ter ASIC or s lave microproc essore . Rep lac e p r in ter p rogram R OMf. Rep lace the E CP

2. Prin ter operation freeze s or be co m es highly erratic during oper ation. Youmust activate the printer from a cold-start to restore operation. Self-testmight work unti l the printer freezes.

a. Check/replace the dc po w er supplyb. Rep lac e p r in ter p rogram R OMc. Rep lac e the p r in ter main microproc essord. Check for therm al in term ittentse . Rep lace the EC P



cA P P E N D I X

Vendors

The following index provides a comprehensive set of resources that you can use tohelp find parts, materials, and even outside service organizations. You can find thisl ist updated regularly on TechNet BBS. This l ist should not be construed as an endorsement of the companies or services outl ined below. You are advised to shoparound and compare p r ices , warran t ies , and tu rnaround t ime befo re commit t ing toany sa le . Sales terms and cond i t ions wil l vary b etw een vend ors . Use cau t ion w henever dealing with mail-order organizations.

Component parts , data, and serviceAm erican C ompu ter Repair, Inc .

(p r in ter module repai r serv ices)

6330 Farm Bureau Rd.Al len town, PA 18106Tel: 215-391-0100Fax: 215-391-0431

All ied E lect ron ics (general par ts on ly )7410 Pebble Dr.Fort Worth , TX 76118Tel: 800-433-5700

Computer Network Serv ices (p r in terrepai r serv ices)

100 Ford Rd.Den vil le, NJ 07834Tel: 201-625-4056Fax: 201-625-9489

Dig i-Key (general par ts on ly )701 Brooks Av e. , SP.O. Box 677Thief River Falls, MN 56701-0677Tel: 800-344-4539F ax :2 1 8 -6 8 1 -3 3 8 0

Electroservice Labs (full-service repaircen te r )

6085 Sikorsky St.Ventura, CA 93003Tel: 805-644-2944Fax: 805-644-5006

Impact ( laser fuser assembly repair)10435 Burn et R d.Suite 114Aust in , TX 78758Tel: 5 12-832-9151Fax: 512-832-9321

Kennsco Componen t Serv ices ( repai rserv ices)2500 Broadway St. , NEMinneapolis, MN 55413Tel: 800-525-5608Fax: 612-623-4489

192



Vendors 195

Texas Instruments, Inc.P.O. Bo x 200 230Austin , TX 78720Tel: 800-527-3500Tel: 817-771-5856

Xante Corp .2559 Emogene S t .Mob i le , A L 36606Tel: 800-926-8839Tel: 205-476-8189

Laser printer supplies and materialsGlobal Computer Supplies1 Harbor Park Dr.Depar tment 52Port Washington, NY 11050Tel: 800-845-6225Tel: 516-625-6200Fax: 516-625-6683

MiscoOne Misco PlazaHolm del , NJ 07733Tel: 800-876-4726Fax: 908-264-5955



Glossary

A C K ( a c k n o w l e d g e ) A handshaking signal sent from printer to com pu ter indicating that the printer has successful ly received a character.

a n o d e Th e pos i tive e lec t ro de of a two- te rm ina l e lec t ron ic dev ice .A S A P ( A d v a n c e d S y s t em s A r c h i t e c t u r e f o r P o s t S c r i p t ) A pr in te r cont ro l le r

des ign that has been t rad ema rked by QM SA S C I I ( A m e r i c a n S t a n d a r d C o d e f o r I n f o r m a t i o n I n t e r c h a n g e ) A standard

set of binary codes that define basic le t ters , numbers, and symbols .A S I C ( a p p l i c a t i o n - s p e c i f i c I C ) A spec ia l ized IC deve loped to se rve a spec i f ic

funct ion (or set of funct ions) in a pr inter.b a s e On e of thre e electro des on a bipolar t ransis tor.b a u d r a t e Th e rate of ser ial data transmissions which is me asu red in PBS (bi ts pe r

s e c on d ) .b i n a r y A num ber system consist ing of only tw o digi ts .b i t m a p A two-d imen s iona l a r ray of do ts tha t com pose an imag e .B I T B LT ( B I T b l o c k - l e v e l t r a n s f e r ) Th e transfer of part of a bi tm ap im age from

one area in the laser pr inter memory to another.B U S Y A handshaking signal sent from printer to computer indicat ing that the

pr in te r cannot accept any mo re cha rac te rs ,c a p a c i t a n c e Th e measure of a device 's abi l ity to s tore an electr ic ch arge , measu red

in farads, microfarads, or picofarads,c a p a c i t o r A devi ce used to s tore an electr ica l charge,

c a t h o d e Th e nega t ive e lec t r ode of a two- te rm ina l e lec t ron ic device ,c lu t c h A mechanism used to swi tch mechanica l force in to or ou t o f a mechanica l

assembly.c o l l e c t o r On e of thre e electro des on a bipolar t ransis tor.c o n t i n u i t y Th e in tegr i ty of a conne c t ion mea sured as a very lo w res i s tance by an

ohmme te r.c o r o n a A f ie ld of conce nt ra ted e lec t r ica l charge produ ced by a la rge vo l tag e po ten

t ia l . Corona wires form one e lec t rode of th i s vo l tage po ten t ia l . There a re twocoronas in a laser pr inter ; the primary corona, and the t ransfer corona.

196



Glossary 197

C P I ( c h a r a c t e r s p e r i n c h ) Th e numb er of charac ters that will f it onto one inch ofhorizontal l ine space, also called character p itch.

C P U ( c e n t r a l p r o c e s s i n g u n i t ) The major controll ing logic element in yourprinter's circuitry.

C T S ( c l e a r to s e n d ) A serial handshaking l ine at the com pu ter usually con ne cte d

to the RTS line of a printer,d a t a An y of eight parallel data l ines that carry binary informa tion from c om pu ter t o

printer.d a t a b u f f e r Tem porary mem ory wher e characters f rom the com puter are s to red by

the printer prior to printing.D C D ( D a t a C a r r i e r D e t e c t ) A serial hand shak ing line usually foun d in serial m o

dem in ter faces .d e v e l o p i n g Th e mov em ent o f toner f rom a tone r supp ly to the la ten t image w ri t ten

to the charged drum surface,d i o d e A two- terminal e lec t ron ic dev ice used to conduct cu rren t in one d i rect ion

only.d p i ( d o t s p e r i n c h ) A laser printer 's resolution ex pre sse d as the num ber of indi

vidually addressable dots that can be placed in both the horizontal and vert icaldirections.

d r i v e r An amplifier used to con vert low -po we r signals in to high -pow er signals.D S R ( D a t a S e t R e a d y ) Th e primary com pu ter signal l ine for hard war e handshak

ing over a serial in terfac e. I t is conn ect ed to the D T R line at the printer.D T R ( D a t a Te r m i n al R e a d y ) Th e prim ary serial printe r signal for hard war e hand

shaking over a serial in terface. I t is connected to the DSR pin at the computer.E C P ( e l e c t r o n i c c o n t r o l p a c k a g e ) A gener ic term referr ing to the e lect ron ic as

sembly used to control a laser printer. The ECP consists of main logic, memory,

drivers, and a control panel. Also called a controller.e m i t t e r On e of thre e electr ode s on a bipolar transistor.E P ( e l e c t r o p h o t o g r a p h i c ) Als o called electrostatic ( s e e E S ) .E P R O M (e r a s a b l e p r o g r a m m a b l e r e a d - o n ly m e m o r y ) An advanced type o f

permanen t memory that can be erased and re-wri t ten to many t imes .E S ( e l e c t r o s t a t i c ) A proces s of creating imag es using the forces of h igh-v oltage to

at t rac t o r repel med ia ( toner) as needed to fo rm the des i red image.e x p o s u r e Th e p rocess o f d ischarg ing the d rum af ter c lean ing to rem ove any e lec

trical charges on the drum photosensit ive surface.f ix in g S ee fus ing .f o n t A chara cter set of part icular size, sty le, and spacing.fo n t sca l in g The process of producing bitmaps of various sizes from a generic

source of size-independent character information.fu s in g Th e proces s of using heat and pressu re to bon d toner to a porou s pap er sur

face.g a t e s Integ rated circuits used to pe rform simple logical oper ations on binary data

in digital systems.G N D ( g r o u n d ) A comm on electrical refe ren ce point for electr onic data signals.G P I B ( g e n e r a l - p u r p o s e i n t e r f a c e b u s ) A parallel communication interface in

tend ed p r imar i ly fo r netw orke d ins t rum entat ion , a lso known as IEEE 488 .



198 Glossary

g r a y s c a l e A ser ies of shades running f rom whi te to b lack . For laser p r in te rs ,sha de s a r e p r odu c e d by c r e a t i ng va r i ous pa t t e r n s o f do t s ( c a l l e d dithering).

i n d u c t a n c e Th e measure of the ab i li ty of a dev ic e to s tore a ma gnet ic cha rge , measured in henrys , mi l lihenrys , o r m icrohenrys .

i n d u c t o r A dev ice used to s tore a ma gnet ic ch arge .

i n i t i a l i z a t i on Resto ring defaul t or s tar t-up cond it ions to the printer due to faul t orpower-up .

l a n d s c a p e Th e orientat io n of chara cters or ima ges on a pa ge that runs the sh ortwa y ( on an 8.5 x 11 inch page , the 8.5 inch sides wo uld be ver t ic al ) .

l a s e r A devic e produ c ing a nar row in tense beam of coheren t , s ing le -wa velengthl igh t w aves .

L C D ( l i q u i d c r y s t a l d i s p l a y ) A d isp lay us ing charac te r images form ed f rom layers of vol tage-polar ized l iquid. In i ts off s tate , the l iquid is c lear. In i ts on state ,the l iquid is opaque.

L E D ( l i g h t - e m i t t i n g d i o d e ) A semicon ducto r devic e des ign ed such tha t photon sof l ight are l iberated when i ts p-n junct ion is forward biased.

l p i ( l i n e s p e r i n c h ) Th e num ber of horiz onta l l ines that fi t into one inch of vert ical page space, a lso known as l ine pi tch.

M B ( M e g a b y t e ) An amou nt of m em ory or s torage area. Each mega byte is 1,048,576

bytes.m i c r o p r o c e s s o r A c om ple x p r og r a mm a b le l og i c de v i c e t ha t w i l l p e r f o r m va r i

ous l og i c a l ope r a t i on s a nd c a l c u l a t i ons ba se d on p r e de t e r mine d p r og r a m ins t ruc t ions .

m o t o r An e lec t rome chan ica l dev ice used to conve r t e lec t r ica l energ y in to mech anica l mo t ion . The re a re severa l type s of mo tors used in laser p r in te rs .

M T B F ( m e a n ti m e b e t w e e n f a i l u r e s ) A mea sure of a devic e 's rel iabi l i ty ex

pressed as t ime or an amount of use.m u l t i m e t e r A versat i le test instrum ent used to test such circui t par am eters as vol t

age , cur ren t , and res i s tance . Also ca l led a DV M or V OM .p a r i t y An ex t ra b i t ad ded to a se r ia l da ta w or d used to check for e r rors in com

mun ic a t i on .p a w l Cu rved p las t ic assembl ies resembl ing c laws tha t gu ide charge d paper th rou gh

the printer.P C L ( p r i n t e r c o m m a n d l a n g u a g e ) A popular p r in te r cont ro l l anguage de ve lo ped

by Hew let t- Pac ka rd that is used or em ulat ed by almost all 300 x 300 dpi resolut ion laser pr inters .

P D L ( p a g e - d e s c r i p t i o n l a n g u a g e ) A reso lu t ion- independent p r in te r l anguagetha t descr ibes the e lem ents of a p r in ted page . Com mo nly used wi th P os tScr ip tor Pos tScr ip t -compat ib le pr in te rs .

P E ( P a p e r E r r o r ) A handshaking signal sent from the printer to te l l the com pu terthat paper is exhausted,

p h o t o s e n s i t i v e A material or de vic e that reacts electr ica l ly wh en exp os ed to f ight,p i e z o e l e c t r i c Th e proper t y of ce r ta in mater ia l s to v ibra te when vol tag e is appl ied

to them.



Glossary 199

p o r t r a i t Th e orientation of characters or ima ges on a pag e that runs the long w ay(o n an 8.5 x 11 inch page, the 11 inch sides wo uld run ve rt ica l) .

p p m ( p a g e s p e r m i n u t e ) Th e maximum speed a t which a laser p r in ter eng ine canmove paper.

R A M ( r a n d o m - a c c e s s m e m o r y ) A tempora ry me mo ry dev ice used to s to re d ig i

tal information.r e g u l a t o r An electro nic dev ice used to cont rol the output of volta ge and currentfrom a power supply.

r e s i s t a n c e Th e measu re of a device 's abil i ty to l imit electric al current, measu red inohms , k i lohms, o r meg ohm s.

r e s i s t o r A dev ice used to l imit the flow of electrical curren t.R E T ( r e s o l u t i o n e n h a n ce m e n t t e c h n o l o g y ) In t rodu ced by Hew let t -Pack ard for

the LaserJet III , RE T imp rove s edg e definit ion by varying the size of dots arou ndthe edg es o f b i tmaps .

R O M ( r e a d - o n l y m e m o r y ) A perma nen t m em ory dev ice used to s to re d ig i tal information.

RT S ( R e q u e s t To S e n d ) A printer serial handshaking l ine usually connected tothe CTS line of a computer.

R x ( R e c e i v e D a t a ) This is the serial input line. Th e printer Rx l ine is con ne cte d tothe computer Tx l ine ,

s c a n n e r In laser printers, the scanner assem bly uses a rotating hex agon al mirror todirect the writ ing laser beam along the photosensit ive drum surface. Dots areformed by turning the laser beam on or off while the beam is being scanned.

S e l e c t A control signal from the com pu ter that pre pare s the printer to re ce ive data,s e p a r a t i o n p a d A soft rubber pad in the paper transfer assembly that prevents

more than one page at a t ime from entering the printer,

so f t fo n t Fon t vecto r o r b i tmap data on d isket te o r o ther com pute r med ia (suc h asC D - R O M ) .

s o l e n o i d An e lect rom echan ical de v ice consis ting o f a co i l o f wire wrap ped arounda core which is free to move.

S t r o b e A handshaking l ine from the com pu ter that tel ls the printer to acc ept va lidparallel data on its data lines.

t h e r m i s t o r A temp eratu re sensing dev ice used to regu la te tem peratu re in the fusing roller assembly.

t o n e r A fine pow de r of p last ic, i ron, and pigme nts used to form im ages in elect rostat ic printing sy stems .

t r a n s f e r Th e p rocess o f a t t rac t ing the dev elop ed image o ff the d rum and on to thecharged paper surface.

t r a n s i s t o r A three-term inal electronic de vice who se output signal is prop ortiona lto its input signal. A transistor can act as an amplifier or a switch.

t r a n s f o r m e r A de vice used to step the volt age and curren t levels of ac signals.T x ( T r a n s m i t D a t a ) This is the data output l ine for serial dev ices . Th e com pu ter

Tx l ine is connected to the printer Rx l ine.



IndexAac supply (see power supplies, ac)ac/dc linear supplies (see power supplies,

ac/dc linear)address lines, main logic board, 174

adjustable resistors, 20alligator leads, 44A N D , 3 8, 39anodes, 27apertures, lasers, 86, 8 6application specific ICs (ASI Cs ), 39, 175ASCII character set, 5, 156-157axial capacitors, 24

BB+K 2522A digital oscilloscope, 59B+KDP-51 logic probe, 58base, 33baud rate, 119BEAM D ETEC TION error message, 120-

121,121,186BEAM LOST error message, 121-122,186

bearing assembly, 13-14, 14binary vs. decimal number systems, 157-

159,158bipolar transistors, 33

static electricity dam age, 72bidirectional data link, 159blacked-out page w ith horizontal white

stripes, 188blacked-out pages, 124-125, 124, 187bleeder resistors, 99, 100block diagram of typical laser printer, 6, 7blotched pages, 124-125, 187Boolean logic, 38breakdown voltage, zener diodes, 29bridge rectifiers, 97-98, 98"bubble" designation, logic gates, 38buffers, 38, 3 9burned-in memory, 170buses, main logic board, 174bushing assembly, 13, 14

ccapacitors, 22-25, 23, 25

axial capacitors, 24bleeder resistors, 99, 100dielectrics, 22, 23electrolytic capacitors, 23-24farad as unit of capacitance, 22fixed capacitors, 23-24housing types for capacitors, 23, 23multimeters testing, 55-56, 55number code for capacitors, 24-25, 25radial capacitors, 24working voltage rating for capacitors, 24

capacity, print capacity, 4carbon-film resistors, 18-19, 19cathodes, 27CD-ROM, 3Centronics parallel interface standard, 3-4character sets, 5charging the drum, 80-81, 81CHECKSUM ERROR message, 118, 186clamping circuit as regulators, 101cleaning and maintenance, 79-80, 178-183

drums, 79-80, 79fusing-roller cleaning pad, 180jammed printer, 182, 183memory overloads, 182-183ozone filter cleaning, 180

paper-feed guide cleaning, 179poor-quality printouts (see poor-quality

printouts)primary corona, 179reassembly o f printer, 180separation pawl, 180static charge in paper, 183static eliminator teeth, 179transfer corona, 178-179transfer guide, 179user-related problems, 182-183

clutches, 16CMOS logic, 58coherent light, lasers, 85, 86coils, 15,17, 25cold solder joints, 48collector, 33color code fo r resistors, 20-22, 21communication interface (see also parallel

interface; serial interface ), 155-169ASCII character set, 156-157bidirectional data link, 159

communication standards, 160control codes, 157data buffer, 161escape sequences, 157handshaking, 159,160,166isolating the comm unication interface, 164line driver, 163line receiver, 163line transceivers, 163links, communication links, 159number systems, binary vs. decimal, 157-

159, 158parallel interface, 159,160-162,160, 161parity bits, 159PRINTER NOT READY error message,

parallel interface, 164PRINTER NOT READY error message, se

rial interface, 167-168serial interface, 159, 161-164,162, 163,

167-169standards, communication standards, 160universal asynchronous receiver/transmit

ter (UART), 163, 164compon ents, 10-41

active com ponents, 18, 27-41bearing & bushing assemblies, 13-14 ,14capacitors, 22-25, 23, 25coils & cores, 15diodes, 27-33, 28electromagnetism explained, 14-15,14,15electromechanical components, 14-18friction redu ction com ponents, 13-14gears, 10,11, 12inductors, 25-27, 26integrated circuits, 38-41, 40lubrication, 13magnetic fields, 14-15,14magnetically permeable materials, 14mechanical parts, 10motors, scanner and main, 17-18 ,17passive components, 18-27pulleys, 12-13,12relays, 15-16,16resistors, 18-22, 19, 21, 22rollers, 13solenoids, 16,16switches or contact sets, 15transformers, 25-27, 26transistors, 33-38, 34

computer/printer commun ication errors,119,186

computer/printer com patibility, 2, 3,11 9baud rate, 119data bits, 119

null modems, 119parity, start, & stop bits, 119contact sets, 15continuity testing, 54-55control codes, 157control panel, 9 ,171-173,172, 190-191

indicator light erratic/inoperative, 173,191inoperative panel, 173,190keypad operation intermittent or defec

tive, 173,191LCD display erratic, 173,191sealed switches, 172-173,172

core, transformers, 15, 94corona, 80-81, 81coupling, magnetic, in transformers, 26cross-sectional diagram of laser printer, 78current measurem ents, multimeters, 52-53,

53

Ddark images, 114,115,182,186data bits, 119data buffer, 161data bus, main logic board, 174-175dc power supply (see power supplies, dc)dc servo motors, 17delivery coupling assembly, paper-control

assembly, 145desoldering, 50, 50dielectrics, 22, 23diodes, 27-33, 28

anodes, 27breakdown voltage, 29cathodes, 27

forward biasing, 28, 28forward current, 27JEDEC specifications, 33laser diodes (see lasers)light emitting diodes (LE D) , 30-31, 30,

34,88-89, 88, 89,148markings on diodes , 32-33, 3 2number code fo r diodes, 32-33, 32optoisolators, 148peak inverse voltage ( P I V ) , 27 , 29polarity of diodes, 27rectification in diodes, 27rectifiers, 27reverse biasing, 28-29, 29semiconductors, 30silicon-controlled rectifiers (SCR), 31, 3 1small-signal applications, 27, 33testing diodes, 30

multimeters, 56-58, 56triacs, 32, 32zener breakdown, 29zener diodes, 29-30, 29

disassembly steps, 74-75, 75distorted images, 132-133,133,188divergence of regular light sources vs.

lasers, 85, 86drain, 35drive linkage, 12-13,12driver circuits, 33-34, 34dropouts, 181drum, 78-79, 79

charging the drum, 80-81, 81

200 Illustrations are in boldface.



Index 201

cleaning, 79-80, 79developing, 82-83, 83erase lamps, 80, 80primary corona & grid, 80-81, 81regulating grid voltage, 81transfer, 83-84, 84writing mechanisms, 81

dual inline packages ( D IP ), 39, 40

duty cycles, 108dynamic RAM (DRAM), 171

Eease-of-use features, 2efficiency, transformers, 96electrolytic capacitors, 23-24electromagnetic interference (EMI), 109electromagnetism, 14-15,14,15

magnetic fields, 14-15,14permeable materials, 14

electromechanical components, 14-18electronic control package (E CP ) (see also

main logic board ), 8,9, 77,116,154-177application specific ICs (ASIC) opera

tions, 175ASCII character set, 156-157

bidirectional data link, 159communication interface (see communi

cation interface; parallel interface; serial interface)

communication standards, 160control codes , 157control panel, 171-173,172data buffer, 161diagram of typical laser printer, 155escape sequences, 157handshaking, 159, 160,166isolating the commun ication interface, 164line driver, receiver, & transceivers, 163links, communication links, 159main logic board, 173-177memory, 169-171microprocessor operations, main logic

board, 174number systems, binary vs. decimal, 157-

159overhead or processing time, 175parallel interface, 159,160-162,160, 161parity bits, 159PRINTER NOT READY error message,

164,167serial interface, 159,161-164,162, 163.

167-169slave microprocesso r, 175system clock/master clock, 175universal asynchronous receiver/transmit

ter (UART), 163-164Electronic Industries Association (EIA), 3electrophotographic printers (see laser

printer basics)electrophotographic process, 2, 78electrostatic discharge (ESD), 71emitter, 33emitter-coupled logic (ECL), static elec

tricity d amage, 72emulation, 5environmental information, 6EP cartridge, 9, 90-92, 91

capacity of typical EP cartridge, 90fogging, 91-92humidity limits, 92NO EP CARTRIDGE message, 152, 190protecting the E P cartridge, 91-92redistributing toner in EP cartridge, 92 ,92sensitivity tabs, 152

shock hazard, high-voltage pow er supply,153

temperatu re limits, 92TON ER LO W message, 152-153,190toner sensors, 152-153,153

Epson, 5Epson ActionLas er 1500 laser printer, 1 54Epson ActionLa ser 1600 laser printer, 93

equipotential bonding, static electricity, 74erasable programmable ROM (EPROM),169-170

erase lamps, 8, 80, 80exclusive NOR (XNOR), 38, 39exclusive OR ( XO R), 38, 39exit area, paper-control assembly, 144-145

Ffaint ima ges, 125-126,125,130-131,187,188farad as unit of capacitance, 22features and specifications, 2-3field-effect transistor (F E T ), 33,3 5, 35filter, ozone (see ozone filter)filters, power supply, 94

high frequency (HF) filters, 102inrush current, 99power supply filters, 98-99, 98ripple, 99testing filters, 104

fixed capacitors, 23-24flip flops, 38flowchart of universal troubleshooting

process, 65-66, 65flux, soldering, 45focusing lasers, 87-88fogged printouts, 91-92fonts, 4-5, 5forward biasing, diodes, 28, 28forward current, 27freezed operation, 177,191frequency measurement

multimeters, 54, 54oscilloscopes, 64, 64

friction redu ction comp onents, 13-14full-wave rectifiers, 97, 97fuses, 103fusing assem bly, 7-8, 84, 84, 122-124

fusing temperature control ineffective,107,149, 185,189

quartz lamp, 123-124SERVICE error message, 123-124,187temperature control, 84thermoprotector (thermal switch), 123

fusing rollerscleaning the cleaning pad, 180paper-control assembly, 144,144

Gg-force limits, 6

gate, 35gears, 10,11,12angled or beveled gears, 10, 11primary shaft or rotation, 10ratio , 12 ,12secondary shaft or rotation, 10,11torque,12

general-purpose interface bus (G PIB ) (seeparallel interface)

Hhalf-wave rectifiers, 96-97, 97hand tools, 43-44handshaking, 159, 160

conditions chart, parallel interface, 166heat sinks, 100-101

heat-shrink tubing, 44henrys as unit of inductance, 25Hewlett-Packard, 5

LaserJet III printer, 1, 5LaserJet IV printer, 77, 116

high frequency (H F) filters, 102high-voltage supply (see power supplies,

high-voltage)

hook-up wire, 44horizontal black lines through page, 129-130, 130, 187-188

Howard W. Sams & Co., 69humidity limits, 92

IIBM, 5IEEE 488 GPIB parallel interface standard, 4image-formation system ( IF S ) , 78-79,116-

117,117poor-quality printouts (see poor-quality

printouts)indicator light erratic/inoperative, 173,191induced ac signals, transformers, 26induction motors, 17inductors, 25-27, 26

henrys as unit of inductance, 25transformers, 25-27, 26

infrared L EDs, 148inrush current, 99Institute of Electrical and Electronic Engi

neers ( IE EE ), 3integrated circuits (I C) , 38-41, 40

A N D , 38 , 39application specific ICs (ASICs), 39, 175Boolean logic, 38"bubble" designation, 38buffers, 38, 39dual inline packages (DIP), 39, 40exclusive NOR (XNOR), 38, 39exclusive OR (XO R) , 38. 39flip flops, 38inverters, 38, 39leadless chip carrier (L CC ), 40logic gates, 38, 39NAN D, NOR & OR, 38, 39pin grid array (P G A) , 41plastic leaded chip carrier (PLLC), 40-41power source connections, 39quad flat pack (QFP), 40quad small outline package (Q SO P), 40,

40regulators, 101.102single inline packages (S IP ) . 39. 40small outline IC (SOIC), 40, 40small outline J-lead ( SO J), 40testing integrated circuits (IC), 41thin quad flat pack (TQFP), 40through-hole ICs, 39-40

truth tables, 38very small outline package (VSOP), 40interfaces (see communication interface;

parallel interface; serial inter face)interlock (see sensor/interlock system)International Consultative Committee for

Telephone & Telegraph (C CIT T), 3inverters, 38, 39

isolation transformer, 70-71, 71 , 95

J

jamm ed printer, 140-145, 182,183, 189JEDEC specifications

diodes, 33transistors, 37, 37

joints, soldered, 46



2 0 2 Index

Kkeypad operation intermittent or defective,

173,191

Llanguages, 5laser delivery system (see also lasers), 119-

122,120,186BEAM D ETE CTIO N error message, 120-

121,121,186BEAM LOST error message, 121-122,186SCAN BUFFER error, 122scan line, 11 9-120,12 0

laser printer basics (see also components) ,1-9,1, 77-92

ASCII characters, 5block diagram of typical laser printer, 6, 7Centronics parallel interface standard, 3-4character sets, 5charging the drum , 80-81, 81cleaning and maintenance, 79-80computer/printer compatibility, 2, 3control panel, 9cross-sectional diagram of laser printer, 78

developing process, 82-83, 83developmen t of laser printers, 1drum, 78-79, 79ease-of-use features, 2electronic control package (ECP), 8, 9, 77electrophotographic printers (EP) de

fined, 2, 78emulation, 5environmental information, 6EP cartridge, 9, 90-92, 91erase lamps, 8, 80, 80features and specifications, 2-3fonts, 4-5,5fusing assembly, 7-8, 84, 84fusing rollers, 84, 84g-force limits, 6IEEE 488 GPIB parallel interface stan

dard, 4image-formation system ( IF S ) , 78-79interface compatibility, 3languages, 5lasers, 85-88, 86, 8 7light emitting diodes (L ED ), 2, 8, 88-89,

88, 89memory, 4motors, scanner and main, 8noncondensing limits, 6operating temperatures, 6operation of laser printers, 2paper specifications, 4paper-control assembly, 8parallel interface, 3peripheral devices, 3physical information: height, weight, etc., 6physical shock and vibration limits, 6powe r requirements, 3, 6, 7power supplies, 6, 7primary corona, 80-81, 81primary grid, 81print capacity, 4print characteristics, 4-5regulating grid voltage, 81relative humidity limits, 6reliability of printers , 2, 5-6resolution, 4RS-232 serial interface standard, 3scan line, 85, 119-120,1 20scanning process, 119-120,120serial interface, 3speed of printing, 2, 4

static eliminator teeth, 83-84, 83storage temperature, 6toner, 7-8, 79-80trace line, 85transfer corona assembly, 83, 83transfer, 83-84, 84writing mechanisms, 8, 79, 81, 84-90, 85

lasers (see also laser delivery system), 85-88 , 86, 87

alignment and focusing, 87-88apertures, 86, 86coherent light, 85, 86diagram of laser/scanning system, 87divergence of regular light sources, 85, 86laser delivery system, 119-122lenses, 87mirrors, 87modulation of beam, 87monochromatic light, 85safety precautions, 121

LCD display erratic, 173,191leadless chip carrier (LCC), 40lenses, lasers, 87life expectancy (see reliability of printers)

light emitting diodes (LED), 2, 8, 30-31,

30,34 , 88-89, 88, 89, 148light images, 114, 115,130-131,182,186,

187,188line driver, 163line receiver, 163line transceivers, 163links, communication links, 159

bidirectional data link, 159parallel links (see parallel interface)serial links (see serial interface)

liquid crystal shutter (L CS ), 89-90, 89logic board (see main logic board)logic families, 58logic gates, 38, 39logic probes, 58-59, 58, 59lubrication, 13

Mmagnetic (see electromagnetism)magnetic coupling, transformers, 26, 94main logic board, 173-177,191

address lines, 174application specific ICs (ASIC) operations,

175buses, 174data bus, 174-175erratic operation, 177,191freezed operation, 177,191microprocessor operations, 174-175, 174overhead or processing time, 175programs, 174read/write (R/W) operations, 175

slave microprocessor, 175system clock/master clock, 175maintenance (see cleaning and maintenance)manuals, schematics, etc., 68-69manufacturers/vendors listing, 192-195master clock, 175mechanical sensors, 147-148memory, 4,169-171

burned in memory, 170dynamic RAM (DRAM), 171erasable programmable ROM (E PR OM ),

169-170nonvolatile or ROM memory, 169-170overloading memory, 182-183permanent mem ory, 169-170programmable read-only memory

( P R O M ) , 169-170

random-access memory (RAM), 169,170-171

read-only memory (ROM), 169-170refreshing memory, 171stat ic RAM (SR AM ), 170temporary memory, 170-171testing and troubleshooting procedures,

171volatile or RAM memory, 170-171

microprocessor operations, main logicboard, 174-175,174

Microsoft Windows, 4mirrors, lasers, 87modems, null modems, 119modulation of lasers beam, 87monochromatic light, lasers, 85MOS transistors, static electricity damage,

72-7.3, 72, 73MOSFET, 35motors, 8,17-18,17,150-151, 189

clutches, 16coils, 17dc servo motors, 17drive linkage, 12-13,12

induction motors, 17phase or phase winding, 17pulleys, 12-13,12rotors, 17SCANNER ERROR message, 151, 189shock hazards, 151solenoids, 16,16stators, 17stepper/stepping motor, 17-18stuck main motor, 151, 189torque, 17variable pitch whirring noise, 151

multimeters, 51-58, 51capacitor testing, 55-56, 55continuity testing, 54-55current measurements, 52-53, 53dynamic tests, 51-52

frequency measurement, 54, 54function setting, 51range setting, 51resistance measuremen t, 54-55, 55semiconductor testing, 56-58, 56static tests, 51-52transistor testing, 57-58, 57volt-ohm-milliammeters (V OM ), 51voltage measurements, 52, 52

NNAND, 38, 39NO EP CA RTR IDGE message, 152, 190noncondensing limits, 6nonvolatile or ROM memory, 169-170NOR, 3 8, 39NP N transistors, 33null modems, 119number code for capacitors, 24-25, 25number systems, binary vs. decimal, 157-

159,158

oohm as unit of resistance, 18op amps, static electricity damage, 72operating temperatures, 6optical sensors, 148, 148optocouplers, 34optoisolators, 34, 34, 148OR , 38, 3 9oscilloscopes, 59-64, 59

accuracy, 61adjustments, 60



Index 203

bandwidth, 60calibration techniques, 61-62, 62controls, 60CRT display, 61frequency measurement, 64, 64housekeeping con trols, 60input impedance, 61specifications, 60-61

startup proce dures, 61-62time base, 61time measurement, 64, 64vertical deflection, 60voltage measu rement, 62-63, 63

overhead or processing time, 175overprinting, 145-146,146,189ozone filter, cleaning, 180

PPAP ER OUT message, 138-140, 149, 188-

189paper-control assembly, 8, 138-146,138,

139,188-189auxiliary PC board, paper-control board,

140delivery coupling assembly, 145exit area, 144-145fusing/exit area rolle rs, 144,144jammed printer, 140-145,189overprinting problems, 145-146,146, 189paper exit sensor, 138PAPER JAM message, 140-145,189PAP ER OUT message, 138-140, 149, 188-

189paper pickup rollers, 138paper pickup/feed mechanism, 141paper tray ID switch, 138-139,138, 139paper-detect sensors, 138paper-feed area, 141-143paper-feed g uide, cleaning, 179paper-sensing arm, 139-140,139path of paper through printer, cross-sec

tional view, 140registration rollers, 138registration/transfer area, 142, 143-144separation pad, 138shock hazards, high-voltage pow er supply,

143,145smudged band on page, 145-146, 146, 189static charge in paper, 183torsion springs for registration rollers, 143

parallel interface, 3,159, 160-162, 160,161,190

Centronics standard, 3-4data buffer, 161handshaking, 160, 166IEEE 488 GPIB parallel interface standard,

4isolating the comm unication interface, 164

pin assignments chart, 164,165PRINTER NOT READY error message,164,190

testing, 164, 165timing signals, 160, 161

parity bits, 119,159PC boards

auxiliary paper-control board, 140cracks, 104,105failure of PC boards, 104-105,104lead pull-through, 104,105trace break, 104, 105

peak inverse voltage (P IV ) , 27, 29permanent mem ory, 169-170permeable materials, 14, 94phase or phase winding, 17

phototransistors, 34physical information: height, weight, etc., 6physical shock and vibration limits, 6pin grid array (PGA) IC, 41plastic leaded chip carrier (PLLC), 40-41pliers, 43PN P transistors, 33polarity of diodes, 27

poor-quality printouts, 124-136,180-182,187-188badly formed charac ters, 182blacked out page with horizontal white

stripes, 134-135, 188blacked out pages, 124-125,124,187blotched pages, 124-125,187dark images, 114, 115, 182,186distorted images, 132-133,133, 188dropouts, 181faint image s, 125-126, 125, 13 0-131, 187,

188fogging, 91-92horizontal black lines through page, 129-

130, 130, 187-188light images, 114,115, 130-131, 182, 186,

187, 188overprinting, 145-146,146,189registration problems, 128-129,128, 187repetitive defects on page, 133-134,134,

182,188right-hand text missing or distorted, 127-

128,128, 187rough or "sue de" print, 131, 131,188sized-wrong images, 136, 188skewed images, 135-136,135. 188smeared print, 131-132,132, 188smudged band on page, 145-146,146, 189speckled pages, 79,126,126,187staining of page, 181vertical black streaks on page, 136,136,

188vertical fade, 180-181vertical lines on page, 181vertical white streaks on page, 126-127,

127,187Postscript, 5potential voltage, 96potentiometers or rheostats, 20power requirements, 3powe r supplies, 93-115

ac supply, 6ac/dc linear, 94-107

ac supply, 96bleede r resistors, 99, 100block diagram, 94, 94core, 94dead unit, 103,184efficiency, transformers efficiency, 96erratic operation, 106-107, 184-185

filters, 94, 98-99, 98, 102, 104fuses, 103fusing temperature control ineffective,

107,185heat sinks, 100-101high frequency (H F ) filters, 102integrated circuit (IC) regulators, 101,

102intermittent operation, 105-106,184isolation transformers, 95magnetic cou pling, 94PC board failure, 104-105,104permeab le m aterials, 94primary voltage, 95rectifiers, 94, 96-98, 97, 98, 104regulated vs. unregulated supplies, 100

regulators, 99-102,100,101, 103-104ripple, 97, 99shock hazards, 103, 105,106, 107step-down transformers, 95step-up transformers, 95taps, 94transformer, 94-96, 94, 95troubleshooting, 102-107

zene r diode clamping circuit, 101bleeder resistors, 99,100dark images, 114, 115,186dc supply, 7dead unit, 103, 110-112, 184, 185duty cycles, 108electromagnetic interference ( EM I), 109erratic operation, 106-107,113-114,184-185fuses, 103fusing temperature control ineffective,

107,185heat sinks, 100-101high voltage, 7, 153

dark images, 114,115, 186light imag es, 114, 115, 186shock hazards, 143, 145,153

inrush current, 99intermittent operation, 105-106, 112-113,

184,185light images, 114, 115,186regulated v s. unregulated supplies, 100requirements for typical laser printer, 3ripple, 97, 99, 109shock hazards

ac/dc linear supply, 103, 105, 106, 107switching supply ( dc ), 108, 111, 112

switching supply (dc), 94, 107-114,108,109, 110, 111, 185

dead unit, 110-112,185duty cycles, 108electromagnetic interference (EM I), 109erratic operation, 113-114,185intermittent operation, 112-113, 185ripple, 109sensing and switching circuits, 108-109shock hazards, 108, 111, 112

troubleshooting list, 184-186power-handling capacity, capacitors, 20primary corona, 80-81, 81

cleaning, 179primary g rid, 81primary voltage, transformers, 95PRIN TER NOT READ Y error message, 164,

167-169, 190PRINTER OPEN message, 149-150, 150. 189programmable read-only memory (PROM),

169-170programs, main logic board, 174pulleys, 12-13, 12

drive linkage, 12-13,12

pulsating dc voltages, rectifiers, 96

Qquad flat pack ( Q FP ), 40quad small outline package (QSOP), 40 ,4 0quartz lamp, 123-124

Rradial capacitors, 24random-access memory (R AM ), 169, 170-

171ratio, gea rra tio, 12, 12read-only memory (ROM), 169-170read/write (R/W) o perations, main logic

board, 175reassembly of p rinter, 180



204 Index

rectifiers, 27 , 94, 96-98, 97, 98bridge rectifiers, 97-98, 98full-wave rectifiers, 97, 97half-wave rectifiers, 96-97, 97pulsating dc voltages, 96ripple, 97, 99silicon-controlled (see silicon-controlled

rectifiers)

testing, 104Wheatstone brid ge rectifiers, 97-98

reference materials, 193reflow soldering, 49refreshing memory, 171registration problems, 12 8-129,128, 187registration rollers, 13,1 38, 142 , 143-144regulating grid voltage, 81regulators, 99-102 ,100 ,101

heat sinks, 100-101integrated circuit (IC) regulators, 101,102regulated vs. unregulated supplies, 100testing, 103-104zener diode clamping circuit, 101

relative humidity limits, 6relays, 15-16,1 6reliability of prin ters, 2, 5-6repetitive defects on page, 133-134,134,

182, 188replacement parts, 67-68resistive sensors, 146resistors, 18-22 ,19 ,21 ,22

adjustable resistors, 20bleeder resistor, 99carbon-film resistors, 18 -19,19color code for resistors, 20-22, 21dissipation of power, 20ohm as unit of resistance, 18potentiometers or rheostats, 20power-handling capacity, 20resistance measurement, multimeters, 54-

55, 5 5surface-mount resistors, 19,19, 22, 22tolerance rating, 20value rating, 20

resolution, 4reverse biasing, diodes, 28-29, 29rheostats, 20right-hand text missing or distorted, 127-

128,128, 187ring conditions, optical sensors, 148ripple , 97, 99, 109rollers, registration, 13rosin-core solder, 45rotors, 17rough or "suede " print, 131,131,188RS-232 serial interface standard, 3,163

sSams Photo/act series, 69

SCAN BUFFER error, 122scan line, 85,119-120, 120SCANN ER ERR OR message, 151,189scanner motor (see motors)scanning process, 119-120,1 20schematics, 68-69screw types and screw drivers, 43-44, 43sealed switches, control panel, 172-173,172semiconductors, 30sensitivity tabs, EP cartridge, 152sensor/interlock system, 146-150

fusing temperature control ineffective,149, 189

infrared L EDs, 148LEDs, 148mechanical sensors, 147-148

optical sensors, 148,148optoisolators, 148PAPER OUT message, 149, 189PRINTER OPEN message, 149-150,150,

189resistive sensors, 146ring conditions, 148temperature-alarm circuits, 146-147,14 7

temperature-proportioning circuit, 147, 147thermistors, 146toner sensors, 152-153,153troubleshooting, 148

separation pad/pawls, 138cleaning, 180

serial interface, 3,159,161-16 4, 162,163,167-169, 190

block diagram, 162connections, 168,168isolating the communication interface, 164line driver, 163line receiver, 163line transceivers, 163PRINTER NOT READY error message,

167-169, 190RS-232 standard, 3, 163universal asynchronous receiver/transmit

t e r (U AR T) , 163,164wiring diagram, 163,163

SERVICE error message, 123-124,187service guidelines (see troubleshooting)shock hazard s, 69-71

ac/dc linear power supply, 103, 105,106,107

high-voltage power supply, 143,14 5, 153isolation transformer use, 70-71, 71motors, 151switching-supply (d c) , 108, 111, 112

silicon-controlled rectifiers ( SC R) , 31, 31static electricity damage, 72trigger signal, 31

single inline packages (S IP ) , 39, 40sized-wrong images, 136,188skewed images, 135-136, 135, 188slave microprocessor, 175small outline IC (S OI C) , 40, 40small outline J-lead (S OJ ), 40small-signal applications, diodes, 27, 33smeared print, 131-132,132 , 188smudged band on page, 145-146,146, 189soft fonts, 5soldering, 44

cold solder joints, 48desoldering, 50, 50flux, 45good solder joints, 48heat required, 46joints, 46poor solder joints, 48

reflow soldering, 49rosin-core solder, 45safety techniques, 47-48solder m aterials, 44, 45soldering irons, 44, 45-47, 46, 47surface-mount d evices, 48-50, 49tinning the tip, 47tips for soldering irons, 45-47, 46, 47vacuum or wick for desoldering, 50wave soldering, 49wettable vs. nonw ettable tips, 46

solenoids, 16, 16source, 35speckled pages, 79, 126,126, 187speed of printing, 2, 4staining of page, 181

standards, communication standards, 160start bits, 119start-up system

baud rate, 119CHECKSUM ERROR message, 118, 186computer/printer communication errors,

119, 186data bits, 119

dead unit, 117-118,186null modems, 119parity bits, 119start bits, 119stop bits, 119warm-up mode continuous, 118, 186

static charge in paper, 183static electricity damage, 71-74, 72. 73static eliminator teeth, 83-84, 83

cleaning, 179stat ic RAM (SR AM ), 170stators, 17step-down transformer, 27step-up transformer, 26-27stepper/stepping motor, 17-18stop bits, 119storage temperature, 6surface-mount d evices

resistors, 19,1 9, 22, 22soldering techniques, 48-50, 49transistors, 36-37

switches or contact sets, 15sealed switches, control panel, 172-173,

172switching supply (s ee pow er supplies,

switching supply)system clock/master clock, 175

TTandy LP400 laser printer, 42Tandy L P800 laser printer, 137tape drives, 3taps, transformers, 94technical data: manuals, schematics, etc.,

68-69temperature limits, 92temperature-alarm circuits, 146-147,147temperature-proportioning circuit, 147,147temporary m emory, 170-171test instruments (see multimeters; oscillo

scopes)thermistors, 146thermoprotector (thermal switch), 123thin quad flat pack (TQFP), 40through-hole ICs, 39-40time measurement, oscilloscopes, 64, 64timing signals, parallel interface, 160,161tinning the soldering tip, 47tolerance rating, capacitors, 20toner, 7-8, 79-80

developing process, 82-83, 83fusing process, 84, 84redistributing toner in EP cartridge, 92, 92TO NE R L OW message, 152-153, 190toner sensors, 152-153,153

tools, hand tools, 43-44torque, 12,17torsion springs for registration rollers, 143trace line, 85transfer corona assembly, 83, 83

cleaning, 178-179transfer guide, cleaning, 179transfer process, 83-84, 84transformers, 25-27, 26

coils, 25core, 94



Index 205

current characteristics, 26-27efficiency rating, 96induced ac signals, 26isolation transformers, 70-71, 95magnetic coupling, 26, 94permeable materials, 94potential voltage, 96pow er supp lies, ac/dc linear, 94-96, 94, 95

primary winding, 25, 95secondary winding, 25, 95step-down transformer, 27, 95step-up transformer, 26-27, 95taps, 94turns ratio, 26voltage characteristics, 26-27

transistor-transistor logic (T T L ), 58transistors, 33-38, 34

base, 33bipolar transistors, 33case styles and sizes, 36, 36collector, 33drain, 35driver circuits, 33-34, 34emitter, 33field effect transistor (F E T ), 33, 35, 35gate, 35JEDEC specifications, 37, 37markings on transistors, 37-38, 37MOS transistors, static electricity damage,

72-73, 72, 73MOSFE T, 35NPN transistors, 33optocouplers, 34optoisolators, 34, 34phototransistors, 34PNP transistors, 33replacing transistors, 35-36source, 35static electricity damage, 72surface-mount transistors, 36-37testing transistors, multimeters, 57-58, 57

triacs, 32, 32trigger signal, SCR, 31troubleshooting, 67-76, 184-191

badly formed characters, 182BEAM D ETEC TION error message, 120-

121,121,186BEAM LOST error message, 121-122, 186blacked out page with horizontal white

stripes, 134-135, 188blacked out pages, 124-125,124, 187blotched pages, 124-125, 187CHECKSUM ERROR message, 118, 186computer/printer communication errors,

119,186control panel, 190-191

indicator light erratic/inoperative, 173,191inoperative panel, 173, 190keypad operation intermittent or d efec

tive, 173,191LCD display erratic, 173,191

dark images, 114, 115, 182,186dead unit, 103,110-112,117-118,184,

185,186disassembly steps, 74-75, 75distorted images, 132-133,133,188dropouts, 181electrostatic discharge ( ES D) , 71EP cartridge, 152,190

NO EP CARTRIDGE message, 152, 190TONER LOW message, 152-153, 190

equipotential bonding, static electricity, 74erratic operation, 1 06-107,113-114,177,

184-185, 191

faint images, 125-12 6,125, 130-131,187,188

flowchart of universal troubleshootingprocess, 65-66, 65

fogged printouts, 91-92freezed operation, 177,191fusing assembly, 122-124,187

SERVICE error message, 123-124, 187

temperature control ineffective, 107,149,185,189horizontal black lines through page, 129-

130, 130, 187-188identifying problem, 66-67image-formation p roblems (see poor-qual

ity printouts)indicator light erratic/inoperative, 173,191intermittent operation, 105-106, 112-113,

184.185isolating problem, 66-67jammed printer, 140-145, 182, 183, 189keypad operation intermittent or defec

tive, 173, 191laser delivery system, 119-122,186

BEAM DET ECT ION error message, 120-121.186

BEAM LOST error message, 121-122,186SCAN BUFFER error, 122

laser safety precautions, 121LCD display erratic, 17 3,191light images, 114, 115, 130-131,182, 186,

187, 188main logic board, 176,191

erratic operation, 177, 191freezed operation, 177, 191no initialization, 176-177, 191

memory, 171memory overloads, 182-183motors, 189

SCA NNE R ERRO R message, 151, 189stuck main motor, 151, 189

NO EP CARTRIDGE message, 152, 190

no initialization, 176-177,191overprinting, 145-146,146,189PAPER JAM message, 140-145,189PA PE R OUT message, 138-140, 149, 188-

189paper problems, 188-189paper-control assembly

overprinting, 145-146, 146, 189smudged band on page, 145-146,146, 189

parallel interface, 164-167,190PRINTER NOT READY error message,

164, 190poor-quality printouts (see poor-quality

printouts)power supplies

ac/dc linear, 184-185high-voltage, 114, 115, 186switching-supply (dc), 185

PRINTER NOT READY error message,164, 167-169, 190

PRINTER OPEN message, 149-150, 150,189

reassembly steps, 75-76registration p roblems, 128-129,128, 187repair vs. replace, 67-68repetitive defects on page, 133-134, 134,

182,188retesting, 68right-hand text missing or distorted, 127-

128,128, 187rough or "suede" print, 131. 131, 188SCAN BUFFER error, 122SCANNER ERROR message, 151,189

sensor/interlock system, 146-150, 189fusing temperature control ineffective,

149,189PAPER OUT message, 149, 189PRINTER OPEN message, 149-150, 150,

189serial interface, 167-169,190shock hazards, 69-71

ac/dc linear pow er supplies, 103, 105,106, 107switching-supply (d c) , 108, 111, 112

sized-wrong images, 136, 188skewed images, 135-136,135,188smeared print, 131-132, 132, 188smudged band on page, 145-146, 146, 189speckled pages, 79, 126,126, 187staining of page, 181start-up problems, 1 17,186static charge in paper, 183static electricity damage, 71-74, 72. 73stuck main motor, 151, 189symptoms, 66technical data: manuals, schematics, etc.,

68-69TON ER L OW message, 152-153,190variable pitch whirring noise, 151vertical black streaks on page, 136,136,

188vertical fade, 180-181vertical lines on page, 181vertical white streaks on page, 126-127,

127, 187warm-up mode continuous, 118, 186

truth tables, 38turns ratio, transformers, 26

uuniversal asynchronous receiver/transmit

ter (U AR T) , 163, 164

Vvalue rating, capacitors, 20vendors/manufacturers listing, 192-195vertical black streaks on page, 136,136,188vertical fade, 180-181vertical lines on page, 181vertical white streaks on page, 126,18 7very small outline package (VSOP), 40volatile or RAM memory, 170-171volt-ohm-milliammeters (VOM), 51voltage measurement

multimeters, 52, 52oscilloscopes, 62-63, 63

voltage regulators (see regulators)

wwarm-up mode continuous, 118,186wave soldering, 49wettable vs. nonwettable soldering tips, 46Wheatstone b ridge rectifiers, 97-98Windows, 4wire cutters, 44working voltage rating for capacitors, 24wrenches, 43writing mechanisms, 8, 79, 81, 84-90, 85

lasers, 85-88, 86, 87light emitting diodes (LED), 88-89, 88, 89liquid crystal shutter (L CS ), 89-90, 89

zzener diodes, 29-30, 29

breakdown voltage, 29clamping circuit as regulators, 101



Abo ut the Author

Steph en J . B ige low is an e lect r ica l eng inee r specia l izing in the repai r o f compu ter and te leco mm unicat ions equ ipment . His ar t ic les f requen t ly appear in Radio Electronics, Popular Electronics,and Mode rn Electronics. H e is the author of seve ral Mc Gr aw-H ill boo ks, including Maintain and

Repair Your Computer Printer and Save a Bundle, Maintain and Repair Your Notebook,Palmtop, or Pen Comp uter, Troubleshooting and Repairing Com puter Printers, an d Troubleshooting and Repairing PC Drives and Memory Systems.



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If you ' r e t i red o f be ing a t the m ercy o f repa i r p ro fess iona l s , th i s bh a s t h e p o w e r t o l ib e r a t e y o u b y t el li n g y o u h o w t o m a i n t a i n ,t roub leshoo t , and repa i r a l l o f the mos t popu la r l a se r and LED pr in teF o r t h o s e wi t h m o d e s t t e c h n i c a l s k i l l s, t h i s ma n u a l ' s s t e p - b y - s t e pi n s t r u c t i o n s , n u m e r o u s i l l u st r a t io n s , a n d h e l p f u l t r o u b l e s h o o t i n g a n drepa i r cha r t s make i t poss ib le to iden t i fy ma l func t ions and co r rec t d o z e n so f sy m p t o m s .

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