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THE PROFESSIONAL MAGAZINE FOR ELECTRONICS AND COMPUTER SERVICING

G1E0'1'11011'0Servicing & Technology JUNE 1994/$3.00

Solving VCR servo system problems

Understanding the RS232(C) serial interface

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Contents

CASSETTELOADING

MOTOR C.B.A.

S144 S143CASSETTE CASSETTEUP SWITCH DOWN

SWITCH

page 46

FEATURES8 Test equipment update

By The ES&T StaffEveryday, we see something thatindicates how consumer electronicshas changed rapidly over the pastdecade and how it will continue tochange. Much of this has benefitedconsumer electronics service cen-ters that are now thriving on serviceof VCRs, camcorders, microwaves,and even computers. What testequipment should a technician useto service these items?

9 The general purposebench oscilloscopeBy Jerry MurphyThis article will propose a set of teststhat can be performed on an oscil-loscope to determine its suitabilityto the needs of the troubleshooterand will also describe how variousvendor's specifications can betranslated into meaningful selectioncriteria.

16 Solving VCR servosystem problemsBy Arthur FlavellThis article will familiarize you withthe components of servo systems andtheir operation in the VHS format.

THE PROFESSIONAL MAGAZINE FOR ELECTRONICS AND COMPUTER SERVICING

EL OTR©D1GServicing Technology

Volume 14, No. 6 June 1994

SHORTCIRCUIT

page 60

24 Understanding the RS232(C)Serial InterfaceBy Gregory W. JonesThis article explores RS232(C)including a discussion of timing,control and data signals, and thenexplains how the signals are used toestablish communications.

46 Readout and tape loadingproblems in RCA/Hitachi/Sears VCRsHere we look at the VCRs manu-factured by Hitachi about eightyears ago that often have failedclock/readout displays. This is usu-ally caused by the failure of thedc/dc converter, which provides24V for the display.

52 Electrical safety forservice personnelBy Ron C. JohnsonOf all the people who work withelectricity on a daily basis, elec-tronic service personnel should bethe most aware of the risks involved.When we don't take that time forconsideration, that's when accidentshappen.

DEPARTMENTS2 Editorial

4 News

6 Literature

22

31

55

56

Products

Profax

Test Your Electronics Knowledge

Audio CornerRadio Broadcast Data System to betest marketed nationally

58 What Do You Know AboutElectronics?Electronics speak

60 Video CornerSolving a VCR short circuit

62 Business CornerPart 9 of the Total QualityManagement series

63 Books

64 Troubleshooting Tips

65 Classifieds/Readers' Exchange

68 Advertisers' Index

ON THE COVERThe world of consumer electronics con-tinues to expand and evolve. Whileexisting test equipment continues to beuseful in troubleshooting, manufactur-ers continue to update their products tohandle the latest in circuit technology.Don't throw away that old DMM oroscilloscope, but don't ignore the offer-ings of the latest most functionaldevices. (Photo courtesy of 3M.)

June 1994 Electronic Servicing & Technology 1

Editorial

Being aninventorisn't what itused to be

One of the things that characterizetoday's world of consumer electronics isthe sheer magnitude of invention thathas gone into it, and continues to causeit to expand and evolve. Take televisionas an obvious example. In little morethan fifty years, television has evolvedfrom a medium of mass communicationthat consisted of gray images on a smallscreen, watched by a relative handful ofviewers, to the bright, colorful, largescreen, stereo sound medium watchedby most people throughout the devel-oped world.

The sheer number of inventions thathad to be conceived, developed andbrought to market to make today's large -screen, color, remote controlled televi-sion, delivered via broadcast, cable, orsatellite, and with stereo sound, possibleis mind boggling. It would be interest-ing to try to come up with a catalog ofthose inventions and developments.Here are just a few:

the color picture tube the NTSC television signal LCD video displays stereo audio TV broadcast projection TV optoelectronics ICs of all kinds video tape recording

A little further research and reflectioncould probably extend this list so that itwould fill this issue and go on and onbeyond it.

But some other areas of consumerelectronics would probably yield aneven longer list of inventions. Take, forexample, the laser and compact discplayers. The patent office must haveentire filing cabinets given over to thestorage of patents related to digitalrecording. There's the recording processof the disc itself, the method by whichthe pits are generated. There's the laserhead, the read process, and the servo sys-tem that keeps everything on track. Andthose are just the obvious areas.

So who are the people who inventedall of these things?

If we look back in history at some of

the historical inventions, we know whothe inventors are. The most obviousexample is Thomas Edison. Everyoneknows that name. Of course part of thereason that we know him is the sheer vol-ume of his inventions, many of whichwere basic inventions-inventions ofentire new technologies.

For example, Edison, known as theWizard of Menlo Park, single-handedlyinvented voice recording and electriclight. He was tireless and persistent inhis quest for materials with which hecould realize his inventions. In his questfor a suitable material for a light bulb fil-ament he sent people all over the world.Of course, another reason we knowEdison so well is that he was a shame-less self promoter.

We know about another inventor,Alexander Graham Bell, because heinvented an entire basic technology:telephony. And he did it pretty much sin-gle handedly.

But how many of us know names likeBrattain, Bardeen and Shockley.They're the men who invented the tran-sistor, an invention that has had a farreaching effect on today's technology.And their names are known and arerecorded in the annals of invention, butthey don't ring through history like thoseof Edison and Bell.

Maybe it's less amazing when some-thing is invented by a committee. Or per-haps it's the fact that most of today'sinventions are developed by unidenti-fied scientists working anonymously forlarge corporations. Or perhaps it'sbecause the sheer volume of technolog-ical change and innovation tends tomask any specific development and itsdeveloper.

Whatever the case, inventions seem tocontinue to flow from the minds ofinventors, and we, the viewers, the lis-teners, the users of this technology, andthe servicers of the products, continue tobenefit from the products that theseinventions make possible.

2 Electronic Servicing & Technology June 1994

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NewsElectronics and appliance studentsto compete at VICA United States

Skill Olympics

Outstanding vocational students inelectronic products servicing and majorappliance technology programs from allacross the country will gather in KansasCity, MO, on June 29 and 30, 1994. Thestudents, all gold -medal winners in theirstates, will participate in the showcase ofvocational -technical education: the VICAUnited States Skill Olympics (USSO).

VICA, the Vocational Industrial Clubsof America, is the national organizationfor students in trade, industrial, technicaland health occupations education. It

sponsors the VICA U.S. Skill Olympicsannually to recognize the achievementsof vocational -technical students, and toencourage them to strive for excellence.Pride in their chosen craft makes for high-ly motivated and quality employees withgood attitudes toward work.

In Kansas City, 52 students will com-pete in electronic products servicing, and24 in major appliance technology. Work-

ing within time limits, the competitors arejudged by industry personnel as they puttheir equipment and skills to work onassigned projects, designed by a techni-cal committee of experts in the respectivefield. The committees acquire equipmentand supplies, obtain qualified judges, andset up the contests.

Walter Seymour, Electronic IndustriesAssociation (EIA), and Gerry Ganguzza,Sharp Electronics Corporation, co-chairthe electronic products servicing nation-al technical committee. Members in-clude: Frank Steckel, El Mueller, DonHatton, Brian Ott, and Elmer Poe, of theEIA; Tom Graff, Thomson ConsumerElectronics; Emmanuel Henry, Matsu-shita Services Company; Mike Brooks,Philips Consumer Electronics; Rich Po-lak, Maxtec International Corporation;Mike Derosa, Sony Corporation of Amer-ica; Don Multerer, Sencore; and SteveBurkhead, Toshiba America, Inc.

The major appliance technology na-tional technical committee is co-chairedby Charlie Roberts, GE Consumer Ser-

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vices, and Jerry Job, Sears Service Center.David Krueger, Sears Roebuck and Com-pany, is a member as well. They are assist-ed by a local committee of educators andindustry personnel.

The top three high school and postsec-ondary winners in each of 54 competi-tions receive gold, silver and bronze me-dallions from VICA. Many receive tools,textbooks, scholarships or special train-ing programs donated by business andlabor organizations, and by trade and pro-fessional associations that support theSkill Olympics.

Kansas site picked for nextETA/SDA seminars

The headquarters of ETA and SDA inGreencastle, IN has announced the loca-tion for the next jointly sponsored tech-nical training school. Neosho Commun-ity College in Chanute, KS will sponsorthe event on July 8 and 9, 1994. NCC'saddress is: 1000 S. Allen St. in Chanute.

The popular SAM-Satellite/Anten-nas/Master Antenna TV systems-schoolwill be produced by the Satellite DealersAssociation. A segment of the school willaddress DBS small -dish installation andservice, in addition to the core elementswhich teach installation, adjustment andmaintenance of C -Band and other TVROproducts. Antenna basics, overcoming in-terference, combining satellites and an-tennas into master antenna and cable sys-tem headends are included in this col-lege -credit two-day course.

The Electronics Technology School isalso a two-day event, running simultane-ously in a separate classroom at NeoshoCommunity College. Topics covered atthis school include TV and VCR servic-ing, computers, fiber optics, telephoneinstallation and servicing, and basic elec-tronics topics such as modem electronicspower supply servicing, and a session onindustrial digital controls.

For the first time, ETA will produce atwo-day school devoted entirely to theFederal Communications CommissionCommercial Radiotelephone OperatorLicense. A portion of this school will bedevoted to outlining the topics includedon the new GMDSS license exams, inaddition to covering all the topics includ-ed on the General Radiotelephone Oper-ator License. The G.R.O.L. is the most



popular license and once acquired allowsthe holder to service and operate radiocommunications equipment used in theaeronautical, maritime or land mobile in-dustries. The FCC Commercial LicenseExaminations will be offered at the con-clusion of the two-day FCC Exam schoolat Neosho Community College. Partici-pation at the school is not a requirementfor sitting for the examinations.

Certified Electronics Technician (CET)exams and Certified Satellite Installerexams will also be offered at the conclu-sion of the schools.

For complete class schedules of thethree schools, hotel and registration infor-mation, call ETA at 317-653-8262, orSDA at 317-653-4301. You can also writeto ETA Schools, 602 N. Jackson, Green-castle, IN 46135.

Home Electronics Installers'Association sets sites for first six

regional education programsCEDIA, the Custom Electronic Design

& Installation Association, is conductingeducational programs for home electron-ics installers in six cities. These programs

began in early May and will run to the endof July, an official announced. Similartwo-day programs covering topics inhome entertainment and systems integra-tion, among other areas, will be held inLos Angeles, Orlando, Chicago, Seattle,Toronto, and Philadelphia. The programsare open to non-members of the associa-tion at a higher cost.

"These regional seminars will givemany installation companies a chance tooffer their employees professional train-ing without leaving their local areas andat modest expense," said Rob Gerhardt,chairman of the association's EducationCommittee. "Based on the initial feed-back, this is going to be a very popularprogram."

Seminar sessions will vary in lengthfrom one to four hours, depending on sub-ject matter, and will include courses pre-sented by CEDIA's Home EntertainmentCouncil and Systems Integration Coun-cil, as well as those presented by individ-ual manufacturers. In order to provide in-dividual instruction, each course presen-tation will be limited to six to eight atten-dees, with sessions frequently repeating

the courses during the two-day event.Attendance fees for CEDIA members

are set at $40, $60 for non-members, andwill cover as many of the manufacturers'courses as the registrant cares to attend.CEDIA's HEC and SIC sessions will beavailable to all at an additional cost.

Specific programs will be announcedat a later date. Dates and cities are:

Seattle, July 10-11Toronto, July 17-18Philadelphia, July 24-25

CEDIA is a national trade associationof companies which specialize in plan-ning or installing electronic systems forthe home-typically, single- or multi -room home entertainment systems, hometheaters, media rooms, home automation,security systems, communication systems,and other residential electronics. The as-sociation was founded in September, 1989,and has approximately 500 members.

For further information, contact Billi-lynne Keller, executive director, CEDIA,8335 Allison Pointe Trail, Indianapolis,IN 46250, or call 1-800-CEDIA-30.

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AMREL LPS-300 Series - Offer Features AndPrices That The Competition Can't Beat! Microprocessor controlled. User friendly keypad data entry. Low output noise rating less than 1 mV. Line/Load regulation rated less than 2mV. Output enable/disable and Power off memory. 2 year warranty. Optional RS -232 interface-capability.

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VIE PROFFSPONAL INAGADME FOR ELF< Tr.LiteratureBrochure describes purification

system for soldering fumesA six -page brochure describing Hexa-

con Electric Company's new Purex Mul-ti -position Arm Fume Purification Sys-tem is now available. This system is wellsuited for handling soldering fumes andis available in a wide range of models toaccommodate any number of operators.

The environmental filter removes allparticulate debris down to 0.3 micron and95% of particles down to 0.01 micron.Harmful toxic gases are purified in a chem-ical cartridge. The expelled air has a purifi-cation factor of 99.997%, and, if desired,can be recycled back into the work place.

The literature describes the company'sreverse airflow which increases filter lifeand efficiency.


Catalog of test equipment, toolsand supplies

This 244 -page catalog from ContactEast lists test instruments and tools forengineers, managers, technicians, and

coniatAt eastSAME DAY SHIPPINGNext nay Delivery. anywhere in the U.S.A.!

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hobbyists. Featured are quality productsfrom brand -name manufacturers for test-ing, repairing, and assembling electronicequipment. Product highlights includenew DMMs and accessories, solderingtools, custom tool kits, EPROM pro-grammers, power supplies, ELF meters,helpful reference books, breadboards,scope meters, datacom tools and testers,adhesives, measuring tools, precisionhand tools, portable and bench top digi-tal storage scopes. Also included are thecompany's lines of communication testequipment, soldering/desoldering sys-tems, static protection products, ozonesafe cleaners, magnifiers, inspectionequipment, workbenches, cases and more.


Power supply brochureFive digital power supplies, and six an-

alog units, are featured in a four -page bro-chure from American Reliance. Each pow-er supply is full -featured and easy -to -use.

The LPS-300 Series linear units are po-sitioned as the most accurate power sup-plies in their class today, according to themanufacturer. All controls are front panelmounted pushbuttons. An RS -232 inter-face is available. Single, dual and tripleoutput models are also available.


Course schedule releasedBMI releases their education calendar

for 1994, including classes and lab in-struction on power quality, power man-agement, and harmonics, taught at theirheadquarters in Santa Clara, CA. Coursesinclude: Problem Solving with the 4800PowerScope, Problem Solving with the8800 PowerScope, The PowerProfiler:Use and Application, The PQNode: Pro-gramming and Analysis, Power Qualityfor the Industry Professional, AdvancedPower Quality Lab, Power Managementfor the Industry Professional, and End -Use Power -Line Harmonics.


ELECTRONICServicing & Technology

Electronic Servicing & Technology is edited for ser-vicing professionals who service consumer electronicsequipment. This includes service technicians, field ser-

vice personnel and avid servicing enthusiasts who repair

and maintain audio, video, computer and other con-sumer electronics equipment.

EDITORIALNils Conrad Persson, EditorLinda Romanello, Assistant Editor

CONSULTING EDITORSHomer L.Davidson, TV Servicing ConsultantVictor Meeldijk, Components ConsultantJohn E. Shepler, Audio ConsultantSam Wilson, Electronics Theory Consultant

PRODUCTIONElizabeth Ryan, Art DirectorBarbara Terzo, Assistant Art DirectorSusan Reale, ArtistEdmond Pesonen, Electronic Composition Mgr.Dorothy Kehrwieder, Production ManagerEmily Kreutz, Advertising ProductionPat Le Blanc, Phototypographer

BUSINESSRichard A. Ross, PublisherJohn Dorr, General ManagerFrank V. Fuzia, ControllerCatherine Ross, Circulation DirectorMelissa Nitschke, Data Processing ManagerCarol Licata, Data ProcessingDenise Pyne, Customer Service

SALES OFFICEElectronic Servicing & Technology76 N. Broadway, Hicksville, NY 11801516-681-2922; FAX 516-681-2926

Diane G. Klusner, Director of AdvertisingEmily Kreutz, Sales Assistant

EDITORIAL CORRESPONDENCE:P.O. Box 12487Overland Park, KS 66212913-492-4857

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Electronic Servicing & Technology (ISSN 0278-9922)is published 13 times a year by CO Communications,Inc. 76 N. Broadway, Hicksville, NY 11801. Telephone(516) 681-2922. Second class postage paid atHicksville, NY and additional offices. Subscriptionprices (payable in US dollars only): Domestic-oneyear $24.75, two years $45. Foreign countries-oneyear $30.75, two years $57. Entire contents copyright1994 by CO Communications, Inc. Electronic Servicing& Technology or CO Communications, Inc. assumes noresponsibility for unsolicited manuscripts. Allow sixweeks for delivery of first issue and for change ofaddress. Printed in the United States of America.

Postmaster: Please send change of address notice toElectronic Servicing & Technology, 76 N. Broadway,Hicksville, NY 11801.

CO Communications, Inc. is publisher of CO The RadioAmateur's Journal, Popular Communications, Micro -Computer Journal, CO Radio Amateur (Spanish CO), COAmateur Radio Equipment Buyer's Guide, CO AmateurRadio Beginner's Buyer's Guide. Popular Communica-tions Communications Guide, and Electronic Servicing& Technology.


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Test equipment updateBy The ES&T Staff

Every day the things we see in the retailstores, in our homes, in newspapers, inmagazines, and other sources present uswith evidence that consumer electronicshas changed rapidly over the past decade,is changing now, and will continue tochange dramatically over at least the re-mainder of our lifetimes. The advances intechnology, the potential for companiesto profit from these new technologies, andthe seemingly insatiable appetites of thepublic for new electronic gadgetry and theinformation, entertainment and conve-nience they provide are driving factors inall of this change.

The effect of technological changeon service

Much of this change has benefited con-sumer electronics service centers thathave had the vision to learn about the newtechnologies as they have been intro-duced. Service centers that once were suf-fering from the continuing decline in thedemand for TV service are now thrivingon service of VCRs, camcorders and mi-crowave ovens. Yet other consumer elec-tronics service centers have opted to takeadvantage of the tremendous growth ofcomputer use by consumers and have be-come consultants/servicers for their cus-tomers in the areas of computer hardwareand software.

The test equipment

All of the test equipment in the typicalconsumer electronics service center isuseful in servicing the new consumerelectronics. A DMM can measure volt-ages in a personal computer, a video gamesystem or a cellular telephone as well asin a TV set. An oscilloscope can displaya waveform in a camcorder or headphonestereo, just as well as in a home stereoamplifier.

But the consumer electronics productsof new technology require that addition-

al test equipment be added to the con-sumer electronic service center. Not onlythat, but because of the increasing com-plexity and variety of today's consumerelectronics products, it is to the advantageof the service technician if the tradition-al instruments he uses feature increasedfunctionality and ease of use offered inthe latest instruments that will allow himto not only make more and more variedtests, but to concentrate on the product be-ing tested and not on setting up or reset-ting, or changing the test equipment in use.

Some of the new test equipmentIt's instructive, and even a little scary,

to take a look at the test procedures re-quired by the latest in consumer elec-tronics products that were not required ofthe older products, and the new test equip-ment that makes it easier to test them.

For example, the CD player has an in-frared laser pickup. Handheld remotecontrols for TVs, VCRs, audio equipmentuse infrared for transmission. Testing ofall of these infrared transmitting devicesis made easier using an infrared detectoror laser power meter.

Thorough servicing of camcorders re-quires a whole host of equipment that aservice center never had to have before:light box, waveform monitor, vector -scope, tension gauges and more.

Servicing of computers and peripher-als also requires its own special set of testequipment and other test products: diag-nostic software, logic probe and more.

More functionality in the traditionaltest equipment

Manufacturers of test equipment, rec-ognizing the changing needs of servicetechnicians and with technology andcomponents that were not available in thepast, have made their products more func-tional, more useful, and often, while mak-ing them easier to use, have sometimeseven lowered the cost.

For example, many modern oscillo-scopes offer some kind of digital readoutof waveform parameters such as voltagelevels, frequency and period. Some ser-vice oriented oscilloscopes offer a syncseparator built in to simplify triggering onthe TV video waveform.

DMMs, too, continue to become moreuseful, frequently smaller and lighter, andeasier to use. For example, as most tech-nicians are already aware, even inexpen-sive DMMs of today offer such advancedfeatures as autoranging, diode check, fre-quency measurement, capacitor valuemeasurement and more.

Some tips for this new technology

It's important to choose the right testequipment. Of utmost importance, ofcourse, is to make sure that the test deviceyou buy will do everything you'll need itto do. Also important is finding a testdevice that works the way you do, thatsupports the service function. The article"Choosing an oscilloscope for servicing"provides readers some food for thoughtwhen choosing a servicing instrument.

Yet to come

But these articles are only the prologueto the story that's still unfolding. Onlyrecently we, have published stories onsuch developing technologies as high def-inition TV (HDTV) as proposed by the"Grand Alliance," and the digital mirrordevice developed by Texas Instruments.And underway right now are actions thatare expected to result in delivery of infor-mation and entertainment to homes viafiber optic cable.

We don't have any information at thistime on what specialized test equipmentservice centers might need in order to ser-vice HDTV or to check fiber optic distri-bution in a home, but we'll be keeping ourears to the ground, and as it happens, we'llkeep readers posted.


The general purpose benchoscilloscopeBy Jerry Murphy

Most people who use an oscilloscopeuse an analog scope. There are two goodreasons for this. One reason is simply thatthere are a lot of analog scopes in exis-tence. The second reason is that the ana-log scope has performance characteristicsthat are well suited to the most commonscope application: troubleshooting.

In order for a digital scope to be a goodgeneral purpose troubleshooting tool itmust give its user superior performanceat an attractive price. Recent advances indigital oscilloscope technology have al-lowed them to move into the trouble-shooting arena. Now the choice of whichscope to use is more complex.

This article will discuss the needs of thetroubleshooter, and propose a set of teststhat can be performed on any oscilloscopeto determine its suitability to the reader'sapplication. It will also describe how var-ious vendor's specifications can be trans-lated into meaningful selection criteria.

TroubleshootingWebster's New World Dictionary of the

American Language defines trouble-shooter as "the person charged with locat-ing and eliminating the source of troublein any flow of work." So, the trouble-shooter is charged with finding the causeof problems. This could also mean thatthe symptom(s) of the problem areknown, but the cause of the problem isalways unknown.

In electronic troubleshooting the per-son performing the task of locating andeliminating the source of the problemusually starts with an in-depth knowledgeof the system's operation. This knowl-edge could be supplemented with theschematic of the system, its specificationsand other documentation.

The troubleshooter is searching forclues to the cause of the problem. So, inthe search for the unknown, the trou-bleshooter must sort through many obser-vations of the system's performance.

Murphy is a marketing engineer at Hewlett PackardPersonal Measurements Operation Colorado Springs, CO

Figure 1. This is the display of a signal on an analog scope using delayed sweep.

The first step in the troubleshootingprocess is to verify that there is, in fact, aproblem. The verification of the problemcould involve the use of measuring instru-ments to check the system under test forproper output or simply watching the dis-play or panel lights of the device undertest. If the problem is reproduced, thetroubleshooter will start the search for thecause of the problem. This search is oftenaided with the use of electronic instru-ments such as a scope and DMM.

Where should troubleshooting start?The troubleshooting process starts at a

point in the system at which the trouble-shooter feels it is operating correctly, andthen the search spreads out into the sys-tem until a suspicious condition is ob-served. It is not uncommon for the trou-bleshooter to make observations at manypoints and to make a decision as to thecorrectness of the observation in only afew seconds.

There may be a written troubleshoot-ing guide or (most likely) the trouble-

shooting procedure is present only in thetroubleshooter's mind. During this pro-cess the troubleshooter's mind is focusedon the circuit under test and not on the testinstruments being used.

If the instrument being used is an oscil-loscope the troubleshooter will simplyglance at the scope's display and quicklymake a judgment of the status of the cir-cuit's operation. Is the display as expect-ed? Does this display represent the causeof the problem or a symptom? What couldbe going on in the circuit to produce thisdisplay? These are some of the questionsthat the troubleshooter will ask as eachcircuit node is probed. The parameters ofthe suspicious signal are not known at thestart of the process.

It is the skill and knowledge of the trou-bleshooter that is used to determine thata fault has been found. The troubleshoot-ing process can be summarized as theapplication of skill and knowledge in theoperation of electronic circuits to isolateand correct faults.

This process is performed in R/D labs


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Figure 2. This is the display of a signal on a digital scope using delayed sweep.

and on production lines, as well as in cus-tomer electronics service centers. Trou-bleshooting is performed by R/D engi-neers and technicians in the developmentof new products, production engineers,technicians, and service technicians.

The troubleshooter's scopeNow that we have an understanding of

the troubleshooting process we can estab-lish the needs of a troubleshooter's scope.In the search for the cause of the problem,many waveforms will be viewed by thetroubleshooter. These waveforms can befound at various locations in the circuit orsystem being tested. Input and outputports, identified test points, and circuitnodes are all places where the trouble-shooter will look.

The waveforms will range from verysimple, such as a power supply's dc level,to very complex control signals in a mi-crocontroller system. The signals viewedin the process of problem isolation canrange from being continuous to infre-quently occurring. Sometimes the ab-sence of a waveform at a circuit node isthe clue to the solution of the problem.

When the troubleshooter places thescope probe on a circuit node, the result-ing display is compared to the trouble-shooter's expected result. The expectedresult is based on a knowledge of the cir-cuit's design and electronic theory. For

example, if a sine wave is observed at theoutput of a power supply's diode bridge,the troubleshooter suspects that the diodebridge is shorted.

So, the troubleshooter's scope musthave a display that accurately reproducesthe waveform at the probe tip. This meansthat the troubleshooter's scope must havea display that is free of aliases and otherdistortions that can be produced in thescope's conversion of the signal at theprobe tip to the display. Nothing can senda troubleshooter off on a time wasting baddirection than a false or alias scope dis-play. This means that if the waveform ischanging the scope's display must followthose changes.

If the waveform is infrequently occur-ring, the display must be bright and clear-ly visible so that the troubleshooter cansee the problem.

Analog scopes are goodtroubleshooting tools

Analog scopes continue to be used introubleshooting applications becausethere are so many of them around andtheir direct beam vector display provideswaveform viewing accuracy. When thewaveform changes, that change is reflect-ed in real time on the scope's display. Thismeans that the analog scope's display isalways alias free and correctly representsthe waveform at the probe tip.

This display is not without drawbacks.At time base settings of 1ms/sec andslower the analog scope's display will beflickering. This flicker may cause eyestrain and the flicker will be worse as thetime base is operated at slower and slow-er sweep speeds (wider time windows).As the signal at the probe tip becomesmore infrequent, the analog scope's dis-play becomes dim. This can also result ifdelayed sweep is used to view the eventof interest (Figure 1). A digital scope willalways produce a bright display indepen-dent of the signal's frequency of repeti-tion rate or the scope's time base setting(Figure 2).

The troubleshooting procedure

Because the troubleshooter's mind isfocused on the problem in the circuitunder test and not on the operation of theinstrumentation, it helps if the trouble-shooter's scope has a control panel thatcan be operated without having to thinkabout it, in other words, a "minds off'front panel. "Minds off' means that thetroubleshooter can make required changesin the scope's front panel without havingto stop the troubleshooting thought pro-cess and think about the scope.

Often when there is a need to make acontrol panel change on the scope, thetroubleshooter will simply grab a knobwithout looking at it. The knob is thentested to see if it is the desired one. Thistest is performed by simply moving theknob a little. If the desired result is notobtained that knob will be returned to itsprevious position and another knob willbe grabbed and tested. Once the desiredreaction is observed, the knob will be ad-justed to obtain the desired result.

A scope that reacts slowly to changesto its front panel controls may cause thetroubleshooter to think that the knob be-ing tested is not the desired knob when infact it might be. A front panel that is basedon icons and menus with a minimum ofknobs may not fit the troubleshooter'sneeds, because the troubleshooter's mindmust shift focus to the menus and iconsto make the desired control panel change.

Negative time triggeringand automation

In some of today's systems the only sta-ble trigger event available is at the end ofthe event of interest. For example, whentroubleshooting a keyboard scan prob-


tern, the problem is knowing the conditionof the keyboard control lines just beforethe key is pressed. The only trigger thatcan be established is the key being pressed.

A digital scope, which can look aheadof the trigger, gives the troubleshooter theability to view the status of the keyboardbefore the key was pressed. This abilityto look ahead of the trigger, sometimescalled negative -time viewing, is an ad-vantage for some applications of the trou-bleshooter's scope.

Measurement automation, that is, thenumeric display of waveform parameters,can be a valuable time saving tool in trou-bleshooting where decisions must bemade in seconds (Figure 3). Simply read-ing Vpp is much easier than countinggraticule lines. Some measurements suchas rise time can be made much faster andto better resolution with this feature thancounting minor divisions of the graticule.Measurement automation not only savestime, but the increased accuracy canlocate problems that might otherwise beoverlooked.

In summary, desirable characteristicsof the troubleshooter's scope include:

bright flicker free display alias free display direct access "minds off' control panel instant control panel response negative time measurement automation

Oscilloscope specificationsThe oscilloscope's performance speci-

fications determine its suitability for aspecific troubleshooting application. Thescope's vertical system must have a wideenough bandwidth to display the highestfrequency signals in the system beingtested. The high frequency componentsof the signals in the system must be con-sidered. For example a 50MHz clock con-tains components well in excess of 200MHz. Such a signal can be viewed witha 100MHz scope but a higher fidelity dis-play will be obtained with a 500MHz scope.

Usually a three -to -one ratio of scopebandwidth to signal speed will producean acceptable display. For higher fidelity,a display that can show the signal's rise -time, overshoot, and other small pertur-bations in the signal, requires a ratio offive to one or greater.

The effects of bandwidth on the dis-played waveform are usually a trade off

of display fidelity and price. Higher band-width gives higher fidelity but at a high-er price. Some people think that they needmore bandwidth than they actually do.This is because high bandwidth analogscopes had more advanced CRTs withbrighter displays than similar models butwith lower bandwidths.

This is not true of digital models be-cause their display is not based on drivingelectrons into the phosphor with greatvelocity. So if you're choosing a digitalscope for a troubleshooting application itisn't necessary to spend extra money to

buy extra bandwidth. Simply choose abandwidth to display the fastest signallikely to be found in the system under test.

The vertical systemThe vertical system needs to have

enough sensitivity to produce a usabledisplay of the smallest signal likely to beencountered. For most digital based sys-tems. a scope with 5mV/div sensitivitywill be acceptable. In analog and mixedsignal systems the scope should be ableto produce at least a one division high dis-

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play of the smallest signal likely to beencountered.

In addition, the vertical system musthave enough dynamic range so that thedisplay won't be distorted in situationswhere the signal is so large that it morethan fills the screen. Don't overlook thefact that a 10:1 divider probe is often usedto view even the smallest signal.

Oscilloscope coupling

A selection of ac and dc coupling is re-quired as well as the ability to limit thechannel's bandwidth to minimize the dis-play of interfering high frequency sig-nals. In digital scopes, averaging can beused to remove noise while maintainingthe channel's full bandwidth. Averagingwill, however, reduce the scope's displayupdate speed.

The troubleshooting scope needs tohave a range of sweep speeds that willproduce a displayed time window thatcovers all of the events of interest in thesystem under test. The narrowest windowwill be used to display events such asclock risetimes.

This equates to a sweep speed that israther fast, something on the order of afew nanoseconds per division. A 100MHzscope needs a maximum sweep speed ofat least 5ns/div in order to display the risetime of common TTL logic. On the otherextreme the scope must have a windowwide enough to display both the starts andends of control cycles. This equates to arather slow sweep speed, something onthe order of seconds per division.

Scope triggeringMost troubleshooting scopes have a

triggering system that picks the triggerpoint from the voltage level and slope(rising or falling) of the signal. The rea-son that more powerful triggering is notfound in most scopes used for trouble-shooting is the added complexity thatcomes with the added power of advancedtriggering. Scopes that are designed forsingle shot analysis must have the ad-vanced triggering features in order to iso-late events but the troubleshooter is moreinterested in viewing as much waveformdata as possible in the minimum time.Often the troubleshooter will be operat-ing the scope in an untriggered mode.

When triggering is required, it must beeasy to set-up and repeat. For example, asignal that requires the use of HF Reject

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Figure 3. Measurement aJtomatior. that is, the digital display of numeric values for wavefcrmparameters whether on the display itself or a separate reaaout, can speed the trouoleshcot.lgprocedure.

to produce a stable display is a symptomof high -frequency interference. That in-terference might be caused by an exter-nal source such as a nearby FM broadcaststation or coupling of a high-speed clock.

Scope performance parametersThe electrical performance needs of a

troubleshooting scope can be summar-ized as follows:

bandwidth at least 3x the highest fre-quency in the system being tested

vertical sensitivity to produce a fulldivision high display of the smallest sig-nal found in the system being tested

Figure 4. Acquisition dead time, also called blird time, is the scope performance 'actor thatdetermines the scope's acil.ty to capture infrequently occurring glitches.


ac and dc coupling as well as band-width limit in the vertical

fast enough sweep speed to displaythe rise time of critical signals

slow enough sweep speed to displaythe full-time window of critical cyclesand events.

stable and repeatable triggering

Understanding banner specificationsThe basic electrical performance spec-

ifications of the troubleshooting oscillo-scope are easy to understand by most peo-ple who are selecting a scope. The digitaloscilloscope adds two new specificationsto the selection process: maximum sam-pling rate and memory depth.

All digital oscilloscope vendors de-scribe their product in terms of its maxi-mum sampling rate but no vendors pro-vide a chart of how their product performsat less than its maximum sweep speed.The fact is that all digital scopes are basedon a fixed memory depth. As a result, asthe sweep speed is reduced from its max-imum setting, the sampling speed must bereduced so that the memory will be filledwith the full time record.

For example, a scope with a 1,000 pointmemory depth and a time base setting of

lms/div must place its 1,000 data pointrecord across a 10ms window. This meansthat at 1 ms/div, this scope must take asample every 1/1000ms or 1 s. This per-formance has no relationship to the scope'smaximum sampling speed specification.That banner specification might be as fastas possible given the technology beingapplied by the vendor.

So the question that must be asked isnot "how fast can the scope sample?," but"how fast will it sample at the sweepspeed that I need to display the signals ofinterest in my troubleshooting applica-tion?" It can be seen from this simpleexample that there is a linear relationshipbetween sampling rate and memory depth.A deeper memory scope will be able tosample faster at a given sweep speed thananother scope with less memory.

A third specification that is added to theselection process with the addition of dig-ital scopes is display update speed. Howmuch data can the scope move from theprobe tip to the display in a given time?Analog scopes connect the probe tip tothe CRT phosphor screen directly. Analogscopes usually have a higher display up-date rate than their digital counterparts,but analog scopes don't operate without

dead time. The dead time of the analogscope is caused by sweep retrace, re -armtime and trigger hold -off. These times canadd up to give the analog scope a 50%display time duty cycle.

Waveform processing indigital scopes

Digital based scopes gather the wave-form at the probe tip and then operate onthe waveform data with a computer (ormicroprocessor). The resulting display isproduced by the CPU operating on thedata it receives from the probe tip. Oscil-loscopes that are based on a single CPUdon't have enough processing power tomove data from the probe tip to the dis-play in a real-time manner. Scopes likethis will operate on a cycle of gather, thenprocess, and then display the waveformdata. You might compare this operationto that of a slide projector. It is easy to un-derstand that changes in the waveformcan be easily missed by such a system.

Only those digital scopes based on mul-tiple processors have enough processingpower to be able to produce real-time dis-plays of the waveform at the probe tip. Amultiple processor scope will operate sothat data is gathered by an acquisition

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Figure 5. An analog oscilloscope is a good troubleshooting tool over a wide range of conditions.There are parts of that range, however, where the display may be dim or flicker.

processor and placed into waveformRAM. A display processor will take thedata from waveform RAM and place itinto the correct position in the bit map ofthe display. A CPU scans the front panelfor control panel inputs, remote controlcommands, and performs software taskssuch as calculations of RMS values.

Such a multiple processor system iscapable of placing data on the CRT atspeed limited only by the CRT's plottingspeed. Such a system will be able to trackchanges in the waveform and it will looklike a real-time analog display of the samewaveform. It is possible to unload thewaveform RAM so fast that there is nodead time in the process.

Capturing glitchesAcquisition dead or blind time is the

performance factor that determines thescope's ability to capture infrequently oc-curring glitches, not maximum samplingspeed (Figure 4). If the scope in questioncan only move small packets of data tothe display in bursts of high-speed trans-fers with a long time to set-up to gatherand transfer the next small packet,changes in the waveform will be missed.Such a scope will have a harder time cap-turing a randomly occurring glitch that iscontained inside another waveform thananother scope that has a lower maximumsampling rate but has a shorter blind time.

To evaluate a scope's ability to be agood troubleshooting tool based on itsbanner specifications the following ques-tions must be asked:

What is the sampling speed as a func-tion of sweep speed?

How deep is the scope's memory? How long is it blind in a typical acqui-

sition cycle?

Troubleshooting scopeperformance tests

Are there some simple tests that can beconducted to determine how well a scopewill be able to perform as a good trouble-shooting tool in your lab (Figure 5)? Theanswer is "yes," and these tests should beadded to any performance specificationthat you or your company might issue asa part of the purchase process. These testsshould be added to the classic electricalperformance tests of bandwidth, sweepspeed, trigger sensitivity and accuracy.The following tests can be used as a guideto development of tests that best fit yourneeds.

1. Display fidelity: Can the oscillo-scope correctly display a changing sig-nal? The amplitude modulated carrier isa good signal for this test. Set the carrierfrequency to a value well within the band-width of the scope, 25MHz is a goodchoice, and set the modulation to a levelthat will be easily recognized as beingcorrect from the scope's display. Either50 or 100% will be good choices.

Simply connect the signal generator'soutput to the scope and press its autoscalekey. The scope will exit its autoscale rou-tine trying to trigger on the carrier fre-quency. Reduce the sweep speed until atleast two cycles of the modulation can beobserved. You might need to raise the trig-ger level to near the top of the modula-tion to get a stable display of the wave-form. Using external triggering to trigger


on the modulating signal will make thetriggering easier to set up.

Does the resulting display look correct?If not, the problem is either caused by atriggering problem or the scope's displaysystem can't follow this changing signal.

Does the display slow down if an auto-matic measurement such as frequency orVPP is selected?

2. Display aliases: As the scope'ssweep speed is moved away from itsfastest settings, will the scope displayfalse or alaised displays?

Connect the scope's probe to the probecalibrator output and press autoscale.Does this display appear to be correct? ifit does, simply reduce the sweep speedone step at a time and check that the dis-play remains correct. Does a lower fre-quency display ever appear in the dis-play? If so, this is an alias of the correctwaveform and the scope in questionmight produce similar incorrect displaysof unknown signals in your system.

3. Control panel reaction time: Howmuch time does the scope require to reactto a control panel change?

Connect the scope's probe to the probecalibrator output and press autoscale. Ifthe resulting display is correct, turn thechannel's position control to move thetrace up one division. Does the trace moveat the same time as the knob or is there adelay between your knob movement andthe resulting trace movement?

Change the channel's deflection factortwo settings. Again does the trace changesize as the volts/division knob is beingchanged or is there a time lag? Performthe same test in the time axis by chang-ing the time per division six positions.Again, does the trace change with the con-trol or is there a time lag?

4. Control panel operation: How manesteps must be performed to make a changein the set-up of the scope? Record thenumber of steps that must be followed toperform the following tasks:

While displaying two channels, movechannel 1 up one division and channel 2down one division. Steps =

While displaying two channels, changethe trigger source from channel 1 to chan-nel 2. Steps =

While running, change the display fromnonstorage to storage. Steps =

Erase a stored display. Steps =

Perform a frequency measurement onthe waveform being displayed on chan-nel one. Steps =

The scope that requires the fewer stepsto perform these common tasks will be aneasier scope to use in day to day trouble-shooting applications where you don'twant to stop and think about the scope'scontrol system.

Closing summaryDon't be misled by some banner spec-

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mance. Such a scope must have the elec-trical performance to correctly gatheryour most critical waveforms and displaythem with accuracy and fidelity that willgive you the information you need tomake the best decisions as to the opera-tion of the system under test.

In addition, the scope you chooseshould have a front panel control systemthat responds instantaneously to yourcommands, allowing you to keep yourmind focused on the circuit under test. Itshould also benefit you by saving youvaluable test time. You won't have to stopthe troubleshooting process to think abouthow to operate the scope.

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Solving VCR servo systemproblemsBy Arthur Flavell

Servo systems are essential to producingstable, noise -free video from a VCR.Their job is to regulate the rotationalspeed of the cylinder and capstan motors.and to keep them synchronized with a sta-ble reference signal. Servo problems maycause symptoms such as jitter in the pic-ture, loss of sync, or noise bands in the re-produced video.

Servicing servo circuits in a VHS VCRrequires an understanding of both circuitoperation and VHS fundamentals. Thisdiscussion will familiarize you with thecomponents of servo systems and theiroperation in the VHS format.

See the sidebar for an analysis of thecomponents of the servo system of a VHSvideo cassette recorder.

VHS formatThe VHS format uses three separate

areas of the video tape on which to recordand play back information (Figure 1). Alinear track along the upper edge of thetape contains the audio information. Vid-eo information is found in the center por-tion of the tape on helical tracks. Eachtrack contains one field of video signal.A linear track along the bottom edge ofthe tape contains CTL, or control track,synchronizing pulses.

To ensure compatibility with NTSC(National Television System Committee)video systems and interchangeabilitywith other VCRs, the timing of the sig-nals recorded on tape must be precise andconsistent. The electronic circuits of theVCR control the timing of signal gener-ation, but constant tape velocity is neces-sary to maintain the timing in the finishedvideotape recording.

Controlling tape transport speed is thejob of the capstan servo. The capstan mustmove the tape at a constant speed so thatthe audio and CTL tracks record and playback properly. It must also ensure that thevideo tracks are aligned under the video

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Figure 1. The VHS format uses three separate areas of the video tape on which to record andplay back information: a linear track along the upper edge of the tape contains the audioinformation, video information on helical tracks in the center portion of the tape and a linear trackalong the bottom edge of the tape contains CTL, or control track, synchronizing pulses.

heads for read and write operations.The cylinder (or drum) servo is respon-

sible for maintaining the correct rotation-al speed of the drum containing the videoheads. This ensures that the correct num-ber of horizontal lines are recorded orplayed back on each track. In addition, itmust position the heads so that read andwrite operations begin and end at theproper place along the track.

Servo operation during playbackIn simplest terms, the servo system is a

feedback loop control circuit. It senses thecondition of the operating system, com-pares it to a reference and generates a cor-rection signal to maintain synchroniza-tion. Operation of the servo system isslightly different in playback and recordmodes. Let's look at the playback func-tion first.

The capstan servo circuit uses two com-parators to control motor speed. Thespeed comparator compares the V SYNCand capstan FG signals. The FG signal isamplified, processed by a Schmitt trigger,and applied to the comparator. The com-parator's output is used to control a pulsewidth modulator.

The PWM's square wave signal is fedthrough an integrator, which smoothes it

into a dc voltage proportional to the dutycycle of the PWM output. This dc volt-age is then applied to a summing amp androuted through the CAP ON switch to thecapstan motor driver. If the comparatordetects a slowing of the motor, the dutycycle of the PWM is increased, providinga higher dc output from the integrator andgreater drive to the motor. If motor speedis too high, the duty cycle of the PWM isdecreased, resulting in reduced drive.

The second comparator in the capstancircuit is the phase comparator, whichacts as a fine speed adjustment. It pro-duces an output that controls a PWM. ThePWM output is processed by an integra-tor and applied to a summing amp, whereit is combined with the signal from thespeed comparator. The capstan phasecomparator uses several inputs to makethe fine adjustments; V Sync, the outputof the tracking multivibrator, CTL and thehead switching signal.

The cylinder motor servo also uses twocomparators, and operates in the samemanner as the capstan circuit. The cylin-der speed comparator uses V Sync and thecylinder FG signal to provide coarsespeed control. Fine speed control is ac-complished by operation of the CYL/CAP phase comparator.


11ow, 1H air

6H +1h

BEGINNING OF VERTICAL BLANKING INTERVAL

BEGINNING OF VERTICAL SYNC

VERTICAL SYNC PULSE

EQUALIZING PULSES(SPACED 1/2H;

HEAD SWITCHING SIGNAL

TIME

HEAD SWITCH T MING

Figure 2. The head amp select circuit sends a switching sigral to control operation of the playback head amplifiers, which alternately turn on andoff so the signal from the head that is currently in contact with the tape is turned on. The proper timing relationship for head switching is shown here.

The H SW GEN is driven by cylinderPG pulses. The output of the generator isa 30Hz square wave which is used by sev-eral other circuits. In the playback mode,this signal supplies inputs to the CAP/CYL phase comparator, the TR MMGEN, the head amp select circuit and theV -lock gen.

The head amp select circuit sends aswitching signal to control operation ofthe playback head amplifiers. The headamps are alternately turned on and off sothe signal from the head that is currentlyin contact with the tape is turned on. Eachhead reads alternate fields of the videosignal. When head switching occurs, vid-eo noise is produced. The timing of theswitch from one head to the other is crit-ical to stable picture production. Figure 2shows the proper timing relationship.

Normal switching in the VHS formattakes place 6H ±1H before the beginningof vertical sync. H is the designation forone horizontal line. This places the switch-ing event in the overscan portion of thepicture, off the bottom of the screen.

If head switching takes place too early,the switch point will be visible in the low-er part of the picture. If switching takesplace too late, it can interfere with repro-duction of the vertical interval, resulting

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in vertical jitter, jumping, or loss of ver-tical sync altogether.

The V -lock gen provides a false verti-cal sync signal which is used duringsearch mode operation. NTSC televisionsor monitors can only reproduce video ata 30Hz frame rate. If the vertical signalstrays too far from this figure, verticalsync is lost and the picture rolls. In searchmode, vertical sync information from thevideo tape is not at the correct frequencyand cannot be used to lock vertical scan-ning. The V -lock generator takes overduring these periods to produce a picturethat is stable.

Servo operation during recordRecord operation of the servo circuits

is similar to playback operation with threemajor differences. The first is the sourceof the V -Sync reference. In record mode,vertical sync information is taken fromthe incoming video. A sync separator andshaper circuit in the video section pro-duces the signal.

The second difference is a change inone of the reference inputs for the CYL/CAP phase comparator. Because the tapeis in the process of being recorded, noCTL signal exists to be used as a refer-ence. The capstan FG signal is counteddown to provide the necessary reference.The select switch operates to connect the

FG signal to the comparator when theVCR is in record mode.

The third difference is in the CTL headcircuit. When the VCR is in the recordmode, the CTL amp is turned on and pass-es a signal from the CTL GEN which isrecorded on the CTL track. At the sametime, the PB ON switch opens.

TroubleshootingSymptoms of servo troubles, such as

jitter, sync problems, or noise bands in thepicture, are often produced by mechani-cal components in the VCR. Before trou-bleshooting electronic circuits, be sure tocheck common trouble areas first. Theseinclude: dirty or blocked video heads,dirty CTL head, tape slippage caused bydirty capstan and pinch roller, or improp-er tape tension because of improper ad-justment, dirty or worn drive belts ordried -out lubricants on the reel spindles.

The next step is to determine if the ser-vos are malfunctioning in the playbackmode, in the record mode, or both. Playa test tape and check for symptoms in thevideo to check the playback mode. Re-cord a test signal and play the tape backin a known -good VCR to check the recordmode. If the symptoms only appear in onemode, concentrate on the circuit elementsthat are exclusively related to that mode.

Determining if a symptom is caused by

a capstan problem or a drum servo prob-lem can sometimes prove difficult. Aquick way to check for off -speed opera-tion is to measure the frequency of the FGsignal from each motor. The service man-ual contains specifications of proper sig-nal frequency and amplitude.

If you find a discrepancy, perform theelectrical alignments for the servo cir-cuits. Depending on the age and model ofthe VCR, these may include: head switch-ing position, tracking set, capstan free runand cylinder free run. If the V -Sync fre-quency is off in playback mode, align-ment of the system control VCO may benecessary. If the trouble symptom persistsafter alignment, troubleshoot the servocircuits for component problems.

Troubleshoot in this sequenceBegin troubleshooting by checking sup-

ply voltages for the servo control IC andthe affected motor drive. Voltages shouldbe checked for proper amplitude and ab-sence of noise and ripple. If problems ex-ist, troubleshoot the power supply circuits.

Check the control inputs to the servocontrol IC. These include: V -Sync, track-ing control, PB FM signal, CTL (in play-back mode), capstan FG, PG shifter andcylinder FG/PG. These should be checkedfor proper amplitude, frequency and wave -shape as specified in the manual. If an

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improper or missing signal is noted, trou-bleshoot the signal's source circuits.

Check the control outputs from the IC.These include: head amp select, false V -Sync, CTL (in record mode), R/S/F andthe outputs of the capstan and cylinderPWMs. If all inputs are correct and faultsare found in one or more outputs, isolatethe defective output before concludingthat the IC is at fault. To perform this checkdesolder the appropriate pin and check theoutput in an unloaded state. If the outputsignal is now present, check the circuit fedby that pin for faults. If the output is stillnot present, the control IC is defective.

If all outputs from the servo control ICappear normal, check the peripheral cir-cuits for proper operation. These include:the integrators, sum amps, capstan andcylinder on switches, motor drivers andthe motors.

Breaking the loop

Occasionally, you will find a situationwhere all inputs and outputs from the con-trol IC appear to be normal and all periph-eral components seem to be operatingproperly, yet the trouble symptom per-sists. This frustrating state can usually betraced to the fact that the servo system isa loop. One section of the system may becorrecting in the right direction, but over-reacting. This causes the circuit to "chaseits tail." To troubleshoot this type of prob-lem, it is necessary to break the loop andobserve the individual circuit elements todetermine which of them is over -reacting.

A convenient spot to break the loop isthe input to the motor drive circuit. Usinga variable dc bench supply, apply the nor-mal control voltage as specified in the ser-vice manual. Vary the voltage slightlyabove and below normal. Observe the re-sponse of the PWM outputs, the integra-tor outputs and the sum amp output as themotor speed changes.

VCR servo circuits are sometimes trou-blesome for technicians. Because of theirreliability, we seldom have to deal withthem and they may not be as familiar tous as other circuits. A scarcity of techni-cal information on the servo operationalso contributes to the problem. Perhapsthis information will help you solve servotroubles you may encounter and reducefrustration at the bench.

System components

Refer to the block diagram (on page20) of servo system elements.

Capstan motor: A direct -drivemotor that rotates the capstan and pro-duces tape motion through the VCR.

FG head: A magnetic pickup that gen-erates a signal from the rotation of thecapstan motor. The FG signal may beproduced by a Hall device in some VCRs.

Capstan motor drive: An IC thatuses low-level inputs to control drivepower to the motor.

Cap on: A solid-state switch operat-ed by the system control circuit. It pro-vides on/off control of the capstanmotor.

R/S/F: An output that controls the di-rection of capstan rotation for differentmodes of operation (Reverse, Stop andForward).

Sum amp: An IC op -amp that addstwo dc input signals into one output.

Integrator: A circuit that converts asquare wave input into a dc output whichis proportional to the duty cycle of theinput signal.

Cylinder motor: A direct -drivemotor that rotates the video head drum.

Cylinder motor driver: An IC thatuses low-level inputs to control drivepower to the motor.

FG/PG Hall IC: A Hall -effect IC thatproduces signals that result from therotation of the cylinder motor. FG is thefield -generator signal and PG is thepulse generator signal.

FG/PG separator: Separates thelower frequency PG pulses from thecombined FG signal.

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V -SYNC

SERVO CONTROL IC

SELECT

RECCTLAMP

CTLAMP

COUNT-11E-11-DOWN

SCHMITT

CAPSTANSPEED

COMPARATOR

TRACKING

PB FM

TO HEADAMP

FALSEV -SYNC

REC CRUTR MM

GEN

ENVDET

HEAD AMPSELECT

CYL/CAPPHASE

COMPARATOR

PBON

R/S/F

FGAMP

CTLHEAD

CAPSTANMOTORDRIVE

CAPSTANMOTOR

PWM

CYLINDERSPEED

COMPARATOR

V -LOCKGEN

4

PWM

PWM

FGHEAD

/77

INTE-GRATOR

INTE-GRATOR

PWM

H. SW

FitGEN

,

FG/PGSEPARATOR

INTE-GRATOR

INTE-GRATOR

PG SHIFTER

SUMAMP

SUMAMP

CYLINDERMOTORDRIVE

CAP ON0-

CYL ON

CYLINDERMOTOR

FG/PGHALL

This block diagram ilustrates the components that make up the VHS VCR servo system.

PG shifter: Controls the timing of thePG signal applied to the head switch gen-erator. The PG shifter determines the po-sition of head switching in relation to thevertical sync signal.

CTL head: A magnetic head thatreads control track pulses during play-back and records them during recordmode operation. CTL pulses occur at the

TV frame rate or 29.97Hz. (This fre-quency is often rounded to 30Hz for con-venience sake.)

Servo control IC: Contains the cir-cuits for signal processing, comparisonand control of the external components.It may be an independent IC or a portionof the system control IC.

REC CTL amp: Amplifies the CTLsignal for recording.

PB on: An electronic switch thatroutes the playback CTL signal to theCTL amp.

CTL amp: Amplifies the playbackCTL signal.


FG amp: Amplifies the FG signalfrom the capstan.

Schmitt trigger: A trigger circuit thatshapes the FG signal into pulses.

Countdown: A circuit that counts theFG pulses down to frame rate.

Select: An electronic switch that cou-ples the playback CTL through in play-back mode and the counted -down FGpulses through in record mode.

Capstan speed comparator: A cir-cuit that compares FG pulses and the Vsync signal. The output is a dc controlsignal proportional to the timing of thetwo input signals. It provides coarsespeed control of the capstan motor.

PWM: Pulse width modulator. A mul-tivibrator circuit that produces a squarewave output. Under normal operatingconditions, the duty cycle of the outputis 50%. Varying the control input aboveand below the normal level causes theduty cycle to vary above and below 50%.

CYL/CAP phase comparator: A cir-cuit that uses several control inputs toproduce a dc control signal output. Thiscircuit provides fine speed control forboth capstan and cylinder motors.

Cylinder speed comparator: A cir-cuit that uses V SYNC and cylinder FGsignals to produce a dc control output.This circuit provides coarse speed con-trol for the cylinder motor.

V sync: Vertical sync, an input thatprovides the reference signal for theservo system. In playback mode, verti-cal sync is supplied by a crystal -con-trolled clock generator in the system con-trol circuit. During record operations, itis provided by the vertical sync signalsof the video signal being recorded.

REC CTL/TR MM GEN: A multi -vibrator oscillator that produces CTLpulses in record mode and a trackingpulse signal during playback.

Tracking: An operator control thatvaries the output of the tracking multivi-brator. The servo system has a limited

range over which it can make speed cor-rections. The tracking control places thecircuit within those limits.

PB FM: The playback FM signal istaken from the playback head amplifier.It is used together with the head switch-ing signal to provide fine control of thetracking generator.

ENV det: The envelope detector iso-lates the individual head outputs from theRF FM head signal.

H SW gen: A multivibrator circuit op-erating at frame rate which supplies asquare wave output for head switchingand other control functions.

HEAD amp select: A buffer amplifierthat provides ON/OFF switching for therecord or playback head amp channels.

V -lock gen: A buffer and shaping ampthat generates the FALSE V -SYNC sig-nal used to keep the on -screen videolocked vertically during forward and re-verse high-speed search functions.

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Understanding the RS232(C)Serial InterfaceBy Gregory W. Jones Jr.

Most personal computers (PCs) have a

serial communications port, which allowsthe PC to communicate with modems,other PCs, peripherals and data commu-nications equipment. Communicationamong the widest variety of these devicesrequires a standard. This recommendedstandard, the RS232 for domestic appli-cations, and the RS232(C), developed bythe EIA in 1969, defines the serial com-munications interface for connecting dataterminal equipments (DTE) and datacommunications equipments (DCE).

RS232(C) defines the control, moni-toring, and timing of serial data signals

and their levels as they should appear oneach of the 25 circuits (pins) of the inter-face. The International Telephone andTelegraph Consultative Committee's(CCITT) standard V24, for European ap-plications, also defines the control, mon-itoring, and timing of serial data signalsand their levels as they should appear oneach of the twenty-five circuits pins of theinterface, but this article will concern itselfwith the EIA RS232 Interface Standard.

This article explores RS232(C), dis-cusses timing, control and data signals.and then explains how the signals are usedto establish communications. There isalso a brief discussion of some of themethods through which analog and digi-tal troubleshooting can be performed on

an inoperative serial communicationsline, and a glossary.

While a detailed knowledge of RS232is not absolutely essential for a computerservice technician, familarity with RS232is useful for those who may find them-selves dealing with serial data transmis-sion problems.

The serial/parallel questionInternally, most processors and periph-

erals transfer a word or character of dataat a time. That is, the total number of bitswhich the device recognizes as a singlecharacter or word are transferred simul-taneously over a parallel data path. Thesize of the circuit's data path, usually re-ferred to as its data bus width, depends

1 --n-1 -FL1_1

is -LI-U -11-1 -EL1 JL1 -1-1-

0 1:1 II

The letter "G" transmitted over aparallel transmission line.

(aJ

The same letter "G" transmittedover a serial transmission line.

(b)

A comparison of the number of data bits that can be transmittedover a parallel or serial data line during a single clock cycle.The parallel transmission requires only one cycle because it cantransmit the entire character at one time. The serial datatransmission requires eight clock cycles to transmit the samecharacter because only one data bit can be sent at a time; oneper clock cycle.

Figure 1. In parallel data transmission, all bits of a computer word are sent simultaneously over a data bus. In serial data transmission, bits aresent one at a time and then assembled into data words at the receiving end.


13 12 11 10 9 8 7 6 6 4 3 2 1

0

o0

o0

00 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 00

25

1 2

24

3

23 22 21 20 19 18 17 16 15

DB-25 Female RS232(C) Connector - Front View.

4 5 6 7 8 9 10 11 12

14

13

o

o0

0

0

0

0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0

0

14 15 16 17 18 19 20 21 22 23 24 25

DB-25 Male RS232(C) Connector - Front View.

These are front views of the male and female 25 -pin (DB-25 Rs232(C) connector.

Figure 2. Pin numbering for RS232(C) connectors.

5 4 3 2

O 0 0

O 0

0 0

O 0

9 8

Female DB-9 Connector - Front View.

2 3

7

4

6

5

O 0 0

O 0

0 0

O 0

6 7

Male DB-9 Connector - Front View.

8 9

These are front views of the male and female 9 -pin DB-9 (IBM-PC/AT) RS232(C)connector.

Figure 3. Some PC manufacturers are usingserial communications port.

upon the manufacturer's design and canbe either 8, 16, 32 or 64 bits wide. FigureI A shows an eight -bit parallel bus for thetransfer of data between devices. A sim-ple piece of flat ribbon cable is an exam-ple of a parallel bus.

9 -pin RS232(C) connectors as their RS232(C)

In this illustration, the letter "G" is be-ing transmitted. Each of the eight wires iscarrying one of the eight bits required torepresent the letter "G". All eight bits aretransmitted simultaneously during a sin-gle transmit clock cycle. If this same pe-

ripheral or processor wishes to commu-nicate with some external device, it might,because of distances or hardware types,have to transmit the data in serial fashion;one bit at a time.

Look now at Figure I B, which showsthe same character "G" being transmittedto a serial device such as a modem, seri-al line -printer, bar-code reader, or anyother I/O device which requires a serialcommunications interface. Notice thateach bit on a serial line must be transmit-ted one at a time. Each time the line is tog-gled on, a "high" appears and remains un-til the end of the clock -cycle. In thisexample, the highs and lows are functionsof time and thus the stream of bits repre-sents several consecutive "ons and offs"over a period of several clock -cycles.

Notice that the first two bits, (1,1) aregenerated by holding the line high for twoclock cycles. This requires that strict tim-ing and control synchronization be main-tained between the sending and receivingdevices so that the receiving device canproperly interpret and disassemble theserial bit -stream into usable characterdata. The device must know where, with-in the bit stream, a character begins andwhere it ends so that the bits don't get all"bunched up" into an unintelligiblestream of garble. Timing and control ofthe serial bit -stream between a DCE andDTE is managed by the signals ex-changed across the timing and control cir-cuits of the RS232 interface, all of whichare discussed in detail later.

Mechanical specificationsThe mechanical specification states

that the RS232 female 25 -pin connector(Figure 2) shall be mounted on the datacommunications equipment (DCE) andthat there shall be a male 25 -pin connec-tor mounted on the data terminal equip-ment (DTE). This is the suggestion madeby the EIA, but in reality, you will findyourself interfacing devices which mayboth have male and female plugs. Thereare many different manufacturers of theRS232 connector and the packaging ofthese connectors will vary (within reason)between manufacturers. Some PC manu-facturers are using 9 -pin RS232 connec-tors as their RS232 serial communica-tions port (Figure 3).

Electrical specificationsThe RS232 interface provides two


channels; a primary channel or signal pathfor the exchange of data, and a secondary,lower speed channel for backup commu-nications, maintenance, quality controland signaling. Many hardware manufac-turers choose not to implement the sec-ondary channel with their products.

The bandwidth of the data channel (cir-cuits BA/BB) is about 4KHz wide and thefrequency range of the signals on thechannel will vary between 500Hz and3300Hz. The EIA's recommended imple-mentation of the primary and secondarychannels are shown in Figures 4 and 5.

The dc signals that appear on any of thepins must be bipolar low voltages, andthey must not be greater than ±25Vdc.The region between ±3Vdc is the level atwhich the transition from "on" to "off"takes place. Figure 6 illustrates the asso-ciation between the different signal statesand the transition between states. Whenthe distance between devices is 50 feet ormore, line -drivers must be used to main-tain the required signal levels.

As you know, dc signals, especiallylow-level dc signals, are susceptible toline loss due to the resistance associatedwith dc transmission lines. Assume thata +10V signal is required to represent a"high." If signal line losses cause us toreceive only 4V or 5V, this could repre-sent a loss of a bit of data. The result ofsuch a loss of signal integrity would causethe data received over such a path to bedistorted and useless. The line driver; adc amplifier installed in series with theline between the DCE and DCT, booststhe input dc signal to its proper level, be-fore it reaches the intended device.

The RS232 standard was written priorto the advent of transistor -transistor logic(TTL) and so the voltage levels on the in-terface are not compatible with TTL.While the accepted supply voltage value(Vc) for most TTL is 5Vdc, with a 5%tolerance of 250mVdc, the levels foundon the RS232 interface may vary from-25Vdc to +25Vdc.

Circuit types and definitionsThe signal types found on the RS232

serial interface primary and secondarychannels have been sub -categorized intotypes, "A" thru "E", as described in thetext that follows. The signals which areon circuits BB, CB, CC, CE, and CF trav-el from the DCE to the DTE while thoseon circuits BA, CA, and CD travel from

Jones/RS232RS232(C) Serial Interface Primary Channel ConnectionsCategory Circuit Pin Circuit

Designator Number Description

GROUNDSAA 1 Protective GroundAB 7 Signal Ground

DATA SIGNALS BA 2 Transmitted DataBB 3 Received Data

CONTROL SIGNALS CA (RTS) 4 Request To SendCB (CTS) 5 Clear To SendCC (DSR) 6 Data Set ReadyCD (DTR) 20 Data Terminal ReadyCE 22 Ring IndicatorCF 8 Carrier DetectCG 21 Modulation DetectCH 23* Speed SelectCI 23* Speed Select

TIMING SIGNALS DA 24 External Transmit ClockDB 15 Transmit ClockDC 17 Receive Clock

OTHER Reserved 9 POS DC Test VoltageReserved 10 NEG DC Test VoltageReserved 11 UnassignedReserved 18 UnassignedReserved 25 Busy

Figure 4. RS232(C) serial interface primary channel circuits.

DTE to DCE. The control signals governthe operation and timing of the transmitand receive data circuits.

Type A: Grounds AA (Pin 1): Protective (frame or chas-

sis) ground.This is the circuit or conductor used to

electrically bond all frames, chassis, andmetallic cabinets so that there is a zerodifference in voltage between them. Thiscircuit may sometimes be attached toearth ground.

AB (Pin 7): Signal ground.Circuit AB is the signal (common re-

turn) ground which provides the 10Vdcreference point for all of the other inter-face circuits except AA. At times, this cir-cuit may be "strapped" to circuit AA, de-pending upon the application.

Type B: Data signals

BA (Pin 2): Transmit data.Circuit BA is the circuit that carries the

data from the DTE to the DCE for digi-tal -to -analog conversion and subsequenttransmission over the telecommunica-tions link. This circuit will not be activat-ed unless the request to send (RTS, cktCA); clear to send (CTS, ckt CB), data setready (DSR, ckt CC); and data terminalready (DTR, ckt CD) circuits have beenenabled with a "high." BA is held in a"mark" condition until those four circuitshave been enabled and also during theinterval between characters during thetransmission of data.

BB (Pin 3): Received data.Circuit BB carries signals received

across the channel from the DCE to the


Category Circuit Pin CircuitDesignator Number Description

GROUNDSAA 1 Protective GroundAB 7 Signal Ground

DATA SIGNALS SBA 14 Sec Transmitted DataSBB 16 Sec Received Data

CONTROL SIGNAL SCA (RTS 19 Sec Request To SendSCB (CTS 13 Sec Clear To SendSCF 12 Sec Carrier Detect

Figure 5. RS232(C) serial interface secondary channel circuits.

+25 vdc

+3 vdc

(On state; Logical 0; 'Space')

(On/Off transition region)

-3 vdc

-25 vdc

fun defined control level)

(Off state; Logical 1; 'Mark)

Figure 6. RS232(C) uses these voltage values to define the "on" and "off" data states.

DTE. This circuit is in the mark state aslong as circuit CF (carrier detect) is off.

Type C: Control signals CA (Pin 4): Request to send (RTS).This circuit carries the signal from the

DTE which instructs the DCE to go intotransmit mode. The DCE then cyclesthrough a series of routines and then re-sponds to the DTE by enabling the clearto send circuit (CTS, CB).

CB (Pin 5): Clear to send (CTS).When the DCE is ready to receive data,

the clear to send signal is sent to the DTEover circuit CB in response to the DTE'srequest to send (RTS). If CA (request to

send) is on and CC (data set ready, orDSR) is on, then the clear to send signalwill be enabled.

CC (Pin 6): Data set ready (DSR).Data set ready is a status indicator used

to determine when the local data set/mo-dem is connected to the communicationschannel, and when all timing and signal-ling functions have been completed.

CD (Pin 20): Data terminal ready(DTR). Indicates that the data terminalequipment is ready to transmit and re-ceive data.

CF (Pin 8): Carrier detect.Indicates that a useful signal is being

received on the channel. If this is in the

off state the received data circuit (BB) isdisabled and placed in the mark state.

CG (Pin 21): Signal quality control.When this signal is in the off condition,

there is a high probability of an error onthe received -data circuit (CB). In somesophisticated data communications net-works this circuit is often used to imple-ment error detection and correction.

CH/CI (Pin 23): Data rate selector.This is a bidirectional circuit which is

used to select and sense different data sig-naling rates. Toggling this signal "on" se-lects the next higher signaling rate. Whenthe highest signal rate is reached, the next"toggle" resets it back to the lowest andthen the process may be repeated if de-sired. The toggling is accomplished bysetting the circuit to high.

Type D: Timing Signals DA (Pin 24): External transmit clock.Provides transmit timing information

from the DTE to the DCE, with the tran-sition period from on to off being the cen-ter of each bit transmitted.

DB (Pin 15): Transmit clock signal.Furnishes the transmit timing signal

from the DCE to the DTE, with the tran-sitions of transmitted signal elementsfrom high -to -low -to -high occurring at thesame time as the transition of DB from itsoff -to -on state.

DD (Pin 17): Receiver timing clock.The DCE uses this circuit to provide

receiver signal timing for signals receivedon the received data circuit (BB).

Type E: Miscellaneous (reserved)The following circuits are reserved or

carry the specified signals:

Pin 9: Positive dc test voltage, Pin 10: Negative dc test voltage, Pin 11: Unassigned, Pin 18: Unassigned, Pin 25: Busy.

Establishing communicationsRS232 provides a means for monitor-

ing, controlling, and timing the signalstates on the RS232C serial interface.These states are:

The on -hook or wait state, duringwhich time the interface is inactive;

The control -state, during which thereis an exchange of control signals for es-tablishing or terminating communications;


Pin Circuit Direction To Remarks

1 Ground bi-directional Provides electrical common ground for DCEand DTE devices.

2 TD DCE Transmitted data from DTE to DCE

3 RD DTE Received data from DCE to DTE

4 RTS DCE Request to send; goes high when terminalis able to send.

5 CTS DTE Clear to send; goes high when DTE on theother end of the link is ready to receive.

6 DSR DTE Data Set Ready; goes high when DCE isready to receive data.

7 Signal Ground bi-directional Provides signal ground return for all datasignals.

8 CD DTE Carrier Detect; goes high when connectionhas been made to another DTE.

20 DTR DCE Data Terminal Ready; goes high when the DTEis on and ready to communicate. Sending DTElow usually caused the modem to disconnectfrom the phone line.

22 RI DTE Ring Indicator; goes high when telephone ringsignal is detected.

Table 1. Common RS232(C) serial communications connections and direction of signal flow.

The data-transmission/data-receptionstate, during which time data is actuallybeing exchanged across the interface.

The following summarizes the way inwhich the DCE device utilizes these sig-nals to establish, maintain, and terminateserial communications:

1. At the local or calling end of the net-work, an operator either dials the tele-phone number or enters commands di-rectly at a terminal. In some cases, thecentral processor at an installation may,during the processing of data, initiate thecall through a special hardware -softwareinterface. Whatever the calling method, itcauses the local DTE to generate a requestto send on CA which then causes the DCEto place a data tone (referred to as F1) onthe data line.

2. The remote or called DCE respondsto the incoming tone (F1) by enabling thering indicator circuit (CE) and then pass-es the incoming RTS to the called DTE.

The called DTE then turns on data setready (CC). At 1.5 seconds after enablingDSR, the called DCE begins a count inmilliseconds. At 150msec after enablingDSR it sends a data tone (referred to asF2) out on the line. At 150msec after F1has started, the receive data circuit BB istaken off hold (mark) and control is trans-ferred to the DTE. At 250msec later, clearto send, (CTS) on circuit CB, is enabledand then carrier detect is enabled. Trans-mitted data circuit BA is then taken offmark and control of the circuit passed tothe called DTE.

3. The local or callingDCE receives theF2 signal and is placed in the data modeeither manually when the operator press-

es the "data" button on the data set, or au-tomatically if auto -call features are ineffect. This causes the local data set readycircuit (CC) to be enabled. At 150msecafter receipt of an F2 signal, the calledDCE enables the received data circuit(BB), then passes control to the calledDTE. IfDSR has been enabled for 1.5sec,the called DCE counts another 265msecand then enables CTS on circuit CB aswell as carrier detect circuit CF, which isturned on by the local or calling DCE.

Signal F1 is turned off, and control oftransmit data circuit BA is passed to thecalling DTE.

When both the called and callingDCE's have passed control of circuits BAand BB to their respective DTE's, a

"handshaking sequence" is initiatedwhich, interpreted as an "ok-to-commu-nicate," causes a series of acknowledge -

(Continued on page 41)


Logic State DC HIGH DC LOWDC Voltage Levels -25 to -3 vdc +3 to +25 vdc

Control Circuit Signals Binary Value 1 0

Signal Type Mark SpaceControl Circuit Status off on

Table 2

RS232 bi-polar voltage levels. Positive high levelsusually refer to signals on the Control Circuits whilecircuits BA and BB usually have negative high logic.

Table 2. RS232(C) Voltage level to signal state relationship.

ment (ACK) signals to be exchanged be-tween the called and calling systems andthe transfer of data commences.

Terminating communicationsAfter the exchange of data has taken

place and the connection is to be terminat-ed, the following sequence takes place:

1. The local DTE disables the data ter-minal ready (CD) circuit, causing thetransmitted -data circuit (BA) to go high,which is interpreted as a space.

2. The remote DTE disables data setready (DSR) circuit (CC), clear -to -send(CB) and carrier detect (CF).

3. The local DTE disables data set ready(DSR) circuit (CC), clear -to -send (CB)and carrier detect (CF).

4. Both devices are now in the on -hookor mark -hold (wait) state.

RS232 signal flow

Although the standard defines 25 cir-cuits for use in serial communications, thecommon 25 -pin DB25 male and femaleconnectors are not part of the standard,which has resulted in the inclusion bymanufacturers of 9 -pin and 15 -pin RS 232serial ports on their devices. Of those 25,only 10 are required for most applica-tions. Those 10 circuits and the signalflow on them are illustrated in Table 1,although for most applications only 10 ofthe 25 are actually required.

Some RS232(C) applications notesDespite the EIA's efforts to standardize

the serial interface, its implementationvaries from vendor to vendor. This caus-

es considerable difficulty for first -timersattempting to configure systems for homeand business use. Another considerationis the relationship between signal levelsand logic states. Take a look at Table 2.A binary or logic 1 should be represent-ed by signal voltages within the range of-25Vdc to -3Vdc, and a logic 0 should berepresented by voltages falling within the+3Vdc to +25Vdc range.

In reality, however, this rule of "nega-tive -high" logic more often than not ap-plies only to circuits BA and BB on pins2 and 3 while the other signals on the in-terface are actually "positive -high." Thisis one of the many reasons that you mustread the equipment manuals to determinethe types of signals required or suppliedby the device's serial communicationsport. This is especially true if you intendto implement handshaking protocols.

Handshaking is a means by which a re-ceiving device may control the flow ofdata being sent to it. However, this mustbe done with care, because the RS232 in-terface was not originally intended to beused for handshaking between a DCE andDTE, but rather between the DTE and theline itself. For example, if you look at pin4, request to send, and pin 5, clear to send,you might get the impression that all youhave to do is switch them on and off insynchronism with each other to controlthe flow of data between two devices.

However, this is not the case. If you turnoff clear to send, thinking to keep it offuntil our receiver is ready to receive moredata, you would really be placing yourDCE in mark -hold, making it necessaryto re-establish the communications link.

Pin Circuit

CD

2 RD

3 TD

4 DTR

5 SG

6 DSR

7 RTS

8 CTS

9 RI

Table 3. The IBM PC/AT implements RS232(C)as shown here. The IBM PC/AT implementationof RS232(C) uses the 9 -pin (DB-9) connectoras its serial communications port.

This led to the development of softwarehandshaking protocols and hardwarehandshaking circuits and also to devicesto work around these problems.

Software handshaking is achieved bycommunications software which embedscontrol characters in the data that it sendsacross the line. The receiving device hassoftware which reads the control charac-


ters and knows when to receive, when towait, etc. Hardware handshaking in-volves the use of RS232 control circuits.most commonly DST, RTS, and CTS, tocontrol signal flow. Hardware handshak-ing is most often accomplished throughthe proper wiring of the data communica-tions cables that connect the devices.

Using a modem eliminatorA modem eliminator cable (also known

as a null modem or modem bypass) is use-ful when connecting terminals to termi-nals or computers to computers. It usesthe control signal generated by one localcircuit to emulate the control signal ex-pected by another local circuit. As men-tioned before, the RS232 standard wasoriginally developed for DTE to DCEcommunications.

When DTE to DTE, terminal to termi-nal communications began, a means hadto be developed whereby both devicesthought they were communicating througha DCE, hence the modem eliminator, acable which has been configured to usesignals generated by the DTE to take theplace of those that it would normally re-ceive from a DCE and vice versa.

In operation, signals that normally flow

Ground

TD

RD

RTS

CTS

DSR

CD

DTR

SG

Ground

TD

RD

RTS

CTS

DSR

CD

DTR

SG

When connecting DCE to DCE or DTE to DTE, a "Null Modem" or "ModemEliminator Cable" is useful because it allows the devices to emulate signalsthey would receive under normal DTE/DCE conditions. The above diagramshows such a cable with more control circuits than you will probably reallyneed. It will handle most situation that you are likely to run into.

Figure 7. A "null modem", or "modem eliminator cable," allows connection from data terminalequipment to data terminal equipment, or data communication equipment to data communicationequipment.

from the local DCE to the local DTE areemulated by taking signals available onthe DTE's RS232 port and routing themback to the circuits which require input

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levels. For example, RTS on circuit CAcan be shunted over to circuit CB whereit will function as a valid CTS signal.

Figure 7 shows a typical modem elim-

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Circle (33) on Reply Card42 Electronic Servicing & Technology June 1994

inator cable. Although it's called amodem eliminator cable, the easiest wayto construct one is to use a male and fe-male RS232 25 -pin connector, with hoods,inside of which you can do your wiringand cross connects. To use it just insert itin series with the RS232 cables of theDTE/DCE device you wish to connect.

We spoke earlier of the different waysin which RS232C has been implemented.One case in point, the IBM PC/AT, incor-porates 9 of the 25 circuits into its RS -232(C) serial port, and provides the nine-pin RS232 connector as the interface. TheAT's Serial Communications Port is illus-trated in Table 3. You can see how the lackof standardization of pin numbering canbe quite confusing when trying to relatethe serial port to the standard. Again, youmust read the manufacturer's manual inorder to determine that company's imple-mentation of the RS232 Standard. The cir-cuits are standard, not the pin numbers orconnector type upon which they appear.

Troubleshooting the interfaceThe process of troubleshooting is a

method of systematic checks and testsused to isolate a problem in order to deter-

mine the best course of action toward res-toration of communications. By perform-ing these tests you will be more likely tocall the right person for the job to be done,i.e. the carrier representative responsiblefor your network lines, the vendor respon-sible for the hardware or software or, in alot of instances, you will determine thatthe operator pushed the wrong button.

There are two parts to data communi-cations troubleshooting; analog testingand digital testing. Analog testing is per-formed on the telephone side of the net-work to determine whether or not the net-work side of the link including the car-rier's equipment is at fault. Here you cantest for such things as phase jitter, delaydistortion, impulse noise and loss of sig-nal levels to name a few. These are thetypes of network that affect data recep-tion at the receiving DTE.

Troubleshooting and isolating these"analog -side" problems can be initiatedat your location under the direction of theresponsible carrier authorities; the localtelephone company, long distance orvalue added network (VAN) carriers.Many of the 212A compatible modemshave provisions for analog and digital

testing and when used in conjunction withanalog testing devices can indicate whatthe source of a problem might be. A deci-Bel meter may also be used to measuresignal levels on the analog side of the lineand an oscilloscope can be used to checkwaveforms for distortion on both the ana-log and digital side.

Digital testing is used to determine thesource of in-house problems which showup. between the local DCE and DTE de-vices. One of the simplest pieces of testequipment for digital side testing is theRS232 breakout box. The RS232 break-out box gives a visual indication of thesignal status of each of the interface's 25pins. It also provides a means of cross -connecting circuits and can be used forpre -installation checks of cables. You canalso use a DMM or oscilloscope to checkthat dc signal levels are within the ± 3Vdcto ±25Vdc range and properly timed.

A data line monitor, on the other handgives you the ability to monitor the actu-al control, timing and data signals travel-ing across the network. Some data linemonitors are capable of complete emula-tion of a DTE, enabling them to be usedfor comprehensive diagnostic testing.

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Another piece of diagnostic equipment,the response time analyzer is useful inchecking the overall response time onpolled, multidropped, or local area net-works where each node has its own rec-ognizable address. Taken as the time be-tween a polling interrupt and an ACK orNACK answerback, response time can he

measured on all of the nodes in a networkto determine the location of loss in over-all response time.

Although there are many more piecesof electronic test equipment and diagnos-tic software that can be used to trouble-shoot the interface between DTE andDCE it is beyond the scope of this article

to discuss them. The point here was to fa-miliarize you with some of the optionsavailable to you for troubleshooting yourcommunications network. For the mostpart, any problems you run across will beeasily diagnosed through the proper useof the RS232 breakout box which you canuse to check cables and RS232 U0 ports.

Glossary of terms

Asynchronous: Often referred to asstart -stop transmission because trans-mission is made one character at a time,with a start bit sent at the beginning ofthe group of bits which make up the char-acter, and a stop bit sent after the last bithas been transmitted.

Bipolar voltage: A dc voltage thatcontains both positive and negative com-ponents as measured above and belowthe zero reference point.

Bit: The most elemental unit withwhich to represent data. The bit, an ab-breviation for "binary digit," has onlytwo possible states; either "on" or "off".

Bit -error -rate: The ratio of the totalbits received in error to the total bitstransmitted. It is expressed as a power often when used to define the maximumallowable bit -error -rate for a given data -communications transmission path.

Bus: Path over which digital signalsmay flow. A parallel bus is a set of simul-taneous signal paths grouped togetheraccording to their function; i.e. a "databus" as opposed to an "address bus." A32 -bit address bus would use 32 wires totransmit 32 bits at the same time.

Carrier: The organization that pro-vides the send/receive path for datatransmission are called carriers.Examples of carriers include AT&T,MCI, GE-TELENET, and SPRINT. Theelectronic signal which is modulated bythe digital data signals sent over atelecommunications network is alsocalled a carrier.

CCITT: The International Consulta-tive Committee on Telephone and Tele-graph. This European counterpart to theEIA is responsible for international corn-

munications applications, and its com-mittee members are representatives ofEuropean communications carriers andpostal telephone service authorities.

Data communications equipment(DCE): This term is used to indicate anyof the various pieces of equipment usedto transmit data in its analog state, over acommunications channel. This includesmodems, multiplexers, port concentra-tors, etc. The DCE takes the digital sig-nal from the DTE, performs a digital -to -analog signal conversion and thentransmits the signal. When receiving, theDCE performs an analog -to -digital con-version and then sends it to the DTE.

Data terminal equipment (DTE): ADTE can be a CRT terminal, computer,printer, remote sensor, or any device thattransmits or receives digital data to orfrom a DCE. The DTE sends digital sig-nals to the DCE for digital -to -analogconversion and transmission. It alsoreceives digital data produced by theDCE from incoming analog signals.

Delay distortion: The analog signalon a data communications line is com-posed of many different frequencies. Astolerances vary along its length, thetransmission line becomes less resonantor responsive to certain frequencies andmore reso nant to others. When thisoccurs, some parts of the data signal failto reach the destination at the same timeas other parts of the signal, causing theoverall signal to be distorted because ofthe delay. This distortion is called delaydistortion.

Electronics Industry Association(EIA): An American trade associationwhich specifies and publishes variousstandards such as those which govern the

interface of computers and communica-tions equipment in North America.

Handshaking: A means by which areceiving device may control the flow ofdata being sent to it. The term hand-shaking applies to the process of estab-lishing timing and control synchroniza-tion between devices. Handshaking maybe accomplished with hardware, soft-ware, or a combination of both.

Impulse noise: The result of signalsradiated by unfiltered electronic orelectromechanical equipment beingoperated near unshielded data commu-nications equipment.

Line driver: A dc amplifier, installedin series with the line between the DCEand DCT to boost the input dc signal toits proper level.

Modem: This is usually required fordata communications between digitaldevices. The term modem is a contrac-tion of the words modulation (the pro-cess of superimposing information upona carrier signal for transmission) and de-modulation (the detection and separationof that information from the carrier sig-nal at the receiving end). This process isused to convert digital square -wave sig-nals into analog signals for transmissionover communications channels such astelephone lines. See also the definitionthat is found under "universal asynchro-nous receiver transmitter (UART)."

Modem eliminator: A cable that hasbeen configured to use signals generat-ed by the DTE to take the place of thosethat it would normally receive from aDCE and vice versa.

Parallel transmission: All of the bitswhich make up the computer's "word"are transmitted simultaneously.


Peripheral equipment: Equipmentwhich is part of the system but notphysically part of the central process-ing unit. Examples include printers,terminals, disk drives, CRTs.

Phase jitter: Momentary instabilityof phase relationship between direc-tion and amplitude of a signal on thecommunications transmission line.

Protocol: This term is often used todefine the rules or sequence of eventswhich must be followed to establishcommunications between devices.

Receive (lock: A timing pulse gen-erated at a certain frequency upon whichthe events related to the reception of data

hits can be synchronized. See discussion

on "Putting it all together."

Serial transmission: Bipolar, bit -by -bit transmission of data over a sin-gle line.

Transmit clock: A timing pulsegenerated at a certain frequency uponwhich the events related to the trans-mission of data bits can be synchro-nized. See discussion on "Putting it alltogether."

Synchronous: Synchronous datatransmissions are made without anyintervals between characters with tim-ing generated by the local and remoteDCE...

Cniversal asynchronous receivertransmitter (UART): The UART isthat part of the modem that allows DTEdevices to communicate with each oth-er over serial data lines. This device.the heart of the modem performs thedigital -to -analog and analog -to -digitalconversions of data signals. Locatedbetween the serial I/O port and the par-allel data bus within the device, it alsoperforms serial to parallel conversionsand parallel to serial conversions.

Word: A pre -defined group of hitsmake a computer word (each comput-er "word" actually represents a char-acter). Expressed in multiples of 8 bitseach, a word can be 8, 16 and with theintroduction of new technology 32 bitswide.

Recommended R

The Distributed System Environment:Some Practical ApproachesGrayce M. BoothCopyright 1981 McGraw-Hill

Modern Data Processors and SystemsDonald EadieCopyright 1971 Prentice -Hall

Data Communications; Facilities,Networks and Systems DesignDixon R. DollCopyright 1978 John Wiley & Sons Inc.

Data Communications;A User's HandbookRAC AL The Electronics Group18500-004 25M881Racal-Vadic 222 Caspian DriveSunnyvale California 94086.408-744-0810

Guidebook to Data CommunicationsCopyright 1977 Hewlett Packard Inc.

Telecommunications SystemEngineering; Analog and DigitalNetwork DesignCopyright 1980 John Wiley & Sons.

Design and Analysis of ComputerCommunications NetworksCopyright 1980 McGraw-Hill.

Status Report on New Standards forDTE/DCE Interface ProtocolsHarold C. FoltsNational Communications SystemCopyright 1979 IEEE "Computer"

Electronics Engineers' Handbookpages 23-76 to 23-82 Second EditionDonald G. Fink; Donald ChristiansenCopyright 1982, 1975 McGraw-Hill.

Microprocessors fromChips to Systems3rd EditionRodnay ZaksCopyright 1980 Sybex Inc.

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Readout and tape loadingproblems in RCA/Hitachi/Sears VCRsIty Victor NIeeldijk

The VCRs that were manufactured byHitachi about eight years ago often havefailed clock/readout displays. The displayitself is usually not the problem, althoughwith age it does get dimmer. This prob-lem is usually caused by the failure of thedc/dc converter, which provides 24V forthe display.

This converter is located on a vertical-ly -mounted circuit board, called the UHF -VHF (or U -V) tuning circuit hoard, locat-ed behind the display circuit board (Fig-ure 1). To remove this circuit board youmay have to push a plastic latch out of theway (Figure 2) and disconnect some ca-bles from a "circuit trap" (Figure 3).

This is not a two-piece connector but aconnector that clamps the cable wires.The wires are released by pushing the out-side of the connector toward the circuitboard. While holding the connector in,pull out the cable. The cable can be rein-serted without releasing the cable trap.

Models with discrete componentsIn older models, which use discrete

components (Figures 4 and 5), a failed

Meeldijk is the Reliability/Maintainability EngineeringManager Diagnostic/Retrieval systems, Inc. Oakland, NJ.

Figure 1. The UHF -VHF (or U -V) tuning circuit board which contains the dc/dc converter thatprovides clock/display power is located behind the clock/display circuit board.

digital transistor Q803 (type DTC124FRCA part number 157959), electrolyticcapacitors C805 (100pF, 25V; open cir-cuited) and C807 (10pF, 16V; changedvalue) are commonly found. There is alsoa semiconductor fuse, ZD801, (marked

Figure 2. You may have to push a plastic latch out of the way beforethe UHF -VHF (or U -V) tuning circuit board can be removed.

N5S, RCA 147464, also NTE 6 I 5P) thatmay have opened.

Other parts that may fail are electrolyt-ic capacitors C803 (47p F, 50V), C806(100p F, 35V), and switching transistorQ804 (type 2SD1266 RCA part number

DISCONNECT RECONNECT

Figure 3. Disconnecting cables from a "circuit trap:


Figure 4. A schematic diagram of an older model VCR that uses discrete components in the do/dc converter.

164231). Other parts that should bechecked are the diodes, fuses, IC andpower transformer in the VCR power sup-ply section.

The semiconductor fuse looks like aTO -92 transistor with two leads, but some-times the third lead (of the lead frame

Figure 5. The discrete component dc/dcconverter.

which is not connected internally) can beseen shorted to one of the other leads. Areplacement device measures as a shortcircuit when checked with an ohmmeter.

There may also be an open winding inthe switching coil L802, (Figure 6) (RCA164232). While this part is still available

Figure 6. An open winding in the switching coil.L802.

as a replacement item, you may be ableto repair the defective coil. If you removethe metal cover of the transformer, usinga thin flat blade screwdriver to pry aroundthe base of the part (Figure 7), you mayfind that the break in the winding is rightat a terminal pin. If that is the case, using

Figure 7. Use a thin blade flat screwdriver topry around the base of the coil to free it fromthe metal shield.


Figure 8. The coil shown in Figure 6 after therepair.

-1,-,

11.1:-- ...--1t ito

(ILONA ta Vol!: iIll il Sod :Milt tvali:" '

Figure 9. In later designs, the discrete com-ponent dc/dc converter has been replaced bya module. Replacement modules have a heatsink on top to improve heat dissipation and re-duce failures of this part.

a fine piece of wire, such as a single strandfrom a 22 gauge wire, you can bridge thegap between the winding and the pin.

Using a magnifying glass and a pair ofpointed tweezers, make a small "J"shaped hook in the exposed transformerwinding, do not pull on the wire whiledoing this as you can easily break it. Thenmake a similar "J" hook in the bridgingwire. Hook the wires together and secure

PLACE VCR IN PLAYMODE, AND PRESS

EJECT BUTTON

EJECTINFORMATION

OK

"CAPST. CONT."AT PINS (IC901)(48) & (49):LO

(47):HI

OK

IC971, IC901,CASSETTE MECHANISM

INSERT CASSETTE

NO

NO

VOLTAGE ATPINS (10901)

(37):HI(38):LO

OK

"CAPST. CONT."AT PINS (IC901)(47) & (49):LO

(48):HI

OK

MOTOROPERATION

NO

NO

STOP

OK BUTVCR STOPS SOON

CASSETTE MECHANISM,S143, S144

SEE"DOES NOT INPUT

FUNCTION SWITCH"

IC901

PLACE VCR INPOWER ON MODE

S143, S144,CASSETTE MECHANISM

IC901

IC902,CASSETTE LOADING

CASSETTE MECHANISM

Figure 10. The cassette loading mechanism troubleshooting chart from the RCA VLT 600-603manual.

them with solder. The other end of thebridge wire can then be wrapped aroundthe terminal pin and soldered in place.This procedure should take about 20 min-utes. Figure 8 shows the repaired coil.

In later designs, the discrete componentdc/dc converter has been replaced by amodule, as in Figure 9. Many replacementpart vendors sell a dc/dc converter repairkit (RCA 163818, Hitachi 5262063) that


Figure 11. The cassette sensing switches (S143 and S144 in an RCA model VLT 600, 601,602) when the right side of the stage mechanism has been removed and turned upside down.

contains an improved version of this mod-ule. The kit contains a module that hasheat sink fins on it, and some replacementcapacitors and a semiconductor fuse. Youmay find, however, that you may needcapacitors in addition to those in the kit.

A tape -stage mechanism problemOften a customer will live with various

VCR malfunctions, finally bringing in theVCR for repair only when a major failureoccurs, such as the display problem de-scribed above. One machine I encoun-

tered exhibited several problems. Whenthis VCR was powered up without a cas-sette, the cassette loaded light came on.Then the stage mechanism would moveout, as if to eject a tape (the stage motoris energized) and the cassette light wouldgo off. The stage would then try to pullin, but be stopped by the cassette loadinglatches (the motor would be energizedand then off). During this last maneuver,the cassette light would come on againand the VCR would finally automatical-ly power off.

It was possible to load a tape only whenthe stage tried to pull in. Attempting toload a tape at any other time caused thegears to lock up, and go out of alignment,especially if the cassette was forced in.During the time the VCR was trying topull in the stage mechanism, if the playbutton was pressed the machine would gointo play mode (remember this is with thestage in an up position, without a cassette).

If you just said to yourself that thisshould not happen because the IR tapeloading sensors should prevent this, youare correct. Just by reviewing the failuresymptoms, you saved a lot of time andpinpointed the failure cause. It should benoted that the cassette loading mecha-nism troubleshooting chart in the RCAmanual does not explicitly list the sensoras a problem area. Figure 10 is the trou-bleshooting chart from the RCA VLT600-603 manual.

Solving the tape -stage problem

This erratic stage behavior is caused bya failed (open circuited) supply sensorwhich sent a message to the VCR micro-processor that there was a tape loaded inthe machine. This sensor is a phototran-sistor with an emitter on the left side and

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S144 S143CASSETTE CASSETTEUP SWITCH DOWN

SWITCH

Figure 12. The cassette loading mechanism diagram from the RCA VLT600-603 manual.

a collector pin on the right. The base isbiased by light. This particular stage de-sign does not use a cassette -in sensor(which is located just above the tape load-ing area).

In this design, the presence of a cas-sette is sensed by the partial rotation ofthe tape -loading mechanism caused by

the user pushing in the cassette. This par-tial rotation of the mechanism results inthe actuation of two switches (S 143, RCA147281), the cassette -down and (S144,RCA 147281), the cassette -up.

These switches, located under the cas-sette loading motor, tell the microproces-sor to activate the stage motor. However,

with a failed end -of -tape sensor, this doesnot occur. Figure 11 shows the switcheswhen the right side of the stage mechanismhas been removed and turned upside down.

There are two levers, or arms, that rideon the gears and actuate the cassette sen-sor switches. The left arm is RCA 162232for VLT 600 and 161701 for VLT601 to603. The right arm is RCA 161702. Fig-ure 12 shows the cassette -loading mech-anism diagram from the RCA VLT600-603 manual.

Without a cassette, the left, or inner-most arm, is down and the right arm is up.You can also see a cutout on the blackgear that the arms are resting on. When acassette is loaded, both arms are up, andthe inner arm, which would otherwise bedown, is held up by part of the stage -load-ing mechanism (the metal arm on gearRCA 162963) pressing on it.

Aligning the gearsTo gain access to the cassette -loading

mechanism, remove the VCR top coverwhich is held in place by screws at theback of the VCR. Remove the front coverby taking out the front screws on the bot-tom of the VCR and then removing the

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Figures 13 and 14. The right side loading gears showing tnat they are aligned by having arrows on the gears point to e,

Figure 15. In this RCA VCR, the owner used a piece of duct tape to holdthe control door in place.

Figure 16. The hinge holding the control door of Figure 15 had beenbroken off.

three screws on the top of the unit. Thestage is secured by two black screws inthe front (where the tape is inserted) andtwo screws that hold it down to the VCRtape transport chassis.

When all the screws are removed, dis-connect the connector going to the stageloading motor printed circuit board. Thestage can then be lifted out from the rear,to clear two front tabs that are in slots inthe front of the VCR chassis. The left sidemechanism can be separated from thestage by removing two screws, one at thefront and one in the middle of the assembly.

To align the gears, look for arrows,which have to point to each other (Figures13 and 14). Be careful when aligning thegears as some of them are spring loadedand are under tension. The photographsshould help you in the alignment process.

Cosmetic fixesWhen all electrical repairs, and check-

outs, are completed you should see ifthere are any little cosmetic repairs thatcan be easily performed (you probably re-member advice like this from the businesscolumns in this magazine). For example,in the RCA machine discussed above, the

Figure 17. The hinge was repaired using a piece of thin gauge aluminum.

customer had the control door held inplace by a piece of duct tape (Figure 15),which looked ugly. The hinge holding thecontrol door in place had been broken off(Figure 16).

I made a new door hinge out of a pieceof thin gauge scrap aluminum, fastenedand screwed it into place (I first drilled asmall starter hole into the plastic and se-cured the screw in the hole with someepoxy, see Figure 17). This took about 15minutes. I then cleaned the unit up andmasked some scratches in the cover withsome silver model paint. While these re-pairs did not do anything to improve the

performance of the VCR they improvedthe appearance of the unit tremendouslyand the customer was pleased.

Note: Thomson Consumer Electronicscan generally supply manuals for ma-chines less than 10 years old: TCE Pub-lications, 1003 Bunsen Way, LouisvilleKY 40299, 502-491-8110. For modelsthat are almost 10 years old, or older, suchas the VLT600HF (a 1985 model), pho-tocopies of manuals can be ordered fromAlexander Graphics, P.O. Box 98, 3658Shady Lane, Plainfield, IN 46168- 0098,317-839-2372.


Electrical safety for servicepersonnelBy Ron C. Johnson

Of all the people who work with elec-tricity on a daily basis, electronic servicepersonnel should be the most aware of therisks involved. Consumer and office elec-tronics products contain both ac and dcvoltages that range from a few mV to over30kV. Every time we open up a piece ofequipment we should take that extra mo-ment to consider how to be safe.

When we don't take that time for con-sideration, that's when accidents happen.Maybe it's human nature; sometimes weget careless, or in a hurry, or sometimesa situation comes along that's new. Wejust don't consider the risks. Whatever thereason, electrical safety is an area that weall need to review from time to time. Thekey to safety is to maintain a balance ofknowledge, attitude and good habits thatwill make sure we leave our workplaceevery day under our own power insteadof on a stretcher.

What follows is a review-some factsabout electricity and its hazards, espe-cially as it applies to consumer electron-ics servicing, and some suggestions onhow you can remain safe at work.

Electricity and the human bodyFirst, how does electrical current affect

the body? Our bodies are mostly made upof water. Actually, water itself is not avery good conductor of electric current,but ionic solutions are. The water inhuman tissue contains sodium, potassiumand chlorine, ions which facilitate themovement of electrochemical stimulusthroughout the body. These electrochem-ical signals control muscles and allow sig-nals to travel from nerves to the brain.

When we receive an electrical shock,current flows through the ionic solutionof cells, overriding the normal low-levelsignals, and causing an effect called depo-larization. The effect can range from amild tingling sensation, to muscle con -

Johnson is a journeyman electronics servicing technicianand an instructor of technology at the Nonhern AlbertaInstitute of Technology in Edmonton. Alberta. Canada.

Figure 1. The type and extent of injury causedby electrical shock depends on a number offactors, including which parts of the body thatthe shock current flows through.

tractions, to fibrillation of the heart, andeventually to burns and tissue damage.(Fibrillation of the heart is a conditionwhere the heart muscle stops pumping ina stable rhythm and quivers randomly. )Table 1 shows the effects of various lev-els of 60Hz ac shock current.

Considering the fact that the resistanceof body tissue is very low, it is fortunatethat the resistance of skin is relativelyhigh. The range of skin resistance is from30052 to 100k52. A skin resistance as lowas mon could occur if the skin was verymoist or if access through the skin oc-curred because of a cut or other injury.Dry, calloused skin might reach 1001M.A more typical value for skin resistancewould be in the range of 2500Q to25,000Q.

Of course, for current to flow, two con-tact points are necessary. The points ofcontact and the route the current takesthrough the body also determine the ef-fects of shock (Also, current density, whichis the magnitude of current through a giv-en cross-sectional area, depends on wherecontact is made.) While 100mA from onearm to the other will cause fibrillation,less than 100pA directly through the heartcan cause the same effect (Figure 1).

The effect of frequencyAn interesting aspect of electrical

Table 1-The Effects of VariousLevels of Shock Current

Current EffectsLevel

100µA Current required directlythrough the heart to causeventricular fibrillation.

1 mA Lowest perceptible currentlevel.

5mA Muscle contractions andspasms which could causesecondary injuries.

20mA

50mA

Strong muscle contractions.Pain. Most victims cannot letgo.

Significant pain.Can cause ventricularfibrillation if duration ismore than a few seconds.

100mA Immediate ventricularfibrillation. Probably fatal.

over Burns, tissue damage,100mA immediate death.

1 A to 6A Current levels used to restartthe heart.

shock is the effect of the frequency. Whileshocks caused by dc voltages and low fre-quency ac can be extremely dangerous,higher frequencies tend to be less haz-ardous. For example, a 100mA current at!kHz, while noticeable, is neither painfulor life threatening. At much higher fre-quencies the person subjected to the samecurrent flow perceives no shock at all.

However, exposure to electromagneticradiation in the ultraviolet, microwaveand nuclear radiation ranges can causedirect damage to human tissue. Whilethese are not examples of electrical shock,


Figure 2. Always use an isolation transformer when servicing any type of electronics equipment

Figure 3. A leakage current check should be performed routinely as a part of any bench ser-vice or service call.

per se, they are definite hazards associat-ed with the electromagnetic spectrum.

An interesting book to read on this sub-ject is the biography of Nicola Tesla, theeccentric genius who invented the ac in-duction motor, and conceived the theoryfor ac power distribution as well. Teslaenjoyed performing spectacular displaysusing high voltages at high frequencies.He would place himself in series with acircuit carrying high levels of high fre-quency current and cause an arc to jumpfrom an electrode he held in his hand. Theother end of the electrode would melt withno harm to him even though all of the cur-rent flowed through his body. Don't trythis at home, folks. While he was unaf-fected, I wouldn't risk it.

Know the risksI think we all realize that the old line:

"What you don't know can't hurt you"doesn't apply in electrical safety. Youcan't see the presence of high voltage in-

side a television chassis but it's still dan-gerous. We have to know enough aboutthe situation to avoid poking our fingersinto the wrong places. If we're confront-ed with a situation for the first time, it'sour responsibility to find out what thehazards are. And, if we still aren't sure,we need to err on the side of caution.

Live chassisOne risk involved with television ser-

vice is the possibility of a live chassis.Whereas most electronic equipment iso-lates the internal circuitry by using apower -supply transformer, many televi-sion sets derive their power supplies di-rectly from the ac line. In so doing theyconnect the neutral side of the line to themetal chassis inside the set and use apolarized two prong line plug. Since theneutral is connected to the system groundin the panel this shouldn't be a problem.

But problems can occur in two ways.First, without an isolating power supply

transformer, all the power supply volt-ages are live with respect to earth ground.If some part of your body is grounded andanother part comes in contact with a sup-ply voltage, you could receive a shock.Lethal voltages exist at several pointswithin a television set. DC voltages in the120V range are used for the B+ supply.Peak to peak voltages in the 1000V rangeon the horizontal and vertical output tran-sistors, and up to 35,000V on the picturetube itself.

The second problem could occur ifsomehow the hot and neutral sides of theincoming ac line were reversed, either byincorrect replacement of the plug or anincorrect connection at the otherend. Thiswould put the television chassis at 120Vacwith respect to earth ground. Again, ifsome part of your body was grounded andyou contacted the chassis, you could beelectrocuted.

Protecting yourself from a hot chassisThere are several ways of protecting

yourself from a hot chassis problem:

When working with the power off,(changing parts or modules), always un-plug the set.

Always check the ac power cord tomake sure it hasn't been modified or tam-pered with. If you're not sure, measure theac voltage from the chassis cold ground tothe third prong ground at the receptacle. Itshould be OV.

Always use a one-to-one ratio isola-tion transformer when servicing any elec-tronic equipment (Figure 2). This re-moves the possibility of a shock hazardfrom the chassis, but it also removes thecurrent path from any of the power sup-plies or higher voltage signals to ground.

Another kind of hot chassis problemcan be created by leakage current in trans-formers and other equipment connectedbetween a voltage and the chassis. Thisproblem can occur in any kind of elec-tronic equipment and should be checkedroutinely as part of any bench check orservice call. In fact, every service manu-al recommends this practice and givesdirections on how to do it.

The test setup for this test is shown inFigure 3. A 1.5k52 resistor with a 0.151iFcapacitor in parallel is connected betweenthe chassis and ground. If the voltageacross the resistor exceeds 0.75Vac


Figure 4. A good way to discharge the picture tube is to use a high -voltage probe. Not only doesit drain off the charge slowly. but you can see the voltage coming down by watching the meter.

Approx. 1000volts peak

Horizontaloutputtransistor

A Flybacktransformer

HV.rectifier

Fromhorizontaldriver

>--

B +120volts

30,000volts

Figure 5. The voltages on the collectors of the horizontal and vertical drive transistors of a tele-vision can be dangerous. Although the average voltage may not be very high, the peak to peakvoltage of the waveform can be as high as 1000V. Care and attention and the use of an isola-tion transformer should help avoid a shock.

(which indicates a leakage current of0.5mA) the equipment is considered un-safe. The leaky component must be locat-ed and replaced.

High voltageAnother very dangerous hazard in tele-

visions is, of course, the high -voltagepower supply and its capacitor: the CRTitself. The high -voltage power supplyruns in excess of 35,000V; nothing to takelightly. Obviously, when you are workingon a live set you have to be careful aboutcoming in contact with the circuitry in andaround the flyback transformer and highvoltage lead. Also, the picture tube acts asa giant high -voltage capacitor and storeselectrical charges after the set is turned off.

It's always a good idea to discharge thepicture tube before working on the set.One way to do that is by using a high -volt-age probe (Figure 4). Not only does itdrain off the charge slowly, but you cansee the voltage coming down by watch-ing the meter.

Some technicians discharge the tube bycrossing two long screwdrivers, one con-nected to the chassis and one to the highvoltage connection on the tube. This willdischarge the tube but is risky with solid-state circuitry so close by. The large (albe-it short duration) current flow can createinduced voltages in adjacent circuitry largeenough to damage sensitive components.

High voltage is also a considerationwhen working on microwave ovens. The

magnetron in a microwave requires be-tween 1.8kV and 4.5kV depending on themodel. Again, stay away from that part ofthe circuit when doing service. And, ofcourse, don't try to measure that level ofvoltage with your DMM. The power sup-plies for these use voltage doublers andhave capacitors which, although they havea bleeder resistor across them, should bedischarged before working on them.

The voltages on the collectors of thehorizontal and vertical drive transistors ofa television can be dangerous as well.Although the average voltage may not bevery high, the peak -to -peak voltage of thewaveform can be as high as 1000V (Fig-ure 5). Care and attention and the use ofan isolation transformer should helpavoid a shock.

Critical componentsMost schematics indicate which com-

ponents must be replaced with the origi-nal equipment manufacturer's part. This,too, is a safety consideration, probably forboth the customers' safety, and also foryour protection from liability in case offire or other accident after a repair is made.Regardless of your opinion of whethert his is needed, it is important that you heedthis warning.

Picture tubesEven with the improvements which

have been made to the manufacture of pic-ture tubes over the last few years, properhandling is still important. Because of theinternal vacuum and the large amount ofsurface area of a picture tube, implosionis a hazard. Always handle picture tubeswith great care, using the proper packag-ing for transport. Never lift or carry thepicture tube by its neck. If a picture tubemust be disposed of, pack it in an appro-priate carton which covers as much of thetube as possible, then break the seal tosafely equalize the pressure.

Never let your guard downCertainly these hazards and suggestions

don't cover every hazardous situationencountered in the service center, but theydo point out a few of the most comon ones.Probably by now you're saying, "Yeah, Iknow all that. I'm safe. . ." Interestingly,while a lack of knowledge is a big con-tributor to accidents in the home, statisticstell us that carelessness is the number onecause of accidents in work situations.


On the job we usually know the risks;they become familiar to us and we getcomplacent. Lack of concentration on thetask at hand, repetition of menial tasks,tiredness, and many other factors contri-bute to carelessness. The following itemsare general suggestions to help you avoidthose kinds of accidents:

Always do your job according to thesafety instructions given by the product'smanufacturer, by your training or by com-mon sense.

Get in the habit of using that isolationtransformer, discharging the picture tubeand handling it with care.

Clean up your work area after the jobis completed.

Make it standard procedure to do aleakage current test as the last step inevery service procedure.

Think before disconnecting or con-necting electronic equipment. Assumethe worst, check it out and then proceed.

When working on live equipment,minimize the risk of shock-removerings, watches and bracelets.

Never allow two parts of your bodyto come in contact with a circuit. If pos-

sible, keep one hand in your pocket whileprobing with a meter.

Safety trainingMany employers provide First Aid

training or support their employees inobtaining such training. A knowledge ofFirst Aid could make the difference be-tween life and death for a fellow worker.Training in artificial resuscitation, theHeimlich maneuver, cardiopulmonary re-suscitation and other procedures are wellworth the investment in time.

Know the fire and emergency proce-dures set out by your employer. If noneexist, your local fire department wouldprobably help in creating them. Be awareof the location of fire extinguishers, theappropriate type for electrical fires andhow to use them. Never use water to putout an electrical fire. In the event of a fire,a quick and appropriate response can savelives and property.

Safety is a habit, and an attitude. Acceptresponsibility for your own and others'safety. Think about the potential risk inall jobs and then take steps to avoid acci-dents. And, don't ever allow yourself totake shortcuts that could endanger yourlife or someone else's.

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Test Your Electronics KnowledgeToolsBy Sam Wilson, WA8RMS

The information for this TYEK is provided by SNAP-ON Tools. crated in Figure 1. Assign the correct letters to the numberedHere is an alphabetical list of the names for the screw slots illus- blanks in the illustration.

A.H.B.I.C.J.D.K.E.L.F.

M.G.N.

Clutch (Type A)ScruloxClutch (Type G)Slab HeadHex CapSlottedMulti SplineTorq SetPhillipsTorxPOZIDRIV

Triple SquareReed and PrinzTri-Wing

(Frearson

Wilson is the electronics theory consultant for ES&'I

(Answers on page 66)


Audio Corner

Radio Broadcast Data Systemto be test marketed nationallyFrom the EIA Executive Report to the Electronics Industries

The Radio Broadcast Data System(RBDS) will be introduced in ten majorU.S. media markets this year as part of anindustry -backed effort to bring this inno-vative digital technology to the nation'sconsumers and broadcasters.

Under the terms of a promotional cam-paign sponsored by the Electronic Indus-tries Association's Consumer ElectronicsGroup (EIA/CEG), radio stations in theselected cities will be given an opportu-nity to test the RBDS technology for 30days at no expense. Details of the planwere unveiled January 7 at the 1994 Inter-national Winter Consumer ElectronicsShow (CES).

According to Gary J. Shapiro, group vicepresident of EIA/CEG, "our manufactur-ers are convinced that the RBDS will de-liver a host of new features that will ben-efit consumers, including the ability to re-lay useful control and display to radios."

Toward that end, the industry programwill target the leading radio stations in tenmarkets: Atlanta, Boston, Dallas, LosAngeles, Miami, New York, Philadel-phia, San Francisco, Seattle -Tacoma, andWashington, D.C. Participating stationswill be provided with RBDS encoders, aswell as a variety of materials designed tohelp the stations promote the new tech-nology to their listeners.

Range of servicesThe RBDS technology, which was

standardized under the aegis of EIA/CEGengineers in January 1993, allows broad-casters to transmit radio text as digitaldata on an inaudible subcarrier. Individ-ual radio stations may purchase an RBDSencoder for as little as $2500.

This should increase the likelihood thatRBDS's growth will accelerate by break-ing the so-called "chicken -egg" circle(i.e., a reluctance on the part of broad-casters to gear up for a new service be-cause so little consumer hardware hasbeen sold, and a corresponding reluctanceon the part of consumers to buy new hard-ware in the absence of programming in

the new media). The chicken -egg prob-lem initially hindered the growth of colorTV in the 1950s and stereo TV in the1980s, and, in the opinion of some indus-try observers, could well complicate theintroduction of high -definition television(HDTV) in the latter part of this decade.

( wistimer featuresAmong the specific services that can be

offered through RBDS broadcasters totheir listeners at home or in their cars are:

display of the station's call letters; tuning by program format (e.g., Top

40, Country, Sports, Oldies, etc.); display of song titles and artist names: the ability to switch to alternate fre-

quencies (e.g. for continuous recep-tion of network programming on longtrips);

automatic announcements of emer-gency and traffic bulletins;

paging information; business and navigation data.

Broadcasters, for example, would beable to send, and consumers receive, texttransmission, enabling radio stations toidentify themselves by call letters and fre-quency, convey information about the re-cording artist and song album/titles, andfeed clock synchronization signals.

In addition, broadcasters could interruptin -car CD and cassette players with trafficor emergency alerts. Another practicalapplication would allow consumers to scanthe car or home radio dial for a particularprogram format (e.g. "Soft Rock"), whichcould be especially useful on road trips, butequally useful in the comfort of the home.

The RBDS technology could also be-come part of a successor to the Emergen-cy Broadcast System. RBDS makes pos-sible a new -generation system that notonly would transmit information whenpeople are listening to their radios, butactually could turn on radios in the mid-dle of the night to alert them to a poten-tially devastating storm, for example.

While car radios would feature an eight -

character display, home RBDS receiverswill offer a 64 -character display makingpossible the transmission of stock quotesand other financial information, sportsscores, and statistics, weather reports, andeven brief commercials.

Auto manufacturers on boardAt the Winter CES in Las Vegas, a num-

ber of consumer electronics manufactur-ers and marketers demonstrated RBDSproducts, including Coupon Radio, Del -co, Denon, Goldstar, Grundig, Interna-tional Jensen, Panasonic and Philips.

Radio stations participating in the pro-motion will be provided with an REAmerica RBDS encoder, distributed byHarris Allied, a worldwide distributor ofbroadcast products. RE America, a man-ufacturer of RBDS encoders based inWestlake, OH, will advise station engi-neers on their use.

Broadcasters will be encouraged topurchase an RBDS encoder after trying itout for 30 days. Each participating stationwill be surveyed for reactions to the newtechnology from the viewpoint of stationpersonnel, broadcasters and listeners.

A cooperative effortRBDS was developed for the United

States and Canada by the National RadioSystems Committee (NRSC), an inter-industry panel including representativesof both EIA and the National Associationof Broadcasters (NAB). The NRSC ischarged with developing standards fortechnologies affecting both transmittingstations and radio receivers.

The RBDS effort is based on the RadioData System (RDS) which has been inoperation within the European Commu-nity since 1984.

The North American RBDS system istailored to the market conditions of theU.S. and Canada. However, it was thegoal of the NRSC to develop a systemcompatible with the European standard sothat a single receiver can be used in bothNorth America and Western Europe.


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Expires

What do you know about electronics?

Electronics speakBy Sam Wilson, WA8RMS

You can review a lot of interesting the-ory by paying attention to the words thatdescribe it.

You will remember that equivalent ser-ies resistance (ESR) is not the series resis-tance of a capacitor. Instead, it is a resis-tance equivalent to the series and parallel(leakage) resistance of a capacitor. Wehave reviewed that concept in a previousWDYKAE?.

Now, let's review the familiar word"superheterodyne." Let's break it intosmall pieces with a little review. Hetero-dyning takes place when two signal fre-quencies are mixed in a non-linear device.

Figure 1 shows what happens. Thereare always four output signal frequenciesavailable in this situation. It wouldn'tmake much sense to go to all of that trou-ble to get signal frequency A or signal fre-quency B because you already have thosefrequencies at the start. So, those possi-ble outputs are sometimes disregarded.But you should always remember thatthey are available.

If the device is linear, no heterodyningcan take place.

The output circuit of the non-linear de-vice or circuit is tuned for frequency A+Bor frequency A -B. All other frequencies

Wilson is the electronics theory consultant for ES&T.

are eliminated. The mixer in the blockdiagram of Figure 2 is not a linear circuit.The two signals heterodyne to producefour outputs: RF frequency, oscillator fre-quency, RF frequency plus the oscillatorfrequency, and RF frequency minus theoscillator frequency. The usual practice isto tune the output to the difference fre-quencies. Tuned circuits in the amplifierpass only the difference frequency.

OK, now we have reviewed heterodyn-ing in the mixer stage. But, why super-heterodyne?

The detector is usually a diode. What-ever it is, it must be a non-linear deviceor circuit, because detection in that stageoccurs as a result of heterodyning.

The two signals of interest are: The car-rier (which is reduced to the intermediatefrequency) and the sideband (which con-tains the audio).

One sideband is eliminated in the detec-tor stage. If you didn't do that the positiveand negative half cycles would cancel.

When the carrier and the sideband het-erodyne in the detector there are four out-put signals available:

Carrier (now at the if frequency) Sideband frequencies (they have the

same audio info.) Carrier plus the sideband frequencies Carrier minus sideband frequencies

The carrier minus the sideband fre-quencies heterodyne to produce the au-dio. (Note: If the upper sidebands are het-erodyning the audio is obtained by sub-tracting the carrier from the sideband.)

The carrier and one of the sideband fre-quencies are usually eliminated by a filtercircuit in the output of the detector stage.

The receiver is a superheterodyne re-ceiver because heterodyning takes placein two places.

If you don't believe that, try substitut-ing a nice linear carbon resistor for the de-tector diode and see what you get.

Mixer, converter ...While we are talking about heterodyn-

ing, here is something to think about.You know that a converter is a circuit

that combines a mixer and oscillator intoa single stage.

OK, what is another name for this typeof converter?

Yes, I know it is referred to as the "firstdetector," but I'm looking fora name thatstarts with an 'A.'

Check your answer at the end of thisinstallment of WDYKAE?

Letters I can't answerOne reason m so far behind in my mail

is that I' m on a fire drill. I have one month

SIGNALFREQUENCY

A

SIGNALFREQUENCY

B

NON-LINEARCoVICE

ORCIRCUIT

SIGNAL FREQUENCIESA + B

SIGNALFREQUENCY

A

SIGNAL- FREQUENCY

B

SIGNAL FREQUENCIESA - B

Figure 1.


to finish two books. Joe Risse (Scranton.PA) and I are co-authors and he is diffi-cult to keep up with.

If I owe you a letter, please be patient.However, if you don't get a reply maybeyou are one of the following:

A reader in Canada sent a letter that Itried to answer. For his address he gave alot of abbreviations. I guessed that mustbe the way they address letters, so I copiedhis abbreviations. The post office inCanada marked my returned envelope"insufficient address."

Another reader sent me a FAX in careof the Kansas office for ES&T. That'sOK, but all he gave me was a FAX num-ber. They mailed his FAX to me, and, Ihave no address to send a reply.

Another reader sent me a technicalquestion and said if I don't know theanswer I should "ask around the office."My wife Norma is the only person whocomes into this office; so, I asked her. Shedidn't know.

We're losing another raceYou need a program to keep track of the

races we are losing. I have a list of thingsto worry about-when I can find that list.

Put this on your list: We (the U.S.) arelosing our first place position in Techno-paradigm shift.

It means that production leads to R&D,and, R&D leads to more production, andproduction leads to more R&D, etc.

As you know our lead in production isdisappearing fast, so, our R&D is goingwith it.

Put that on your list.

Sound offI have been corresponding with a very

nice man in Ghana, West Africa. He hasa serious problem with pests-like bugs.He sent the schematic for a sonic bugeliminator that didn't work.

He asked me for the plans of one thatwould work. I have spent hours goingthrough articles and schematics trying toget him an answer.

The problem is that all of the materialI've turned up say the devices don't work.

In desperation I ordered a commercialunit for him. It takes 4 to 6 weeks to getit. In the meantime, if any reader has anyinformation, or, a schematic of a unit,please send it to me in care of this maga-zine. put it in this column, and alsosend it to him.

His name is Joseph Anie. The way heputs it, it is very difficult to work in his

Figure 2.

shop because of the pests. I feel confidentin asking for your help. After all, manytechnicians deal daily with bugs!

Brain deadAs a technician you are certain to be

aware of computer viruses. The recentMichelangelo virus has focused attentionon the virus problem.

There is a sickness about a computervirus, but the sickness isn't in the com-puter. It's in the people that generate them.

In 1956 there were only five known vi-ruses. Today there are over 500. Virusesare now being discovered at the rate oftwo new viruses per day!! That informa-tion comes from the National ComputerSecurity Association and Marty Baumannof USA Today.

A meaningless termWhen a term is made up to describe

something in electronics that term shouldat least be technically correct. Unfortun-ately, there are people who seem to haveenough influence to make up words thatare wrong. For some reason that I willnever understand, those terms catch on.

I think every child in the first gradeknows that when you multiply RMS volt-age by RMS current you get average power.

Whoever it was that decided to call itRMS Power needs a lot of help. Technic-ally, there is no such thing as RMS power.The term is meaningless.

Here is another one that makes no senseat all: white noise. Now, white noise isnoise with a frequency range of nearlyzero hertz to somewhere in the GHz fre-

quency. Noise throughout that range isnearly constant in a discussion of whitenoise. However, when the noise is gener-ated by an electronic device, such as bipo-lar transistor, the noise is not evenly dis-tributed. So, it's not white noise. That isnot the issue here.

The concept of white noise is that itsamplitude does not go through high am-plitude variations within a given rangeof frequencies.

Now, if you have had experience try-ing to get white on the screen of a colortelevision receiver, do you mix equal val-ues of blue, red and green?

Hardly. If you do that, you get an uglycolor. Of course, beauty is in the eye ofthe beholder-right? OK, try this little ex-periment. Set the color on your color TVCRT so that blue, red and green haveequal amp-litudes. What do you thinkabout that color?

The point is this; white color is notmade up of even values of colors through-out the color spectrum.

I know what you are going to say. Theproblem is in the human eye. Well, I spenta long time in the library trying to answerthis question: Is white an even mixture ofthe frequencies of color regardless ofwhat a human sees when observing it, or,is white only white if a human perceivesit to be white?

I believe white is only white if it is per-ceived by a human as white. If you agree,then you will also agree that the termwhite noise is silly.

The answer to the question asked in thefirst part of this article: Autodyne mixer.


Video Corner--

Solving a VCR short circuitBy Steve Babbert

Never underestimate the value of yoursenses when trying to solve an electron-ics problem. Sometimes your senses canbe of greater value than the most elabo-rate test bench. My senses of sight andsmell recently helped me to make a repairthat otherwise might have been an impos-sible one.

A friend brought me a Radio ShackVCR (Model 18) that had been aban-doned in a house. The FCC ID number ofthis unit was BEJ9QK16507. These IDnumbers were discussed in the Decemberissue of ES&T (Page 8). The chart ofFCC ID number prefix vs manufacturerin that particular issue shows that thisVCR was made by Goldstar.

Erratic operationThe VCR would play for a while and

then shut down. Sometimes it would notplay at all. I knew that it must have had aserious problem or it wouldn't have beenleft behind. It wasn't very old and lookedgood on the outside.

Because the problem was intermittent,I removed the cover of the VCR andbegan looking for loose connectors.Before long I found that a small boardattached to the right hand side of the tapetransport mechanism was sensitive(Figure 1). When I applied pressure to theboard, the machine would shut down. Itmade no difference what mode the VCRwas in. Each time this happened, I wouldnotice a very faint odor that smelled likemotor brushes sparking; however, I

couldn't see or hear any spark discharge.Every time the unit shut down, I had toreset the controller IC by first unpluggingthe VCR and then plugging it back in.

Someone else had been here beforeI had to remove the tape transport

mechanism to get a closer look at thesmall circuit board. As I was doing this.I noticed that some plastic screw wells

Babben is an independent consumer electronics servicingtechnician.

Figure 1. Here you can see the circuit board mounted on the right hand side of the tape trans-port mechanism. Any pressure applied to the board or the connecting wires would cause the unitto shut down.

were stripped, probably as the result ofrepeated disassembly and reassembly. Atthis point, I was pretty sure that someoneelse had been on the same track I was on.

With the tape transport mechanism

removed, and in its fully unloaded posi-tion, I marked the gears with a permanentmarker to insure proper reassembly. NowI could remove the motor, worm gear, andidler gear that covered the board. Finally,

SHORT/CIRCUIT

Figure 2. With the board removed, close observation will reveal a tiny burned spot where thetrace came into contact with ground.


I removed the board itself. Close inspec-tion of the board revealed nothing unusu-al, and continuity checks showed no opentraces or bad connector contacts.

The only component on the board wasthe Take -Up Reel End Sensor. Other thanthis sensor, the only things on the boardare the traces that route signals from thetape sensors and switches to the systemcontrol. The next step was to power upthe machine with the board removed fromits mounting support. The purpose of thiswas to help localize the problem.

The VCR did seem to power up withthe board removed. I repeatedly flexedthe board and worked the connectors, butit never shut down when the board wasremoved. I even applied pressure to themounting support and surrounding area,and again, it never shut down. Had I dis-lodged a particle from a connector whenI removed the board? Maybe I brokethrough some oxide on one of the pins. Itouched up the solder connections, to besafe, and then reassembled the entire unit.

The problem reappearsI put the VCR into the play mode and

everything operated perfectly. I appliedpressure to the board, and just as before,the machine shut down. Again there wasthe faint smell of a spark discharge. NowI understood why the screw wells werestripped; someone had been down thisroad several times. Very possibly the unithad been diagnosed as too costly to repair.

At this point, I became more deter-mined than ever to locate the source ofthis strange problem. I realized I had tofind this problem with the unit assembled.I moved my lights around so that I couldbetter view the area from different angles,and finally, I got lucky.

The next time I caused it to shut down,I saw a small spark between the mount-ing tab at the top of the board and the metalmounting support that held the board inplace. Even with my 20/20 vision, I sawthe faint spark only because a shadowcovered the area. Now I knew that one ofthe traces was coming into contact withthe mounting support, but I didn't knowhow this was possible?

Once again, there was no choice but toremove the board from the transportassembly, but at least now I knew exact-ly what section of the board to focus on.

Figure 3. With the unit reassembled, and the paper it sulator in place, the problem was correct-ed. Here are two views of the PC board with the insu ating cardboard in place.

I removes the board and placed it undera good ligit that revealed a type of designflaw that I'd never seen befcre. A circuittrace was routed along the tab of the boardthat fits into the metal support. There wasonly a thin layer of enamel to insulate thistrace from ground. A very small burnedspot is visible in Figure 2.

The purpose of this trace is to route Vto the Tape End Sensor. When this tracecame into contact with the metal support,Vec was pulled to ground. No doubt a fail-safe circuit caused the unit to shut down.

When the load motor was actuated,friction eventually scraped away a bit ofthe insulating enamel. since there was alot of mechanical movement in this area.I wondered how often this problem hadoccurred in VCRs using this transportmechanism. I had a Sears VCR on anoth-

er bench that used the same mechanism,but it showed no signs of this problem.

Correcting the problemThere are any number of ways that one

could correct this problem, but I chose theleast costly and time consuming route. Ireassembled the machine, and folded astrip of paper cut from a business card.You can see the folded strip in Figure 2.I then slipped the paper insulator betweenthe board and the support. Next, I foldedthe ends over the mounting support tokeep it from slipping down through thesupport opening. The paper insulatorwould be held in place by the wires oncethey were routed into their normal posi-tion. Now I make this modification when-ever I service a VCR that has a chassis ofthis style.


Business Corner

Will Total Quality Managementwork for you?-Part 9By John A. Ross

This is the ninth part in the BusinessCorner series on W. Edwards Deming'sTotal Quality Management.

TQM point 9Break down the barriers between de-

partments. Does your service organiza-tion contain separate, distinct depart-ments? Most modern organizations con-sist of interrelated units that direct theirefforts toward an overall organizationalgoal. Obviously, cooperation and com-munication among those departmentslead to happier employees and higherprofits. Yet, if the departments within theorganization independently drive towardgoals and have different ideas about theoverall goals of the organization, the re-sulting competition can cause several dif-ferent types of problems to arise.

The lack of meaningful communicationsbetween departments, the uneven allo-cation of resources among departments.or perceived favoritism can raise barriersand cause even the best of teams to fail.

Meaningful communicationWhen we talk to each other about the

weather, we communicate, but in lessthan meaningful terms. Meaningful com-munications between departments in-volve conversations about issues such asservicing techniques, responses to cus-tomer inquiries, personnel policies, andtechnological innovations. Additionally,meaningful communications can includethe establishment of a family -like atmos-phere throughout the organization. Thus,when we speak about meaningful com-munications in this context, we have thegoal of firmly rooting in-house loyaltiesand cooperation throughout the entireorganization.

In the end, a paucity or the completeabsence of meaningful communicationbetween all units of an organization de -

Ross is a technical writer and microcomputer consultantfor Ft. Hays State University. !Lc,. KS.

tracts from the best uses of resources,sometimes causes a duplication of efforts,or sends departments in opposite direc-tions. If the service and sales departmentsof an electronics retailer do not communi-cate about major service problems andtypes of equipment sold, the companymay find that customer dissatisfactioncoincides with the increasing sales of anelectronic product that fails regularly.Yet, because the departments have differ-ent priorities and few things in common,departmental managers and companyowners may never make the connection.

Sometimes, a manager or owner will

lack knowledge about a department'sneeds or tasks needed to efficiently allo-cate resources for the department. The un-even allocation of resources unnecessar-ily creates barriers within the organiza-tion. For subordinates, one easy solutionto this problem is to provide informationabout departmental activities and tasks toyour supervisors or the ownership.

An example of this tactic would be thepresentation of a monthly or annual reportshowing a comparison between the num-ber of activities and the number of man-hours needed to complete the activities.Figure 1 shows a brief example of an an -


Books1992

Total number of microcomputers 889

.12_a9

1030

Total number of microcomputer peripherals(printers, scanners, modems, other)

460 528

Total number of microcomputer -related devices 1349 1558

* Numbers provided by Budget and Planning-January 4, 1994

System purchase dates range from /986 to 1993

1993 Requests for computing services received and completed by themicrocomputer services section

Total number of requests received: 485Total number of requests completed: 477

Completed requests by category

Total number of administrative requests completed: 228Total number of academic requests completed: 249Total number of requests completed: 477

Total number of hardware -related requests completed: 233Total number of software -related requests completed: 244Total number of requests completed: 477

Figure 1. Microcomputer system and peripheral device yearly purchase totals*

nual report. This type of information be-comes especially valuable when a depart-ment head finds it necessary to justify ad-ditional resource or budget requests.

The squeaky wheelIt is an unfortunate reality in our soci-

ety that the "squeaky wheel gets the oil."Many times, assertive managers, or man-agers who are aware of the preferencesshown by the ownership win the resourcebattle. However, many of us cannot turnon the charm or become competitive whenwe need those characteristics. A carefulassessment of needed resources, a con-stant flow of information about depart-mental activities, and factual justifica-tions not only show organizational skillsbut help managers make good decisions.

The uneven allocations of resources orbudgets throughout any organizationsends a clear message to every depart-ment head. Departments that receiveplenty feel rewarded, while departmentsthat receive less begin to doubt their

worth for the organization. Certainly, un-even resource allocation can spark un-healthy competition between depart-ments, the perception of favoritism, andthe type of jealousy that provokes con-flict. Managers and owners need to beaware of the power of their actions whenallocating scarce budget resources. Here,managers and owners must have the sameawareness of departmental needs and jus-tifications when making decisions. Inaddition, they need to remain aware of thevalues of both the entire organization andthe individual departments.

as you can see, breaking down depart-mental barriers takes us back to human -to -human communications and a deeperunderstanding of the organization. In ad-dition, breaking down those barriers be-comes a matter of everyone in the orga-nization paying attention to both the "bigpicture" and those sometimes hidden "lit-tle pictures." Thus the practice of manag-ing becomes an art as it intertwines withlistening, working and learning.

Exploring The Internet, By CarlMalamud, Prentice Hall, 360 pages,$26.95

The global village is here and is docu-mented in a professional reference book,Exploring the Internet: A Technical Trav-elogue. In an easy -to -understand, narra-tive style, Carl Malamud chronicles hisInternet journey, circling the globe threetimes, visiting 56 cities in 21 countries insix months.

Carl Malamud, noted author in thecommunications field, takes readers on avoyage of discovery in this book. He pro-vides readers with technically sophisti-cated descriptions of everything fromnew networking research to efforts tobuild national and international infra-structures. The Internet is a technicaltravelogue aimed at networking profes-sionals and anyone who wants a behind -the -scenes look at the people, laborato-ries and institutions that make up theglobal village of the Internet.

"This book demonstrates what many ofus have long felt: the worldwide networkis here. Interoperability is not some imag-inary goal at vendor briefings, but a con-crete part of networks all over the world,"says Dan Lynch, Founder and President,Interop Company, in the foreword. "Aswe read about the worldwide spread ofthe Internet, we get to appreciate its diver-sity and its usefulness to millions."

In this book, Malamud takes us on hisnarrative journey, three times around theglobe. A few places visited by Malamudinclude: Hong Kong, where Malamud ex-plored the Royal Hong Kong Jockey Clubwhere a massive network handles mil-lions of bets each race day, Amsterdam,where he witnessed the birth of EBONE,a long-awaited pan-European researchbackbone, Tokyo, where he takes a be-hind -the -scenes look at the world's mostexpensive building, Tokyo City Hall, andintroduces you to Jun Murai, the InternetSamurai, one of Japan's leading research-ers and the architect of Japan's nationalnetwork, Singapore, where Malamuddigs into the government's highly ambi-tious plans to make an "Intelligent Is-land," a digital port for the network high-ways, and Geneva, where he wages battlewith the international standards cartel inan effort to make standards documentsmore widely available.

Prentice Hall, Simon & Schuster Education Group,Englewood Cliffs. NJ 07632


Troubleshooting TipTT0194 Emerson Color TV Dead SetDavid NI. Luckner

Have you ever thought that a serviceprocedure was going to be simple andstraightforward then find that other prob-lems arise as you're trying to completethe job? I am sure every service techni-cian has experienced this situation before.Here's a description of one service jobthat I performed that looked simple at theoutset, but became complicated.

A customer brought an Emerson TVinto the shop with the complaint that theset was dead. After pulling the back off,I checked the obvious areas such as thepower supply and horizontal output.

The 2SD1555 horizontal output tran-sistor was shorted, and a resistor in thepower supply, R502, 3.9L1, was open.After replacing the horizontal output tran-sistor with a universal replacement andR502 in the supply, I applied power to theset at reduced line voltage for about fiveminutes using a variable ac supply, thenshut down the supply.

The high voltage transformer and H.O.T.heat sink were very hot to the touch, so Idecided to replace the transformer. Witha replacement transformer installed, Iagain ran the set at reduced line voltagefor about five minutes. This time the re-placement transformer was running cool,but the horizontal output transistor wasexcessively hot.

I increased the line voltage to check theraster for any defects, and noticed a blackborder coming in on the right side with acripple. There was also a faint drive linepresent in the center of the screen. In thepast I've found that defects in the hori-zontal drive can cause such symptoms.

After careful inspection, I found thatC446, a 11.1F decoupling capacitor on theprimary of the horizontal drive trans-former, was open. After I replaced C446,the raster was greatly improved, but aslight black border still remained on theright side.

I turned my attention to the universalreplacement transistor, wondering ifthere might be a compatibility problem.

Luckner is an independent servicing technician.

In the past I've found that in some casesuniversal replacement components maynot be close enough to the original to per-form as necessary, so I decided to substi-

tute the replacement with the exact partnumber, 2SD1555. When I turned the seton after making this change, the set oper-ated perfectly.


ClassifiedClassified advertising is available by the word or per column inch.By -the word. $1.65 per word, per insertion, pre -paid Minimum charge is $35 per insertion. Initials and abbre-viations count as full words. Indicate free category heading (For Sale, Business Opportunities, Miscellaneous,Wanted). Blind ads (replies sent to ES&T for forwarding) are $40 additional. No agency discounts are allowedfor classified advertising by the word. Contact Linda Romanello at 516-681-2922 to place your classified ad(by -the -word). Mastercard, VISA, American Express and Discover are accepted for FAX or mail orders.Per column Inch (classified Display): $235 per column inch, per insertion, with frequency discounts avail-able, 1" minimum, billed at 1/4" increments after that 10" maximum per ad. Blind ads are $40 addition. ReaderService Number $25 additional to cover processing and handling costs. (Free to 4 -inch or larger ads.) Formore information regarding classified display advertising please call 516-681-2922. Optional color (deter-mined by magazine) $150 additional per insertion.

Send your order, materials and payments to:Electronic Servicing & Technology, 76 North Broadway, Hicksville, New York 11801

Attn: Linda RomanelloPhone: 516-681-2922 FAX: 516-681-2926

FOR SALE

» TIPS 1/2 HALF 1/2 PRICE SALE «THE TIP COMPUTER PROGRAM is now on salefor HALF price. Over 13,000 TIPS & growing !!!One of the LARGEST tip databases in the WORLD- TV's, VCR's, CAMCORDER's plus other elec-tronic equipment. HIGH QUALITY PROFES-SIONAL SOLUTIONS with up to 8 LINES of prob-lem solving information. ADD your own tips tothe program and find them FAST !!! Stop writingYOUR valuable repair tips on schematics. PRINTany tip. Special DATA EXCHANGE & UPDATEfeature INCLUDED with every program.Exchange tips with others that have the TIPProgram. We have 12 years computer program-ming experience -avoid those other programs.Don't even consider buying another technical tipprogram until you have seen our FREE Demo.Our clients say this is the BEST technical tipdatabase program on the market today. Pays foritself with 1 or 2 repairs. Works on ANY IBM com-patible computer with a hard drive INCLUDING486, 386, AND older 286 & XT models. Updates& Network versions available. Used by National,State & Local Electronic Association Members &Officers. Only $150.00 !!! (High density disks -specify size). Prepaid orders (check/money order)within the U.S.A. are shipped FREE. Call for:C.O.D., Double density disks. and shipping out-side U.S.A. Have your own tips? Ask about ourTIP EXCHANGE policy with FREE Updates.Already have a tip program? Ask about ourCompetitive Upgrade Discount!!! P.S.-can'tfind the right software for your business or otherinterests? We write CUSTOM SOFTWARE to fityour needs. CALL NOW !!! (516) 643-7740HIGHER INTELLIGENCE SOFTWARE, 60Farmington Lane, Melville, NY 11747.

ZENITH MODULE REPAIRSSave Money Repair Your Own ZENITH Modules248 CURES 9-124 THRU A-9740 LASER PRINT-OUT $29.95 INCLUDES S/H MAIL CHECK TO:Electronic Software Developers Inc. 826 S.Main Street, South Farmingdale, NY 11735.

TV CASE HISTORIES: Booklet with 2,150+histories. Satisfaction assured. Only $45 (plus$2.90 for 2 -day priority). Mike's Repair Service,P.O. Box 217, Aberdeen Proving Ground, MD21005. Same mailing address 30 years. SendSASE for samples. 410-272-4984, 11 am-9pm.

Sencore SC3080 Waveform Analyzer, manuals &leads included. Excellent condition. $1950.00. Call1-916-283-3912.

FOR SALE

VHS -VCR Repair Solution Sets I - IX. Each con-tains 150 symptoms and cures, cross referencechart, free assistance, $19.95 each, all nine$49.95. Visa/MC. Eagle Electronics, 52053Locks Lane, Granger. IN 46530.

Earn $$$ Repairing Zenith TV's to componentlevel -now over 1000 models 1983-1993.Guaranteed effective and reliable cost cuttingsymptom/cure format. Buy manual and receiveFREE report, "Repair Shorted CRT's Fast" (rasterone color) $89.95. Demo Sampler $19.95. ZMEX,807 Queen Palm Lane, Sarasota, FL 34243.

DON'T BE MISLED BY OTHERTECHNICAL TIP PROGRAMS - SERVICE TIPSIS DIFFERENT FROM ALL OTHERSIT ORGANIZES 15,700 CURRENT SERVICETIPS IN ALPHABETICAL ORDER BYModel/Brand & Symptom with ConciseSolutions, Part Numbers, Values & Locationson ALL CONSUMER ELECTRONIC EQUIP-MENT. Created by 2 CET'S with over 20 YEARSexperience arid a computer programmer, IT'S ANINDISPENSABLE TOOL FOR ANY SERVICECENTER. Members of electronics associationssuch as NESDA-NYS, NESDA, METSDA, TESA-LI use this program and confirm there is no bet-ter program available today. DUE TO THE OVER-WHELMING RESPONSE TO OUR PREVIOUSADS, NOW for a LIMITED TIME ONLY SER-VICE TIPS is available for the low price of$129.95 Demo Disk & Quarterly Updates arealso Available - Call or Write Electronic SoftwareDevelopers, Inc. 826 So. Main Street, SouthFarmingdale NY 11735 Phone (516) 221-6403.

TEST EQUIPMENT BOUGHT & SOLD: OSCIL-LOSCOPES, ETC. 1-408-738-4206. FAX 1-408-736-6946.

TECHNICIANS, FINALLY A BREAKTHROUGH INSMD REMOVAL! CHIP QUIKTM SMD REMOVALKIT. NO DAMAGE TO CHIP OR BOARD LOWOPERATING TEMPERATURE EASY TO USEINEXPENSIVE. $12.00 per kit + $2.00 S&H. 24 hr.Fax for credit card orders (508) 879-7727. Infoavailable, call CHIP QUIK, 3 SECOND STREET,FRAMINGHAM, MA 01701, 1 -800 -836 -CHIP.

REDUCED 85%. Diehl Mark III $69. Diehl Mark VHorizontal circuit tester $179. New. Conductivecoating for remote control keypacs $9.99 ppd.WEEC, 2411 Nob Hill Road, Madison, WI 53713.608-238-4629, 608-233-9741.

FOR SALE

HELLO! Doctors Do It! Airlines Do It! Lawyers DoIt! TVNCR TECHNICIANS SHOULD DO IT!In any profession, knowledge coupled withthe ability to quickly access case or service his-tory PAYS BIG!! A doctor's ability to diagnose quick-ly and confidently is largely attributed to use of andaccess to data and information of previously diag-nosed symptoms. Lawyers can predict the outcomeof the r present case by using prior case historiesexhibiting similar symptoms. A productive TVNCRtechnician will not rely on memory alone to diagnosesymptoms and part numbers. Instead they will callupon past case histories, which will enable the smartservicer to avoid the costly mistakes of mis-diag-nosec part orders and loss of extremely valuabletime ... Unlike other programs, our Tech -Tips arenot unverified mail -ins. We keep our programbase quantity at 10,000 repairs in which oldrepairs are removed and replaced daily with 2000new repairs through each update period. Thisassures you of having the repairs occurringtoday and not some unbelievable quantity of, for-ever gone, non-profitable repairs other tip pro-grams may offer on a one shot deal. From the orig-inal world wide Tech -Tip program comes 3 milliondollars worth of actual solid repairs. 1987 throughJune 1994. Whether you are computerized or not,our Tech -Tip program provides the most current,comprehensive published paper format that there istoday You may also wish to choose our Tech -Tipcomputer program, licensed World Wide under(PARADOX) by The Boreland International Co.We're not fooling around and you shouldn't either.Remember it's not the quantity of tips but theconslant update quality of part numbers andsolutions that makes the difference. Our initial10,000 TECH -TIPS cover VCR -VCP-TV/ VCRcombo CD LD-FAX-Camcorder-VHS/ SVH/ C/8MM-Stereo-Monitors-Amps Projection Front and Rear -Micro wave -Satellite. Category is easily cross-refer-encec first by Model, Chassis or FCC ID Numberand printed out or simply scanned by Make andCategory. Unlike other programs, we provide OEMPart numbers and substitutes. Our on -going pro-gram is actual day-to-day repairs written on theBench by technicians for technicians. No needfor you to spend time updating your data base orpaper binders. We send you a complete revised pro-gram at each update of your choice (Computer orPaper Format). Outside technicians and studentslove the portability and say it is a must in the fieldwhere time is very critical. As an added Extra, weprovide an FCC ID Cross -Reference list which savesthe expense of purchasing Service Manuals you mayalready have and just don't know it. This programwas produced by Ed Erickson, technician and ownerof TV -Man Sales & Service, Inc., and President ofThe Professional Electronics Association of SouthFlorida. Our program start up fee for the past 3years has been 6350.00 and for a good reason.Our on -going program really works, with a 30 -day money back guarantee on our computer pro-gram. The PHONE CALL is "FREE" and so is theDisk or Paper Sample Demonstration. CALL (800)474-3588 or Write TV -Man (TECH -TIPS), 7040 W.PALMETTO PK. RD. #2-112, Boca Raton, FL.33433.

Over 1000 VCRs & TVs and misc. audio equipmentfor parts or repair. $10,000 for all or $20.00 each.Madison, WI. 608-244-5767.

Sencore Equipment for Computer Monitor RepairBusiness. Also Schematics and Database. Excel -lent potential for technical individual. Limitedfinancing. Information: (800) 979-1969 Days.


Classified

FOR SALE

SENCORE ELECTRONIC TESTING EQUIP-MENT. Excellent condition, hardly used. SC3080Waveform Analyzer System with 50 to 1 HV probe,$2700.00; VA62A Video Analyzer. $2800.00;PR57 Variable Isolation Transformer, $450.00; orwhole package for $5500.00 Cdn. 1-604-743-4224, Vancouver Island, Canada.

BUSINESS OPPORTUNITIES

MOVE TO THE LARGEST REPAIR PROFITMARKET IN THE COUNTRY - SUNNY SOUTHFLORIDA Think about it, our clients bring theirREPAIRS & MONEY from all parts of the world.Annual Gross 375,000k, take home 80 - 95,000k.Owner retiring, Fully staffed & operated in thelargest growth area of Ft. Lauderdale. Established12 years. No competition. Authorized 15 majorbrands. Totally computerized. Simple to operateBusiness. Price is 150,000k, Owner financingavailable. (305) 474-2677.

Couple retiring after 10 years at ElectronicSales/Service shop with service contracts, repeatersights in no crime area, near 2 lakes. 300K gross.Authorized dealership on several brands, no com-petition. 175K, possible financing. (913) 428-3601.

MISCELLANEOUS

A.T.S. SPECIALTY REPAIR PRODUCTS for VCRand copier type equipment. Products includeINSTA-PLASTIK GEAR AND PLASTIC REPAIR.Proven in over 3000 service centers, INSTA-P L AS-TIK field repairs broken parts, nylon gearsand teeth. (800) 343-3530.

PROMS Programmed & Tested. For Info., MailSASE To The PROM Service Co., P.O. Box 142,Council Bluffs IA., 51502

Test YourElectronicsKnowledgeAnswers to the quiz(continued from page 55)

1. C 8. M

2. J 9. D

3. E 10. B

4. F 11. N

5. G 12. L

6. A 13. H

7. K 14. 1

Readers' Exchange

Readers' Exchange has been reinstated as a freeservice.

The following restrictions apply to Readers'Exchange:

Only individual readers may use Readers'Exchange, and items must be restricted to thosethat are ordinarily associated with consumer elec-tronics as a business or hobby. If you're in busi-ness to sell the item(s) you want to offer for sale,the appropriate place for your message is in a paidadvertisement, not Readers' Exchange.

Readers' Exchange items must be restrictedto no more than three items each for wanted andfor sale, and may be no more than approximatelyfour magazine column lines in length (about 20words).

All submissions must be typed or printedclearly!

Send your Readers' Exchange submissions to:

Readers' ExchangeElectronic Servicing &

Technology76 N. Broadway

Hicksville, NY 11801

WANTED

Looking for picture tube #21FP4A or21KP4A for a Dumont TV Model RA165,a service manual or a Sams #I89-7, a copywill do. Also, a service manual for a ZenithVCR Model #VR1805. Contact John Carr,John's Repair Service, 142 Jackson PL,Philadelphia, PA 19148.

DC/dc converter modules for AudiovoxModel HCD-100 car stereo. Contact BobCrosswhite, PO Box 1907, Breckenridge,CO 80424. ( 303) 453-1109.

Source of control deck schematics and chips(new/used) for Amana, Tappan, Sears andMagic Chef microwaves -and Sams col-lection above #1535. Contact W. Worley,305 Hickory Bend. Enterprise, AL 36330.(205) 347-5281.

Service manual/schematic for JFD Model7200, field strength meter. Contact MarvinMoss, Box 28601, Atlanta, GA 30358.

Need schematic for System AudioElectronics Inc., (formerly of Los Angeles)integrated amplifier Model # A14. ContactWalter V Ciecierski, CCC Consultants, 65-1 North Branford Rd., Brandford, CT06405. (203) 481-5275.

Videographers want to upgrade from singleCCD S -VHS camcorders and VCRs tooperational 3 chip S -VHS cameras; a wave-form monitor-vectorscope; S -VHS SEG;and editing equipment with S -VHS VCRs(2 or 3). Should be guaranteed or priced assalvage. Service lit. a plus! Send prices andinfo. to: PO Box 1062, Russell Springs, KY42629. (502) 378-6201, 9-6 CT

Power supply board stock #01243602 or01283601 for Sansui VCR Model #S -V7.Contact J. Powell, 299 Charleston Rd.,Willingboro, NJ 08046. (609) 877-9143.

Schematic for Panasonic Model #CT -3003.Contact (912) 368-3874 or write MarcelBouchard, PO Box 85, Walthourville, GA31333.

Need schematic/service manual for:Pioneer stereo receiver, Model #SX-5590.Contact Robert Niles, 401 Holiday Terrace,Brandon, FL 33511. (813) 681-3334.

IC wanted for a Magnavox TS -9 tun-ing sys. 6125550002 or 483520987597 orMAB8441T025. These are same ICs butno longer avail. Will accept new/used.State price wanted. Contact Larry (918)455-1041).

Schematics for GTE portable telephoneModel HAC31500 and Phone Mate answer-ing machine, Model 7000. Contact W.M. D.Browning, 311 Saipan, San Antonio, TX78221. (210) 923-7879 (Hobby).

Cassette back tension guage in good condi-tion. Need meter for Hickok 225 or as -iscomplete. Contact 216-297-1491, PortageTV 414 Cleveland Rd., Ravenna, OH 44266.

Flyback for a Sharp I 3LM56 TV. Part #F0015PE-M. B&K 520B transistor tester.Pioneer SX-1000 receiver, working or not.Contact Dan Cutler, PO Box 1203,Douglas, WY 82633. (307) 358-6219.

For a Magnavox 25 inch TV chassis # E34-18. A yoke #361838001. The picture tube# 25VGMP22. Contact Bob Daffin, (215)739-5819.

Need tuner for RCA GD665H MST 001Ain working condition. Contact D.M. Noble,2173 Kensington Rd., Salina, KS 67401.(913) 827-3917.


WANTED

Photocopy schematic of pre -amp and out-put or information of a Telefunken stereoreceiver, Model #TR-717. Contact TerryOnda, 118 Badger St., Saskatoon, Sask.,S7N 2X4. Canada.

FOR SALE

A B&K transistor checker Model #510 withleads, $100.00. Also, a Fluke DMM Model#8010a ac powered with leads, $100.00.Contact John Carr, John's Repair Service,142 Jackson Pl., Philadelphia, PA 19148.

Sencore SC61 $1300.00, B&K 1250 NTSCpattern generator $350.00, Huntron 1005tracker $300.00, 123 Sharp factory TVmanuals 1986-1992 $400.00, plus lotsmore. Contact Steve at 908-281-0300.

Tektronix 2245A 4 channel oscilloscope.100MHz digital readout with pouch andcover, tilt swivel base, 4 probes, includinghigh voltage and service manual. Like new,recent factory calibration 1750 or best offer.Contact Bruce at (516) 437-2277.

Sencore FC45 frequency counter 230MHz$199.00, SM152 sweep marker generator$125.00, PS29 scope MHz usable to10MHz $149.00, CG153 color generator$49.00, FE21 field effect multimeter N.R.$35.00, plus more. Contact Al Nikora Sr.,5298 Argyle Ct., Sterling Hgts., MI 48310.(313) 268-6938. Call after 6:00 p.m.

Electronic Technician Magazine, 55 copies,1965 to 1969. All for $25.00. Also, VideoMaker Magazine, all since the beginning,64 copies. $30.00 Contact Daniel Seidler,3721 W. 80 St., Chicago, IL 60652. (312)284-8221.

Vacuum tube voltmeter $30.00, triplettmulti -tester $7.50, Sperry pocket sizemulti -tester $7.50 new. All in workingcond. Contact Joe Oracki, 3502 E.Northern Parkway, Baltimore, MD 21206.(410) 254-0284.

Sams Photofacts: folders, AR, TSM andVCR. Send a SASE for list and prices.Contact Ray Schmidt, Schmidt Electronics,607 First Ave. S.W., Austin, MN 55912. 507-433-0069.

Sencore TC28 tube and transistor tester$110.00. Contact (216) 721-9435.

Sencore VA62A and VC63 in original box$1500.00 plus UPS. Contact A.G. Tannen-baum, PO Box 110, E. Rockaway, NY11518. (516) 887-0057.

Sencore SC61 waveform analyzer, rarelyused, excellent condition $1500.00, plusshipping. Complete with original box,includes: probes, accessories and manuals.Contact Manuel Amaral, 1996 Shaw-mut Ave., New Bedford, MA 02747. (508)995-7101.

Sencore SC61 waveform analyzer$1990.00 plus shipping. Contact SalCribari, 1312 Well Dr., Camp Hill, PA17011. (717) 763-1855.

For sale or for exchange for other testequipment, Sams Fairchild square wavegenerator S150.00 or best offer. Heathkitscope $85.00, two color bar generator$85.00. Contact Gary Barily, 84-39 120thSt., Jamaica, NY 11415-3106.

Telequipment Model D54 osciloscope$300.00, Dumont 401B scope. Make offer.Contact Stever Ignacki, RT2, Box 120D,Chatham, VA 24531.

Sencore SC61 waveform analyzer, excel-lent condition, used one year, was pur-chased new $1600.00. Complete with allprobes, accessories and manuals in originalbox. Includes shipping. Contact Kent Wilke,3724 El -Char La., St. Louis, MO 63125.(314) 892-5682 days.

VA62, NT64, VC63, VC93. all cables,books, tapes, and cartons in absolute "ten"condition. Will ship COD $3400.00 firm.Contact day or night to S.L. Taylor, PO Box616, Marmora, NJ 08223.

Best offer for Sencore equipment SG165stereo analyzer, used less than 24 hours.Also, much more available. For completelist send SASE (legal size) to Fred Ingersoll,6845 Lathers, Garden City, MI 48135.

Sencore VA62, NT64, ST65, together withbooks and most cables. Contact Myles,(515) 752-0960.

Heathkit equipment, capacitance meter IT -2250, audio generator IG-5282, RF oscilla-tor IG-5280. All like new tih manuals$35.00 each, $100.00 for all. ContactGerald, (919) 745-4493.

VA62 video analyzer $2300.00, Sencorecricket transistor tester $415.00. Both inexcellent condition. Contact (905) 354-5465 (Niagara Falls, Ontario Canada).

Leader dual -trace oscilloscope 35MHz,nice shape, $200.00. B&K model 801capacitor analyst, $35.00. Isotap ModelWP -27A $25.00. Contact G. Inman (612)535-3933.

Complete test bench, 14 pieces total. TenSencore pieces. Sams and variety manu-facturer service manuals and parts. Bestoffer. Will sell separately. SASE to DennisPuvalowski, 1910 Sand Beach Rd., BadAxe,MI 48413 or call (517) 269-7709.

Trenton State College Library offers SamsPhotofact auto radio series and auto servicedata no 156 to 339, 1973 to Jan. 1984 andSams Photofact service no. 26 to 194, 196to 309 (1994) free. You pay for the ship-ping. Contact Kitty Shu (609) 771-3482 orFAX (609) 771-3299.

Sylvania test jig -Model #CK3000 withadapters and cables $100.00 plus shipping.Call (708) 891-5550.

Electronic Technician Magazine, 55 copies.1965 to 1969. All for $25.00 postage includ-ed or trade for old radio books. ConactDaniel Seidler, 3721 W 80 St., Chicago, IL60652. (312) 284-8221.

Sencore VA62 video analyzer $1200.00,Sencore SC61 waveform analyzer.$1200.00. Contact (717) 769-7301.

Tentel gauges TQ-600, TSH-VS, and tapetension guage for sale. Almost new, com-plete with instructional tapes and literature.All three for $500.00 plus shipping. ContactSteve Mack, 1335 Ave. E Billings, MT59102.

Sencore SC3100, VG9 I TVA 92 and B.I.FG2A with manuals and cables. One yearold $6500.00 for all. Contact JackSoman,PO Box 422, Bolinas, CA 94924. (415)868-2930.

B & K Equipment 1077BTV analyst, 415Marker Generator, 1477 dual trace oscillo-scope, 465 CRT tester, 520 transistor tester,283 digital multimeter, 1245 color genera-tor. $1,200 plus shipping. Dales, RT 1, Box236, Mtn View, MO 65548


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Wholesale Electronics 68 5 800/351-2233

We'd like to see your company listed here too. Call DianeKlusner at 516/681-2922 to work out an advertisingprogram tailored to suit your needs.

SALES OFFICE

PHONE (516) 681-2922

FAX (516) 681-2926


PHILIPSTECHNICALTECHNICALTRAINING

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Servicing & TechnologyCS -6030 1000Wz, 4Ch. Delayed Sweep. Video 2449.00 QUOTES Triggering System,...

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Transcript of Servicing & TechnologyCS -6030 1000Wz, 4Ch. Delayed Sweep. Video 2449.00 QUOTES Triggering System,...