LCD Flat panel TV troubleshooting guide.doc

7/30/2019 LCD Flat panel TV troubleshooting guide.doc

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LCD Flat panel TV troubleshooting guide

This is the basic guide that can be applied

to most LCD flat panel TV.By Bud MartinFirst, try unplugging it from the outlet forabout an hour to see if the TV will reset

itself since the minute the TV is pluggedinto the wall, the power supply andprocessor will be running 24/7 and theEPROM can get corrupted by spikes andsurges. Try using good quality surgesuppressor to see if it will help if it happenoften.Look for burnt brown hot spots on thecircuit boards, capacitors with

leaking/bulging top or seals. Loose wiresand connectors. Blown fuses.

Notes: We are dealing with highvoltage that can kill you! so if you arenot sure what you are doing, pleasehave it fixed by the pro that has

proper tools and safety equipment.Try to use GFI outlet and isolationtransformer when work around the

TV set and DO NOT DEFEAT THEGROUND PIN OF THE OUTLET/POWER


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CORD.

1) The power supply module (There

are 4 regulated output voltages:-Standby 5vdc, switched 5vdc,12~16vdc, and 24vdc).

The power supply converts the120/230vac 50/60Hz into DC, then the

high frequency (50~100kHz) switchingcircuits convert it back to AC but at highfrequency which allows the power supplyto use smaller and much lighter weighttransformers (Ferrite core instead ofheavy iron core), then this high frequency

AC voltage is fed to the rectifier Diodes toget the DC output and filtered by DC filtercapacitors that must have low ESR(Equivalent Series Resistance), if generalpurpose electrolytic caps are use, they willfail within a year or less due to high ESR.

The power supply has Standby 5vdc(5VSB, this is a MUST have beforeanything else will work, when thisvoltage is not present, there will be


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no any indicator light on the frontpanel) which is always on the minute theTV is plugged into the AC outlet, so it is

on 24/7 and that is why the power supplywill fail even though you may use the a TVnot very often. It will see all the voltagespikes and surges 24/7.This 5vdc is used to power the Processorand the Infrared remote receiver module

which will be waiting for the commandfrom the remote control or from the powerswitch, when the Power on command isreceived, the Processor will send the turnon signal (3.3~5vdc) to the power supplymodule (PSON pin) to the switch on

the main 5vdc for the rest of the logicand Tuner circuits,12~16vdc for thesound amplifiers (mostly Class D amplifierfor efficiency and small size) and for theT-CON board (interface board between theLCD panel, some T-CON uses 5vdc, andthe Logic board), and 24vdc for the Backlights inverter board (if there is no 24vdcthen no back light).

Common failures:Blown main AC fuse due to shorted out


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Diodes, transistors.Bad capacitors leaking or bulging tops:Shorted Diodes, transistors in the DC

output side.

Testing the power supply output withdummy load:24vdc testing: Disconnect the wires thatgo to the inverter board first. You can use

two #1157 (using both Lo & Hi filamentsby paralleling them at the base) car lightbulbs connected in series so they canhandle the 24vdc, this will put light load(about 70w) on the 24vdc output, thevoltage out should be steady at 24vdc

(+/-2v), if not, the power supply hasregulation problem.

12vdc testing: disconnect the 12vdc TVload from the power supply board andattach one #1157 (using just the Hifilament), the 12vdc output should stay at12v (+/-1v), if not, the power supply hasregulation problem.

You should see the lamps go on when theTV power switch is ON and go OFF when


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the TV is powered OFF.

2) Main Logic board (It has Processor,

Tuner, Sound Processor, Sourcesselector, Power amplifiers forspeakers, interface port to the T-CONboard for pictures).This logic board sends out the turn onsignals (3.3~5vdc) to turn on the rest of

the Power supply module (Switched main5vdc, 12~16vdc, and 24vdc), Inverterbacklight ON (if no back light on commandthen there is no back lights),Logic board supplies 5 or 12vdc to T-CONboard B+ pin through switched transistor

if T-CON board does not get the voltagefrom the logic board, you will getwhite/gray glowing screen withoutpictures.Back lights Dimming (PWM) control signal.

3) Back lights Inverter board. Its jobis to provide the drive voltage for theCCFL (Cold Cathode FluorescentLamp) to give white light for thescreen.


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This board is fed by the switched 24vdc(VBL) from the power supply module, italso get the Back lights on (BL on) control

signal and the Dimming (Dim/PWM/)signal from the logic board. So for it towork, it needs 24vdc and the BL_ONsignal.

Common failures:Blown surface mounted fuses due to

shorted transistors/ICs, Transformers.Bad capacitors.

4) The T-CON (Timing Control or LCDcontroller) board (on the LCD panel

assembly). It transfer the universal

image signal of TTL and LVDS signal tospecific TFT (Thin Film Transistor) LCD.This is the interface board that talks to theLogic board and the LCD panel to producepictures on the screen. White/grayglowing screen when it fails or not gettingthe voltage from the logic board (the 5 or12vdc is supplied by switched transistoron the logic board). Notes: T-CON boardis mounted under the metal covershield.


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Common failures:Scrambled pictures, Lines, multiplepictures, color smears/blotches, rolling

pictures after warm up.Blown surface mounted fuse.No pictures, very dark screen almost likethere is no backlights.

5) LCD panel assembly to display

pictures. The LCD pixels will block orunblock the back lights from the backof the panel to the front of the panel.This is the most expensive part of theTV, it will be more than the TV is

worth.

As you can see, you must have all theabove Voltages and signals for the TV towork properly. This may help younarrowing down the bad modules insteadof buying the wrong module just to findout later that it did not solve the problem.Hope this will help you out.

Sound problem:Let verify first to narrow down to where


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the problem can be. First, check the onscreen sound set up menu to make surethe speakers are not set to OFF. If the

setting is OK, then attach the powered PCspeakers to the Audio Line out RCA jacksto see if you can get the sound there, ifyou do, then the problem can be in thepower amp circuits, if you do not get thesound at RCA jacks, then the problem can

be in the sound processor circuit. To getthe sound repair, it can be from $150 andup, so if you can put up with using thegood set of Powered PC speakers whichactually give better sound than the tinyspeakers inside the TV, I would keep

using it that way, you may want to findout if the on screen sound setup will allowyou to set the audio line out to beVARIABLE which means the sound outputfeeding the PC speakers can be variedusing the TV remote control, then all youhave to do is to set the PC volume toabout half way up and set the TV volumeto what ever you want.

Inverter board connector: you can


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also see the Fuse 4A/160V

Power supply connector:


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T-CON board: (metal cover shiledingis removed to show the bd.)

Inverter board:


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Logic board:

T-CON board is dead, no power orblown fuse on the board, screen turns

WHITE, must see it in darkened room,otherwise you may mistaken it forfailed backlight.


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Bad T-CON board after warm up, youget lines, multiple pictures, strangecolors blotches, etc.

Bad caps (leaking/bulging tops).Make sure to replace bad cap withgood cap such as PANASONIC FM orFC series from DIGIKEY, they are LowESR (Equivalent Series Resistance)


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which is a must for switch modepower supply, and 105c rating. Do notuse generic electrolytic caps.

http://en.wikipedia.org/wiki/Capacitor_plague

Bad T-CON board or power supplyproblem
http://en.wikipedia.org/wiki/Capacitor_plaguehttp://en.wikipedia.org/wiki/Capacitor_plaguehttp://en.wikipedia.org/wiki/Capacitor_plaguehttp://en.wikipedia.org/wiki/Capacitor_plague


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24vdc power supply output testing usingtwo 12vdc car lamps in series.

Bad LCD panel, vertical 1~2 pixel widelines

LCD Flat panel TV troubleshooting guide.doc

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