LCD Flat panel TV troubleshooting guide by Bud Martin 

Troubleshooting LCD flat panel TV.

This is the basic guide that can be applied to most LCD flat panel TV.
First, try unplugging it from the outlet for about an hour to see if the TV will reset itself since the minute
the TV is plugged into the wall, the power supply and processor will be running 24/7 and the EPROM can
get corrupted by spikes and surges. Try using good quality surge suppressor to see if it will help if it
happen often.

Look for burnt brown hot spots on the circuit boards, capacitors with leaking/bulging top or seals. Loose wires and connectors. Blown fuses.

Notes: We are dealing with high voltage that can kill you! so if you are not sure what you are
doing, please have it fixed by the pro that has proper tools and safety equipment. Try to use GFI
outlet and isolation transformer when work around the TV set and DO NOT DEFEAT THE GROUND
PIN OF THE OUTLET/POWER CORD.

Back ground on LCD TV: http://en.wikipedia.org/wiki/LCD_television

1) The power supply module (There are 4 regulated output voltages:- Standby 5vdc, switched
5vdc, 12~16vdc, and 24vdc).


The power supply converts the 120/230vac 50/60Hz into DC, then the high frequency (50~100kHz)
switching circuits convert it back to AC but at high frequency which allows the power supply to use
smaller and much lighter weight transformers (Ferrite core instead of heavy iron core), then this high
frequency AC voltage is fed to the rectifier Diodes to get the DC output and filtered by DC filter capacitors
that must have low ESR (Equivalent Series Resistance), if general purpose electrolytic caps are use, they
will fail within a year or less due to high ESR.

The power supply has Standby 5vdc (5VSB, this is a MUST have before anything else will work,
when this voltage is not present, there will be no any indicator light on the front panel) which is
always on the minute the TV is plugged into the AC outlet, so it is on 24/7 and that is why the power
supply will fail even though you may use the a TV not very often. It will see all the voltage spikes and
surges 24/7. 

This 5vdc is used to power the Processor and the Infrared remote receiver module which will be waiting
for the command from the remote control or from the power switch, when the Power on command is
received, the Processor will send the turn on signal (3.3~5vdc) to the power supply module (PSON pin) to the switch on the main 5vdc for the rest of the logic and Tuner circuits, 12~16vdc for the sound
amplifiers (mostly Class D amplifier for efficiency and small size) and for the T-CON board (interface
board between the LCD panel and the Logic board), and 24vdc for the Back lights inverter board (if there is no 24vdc then no back light).

Common failures:
Blown main AC fuse due to shorted out Diodes, transistors.
Bad capacitors leaking or bulging tops: http://en.wikipedia.org/wiki/Capacitor_plague 
Shorted Diodes, transistors in the DC output side.

Testing the power supply output with dummy load:
24vdc testing: Disconnect the wires that go to the inverter board first. You can use two #1157 (using
both Lo & Hi filaments by paralleling them at the base) car light bulbs connected in series so they can
handle the 24vdc, this will put light load (about 70w) on the 24vdc output, the voltage out should be
steady at 24vdc (+/-2v), if not, the power supply has regulation problem. 
12vdc testing: disconnect the 12vdc TV load from the power supply board and attach one #1157 (using
just the Hi filament), the 12vdc output should stay at 12v (+/-1v), if not, the power supply has regulation
problem.

You should see the lamps go on when the TV power switch is ON and go OFF when the TV is powered
OFF.

2) Main Logic board (It has Processor, Tuner, Sound Processor, Sources selector, Power
amplifiers for speakers, interface port to the T-CON board for pictures). 

This logic board sends out the turn on signals (3.3~5vdc) to turn on the rest of the Power supply module
(Switched main 5vdc, 12~16vdc, and 24vdc), Inverter backlight ON (if no back light on command then
there is no back lights), 
Logic board supplies 5 or 12vdc to T-CON board B+ pin through switched transistor if T-CON board does
not get the voltage from the logic board, you will get white/gray glowing screen without pictures.
Back lights Dimming (PWM) control signal.

3) Back lights Inverter board. Its job is to provide the drive voltage for the CCFL (Cold Cathode
Fluorescent Lamp) to give white light for the screen.

This board is fed by the switched 24vdc (VBL) from the power supply module, it also get the Back lights
on (BL on) control signal and the Dimming (Dim/PWM/) signal from the logic board. So for it to work, it
needs 24vdc and the BL_ON signal.
Common failures:
Blown surface mounted fuses due to shorted transistors/ICs, Transformers.
Bad capacitors.

4) The T-CON (Timing Control or LCD controller) board (on the LCD panel assembly). It transfer the
universal image signal of TTL and LVDS signal to specific TFT (Thin Film Transistor) LCD.

This is the interface board that talks to the Logic board and the LCD panel to produce pictures on the
screen. White/gray glowing screen when it fails or not getting the voltage from the logic board (the 5 or 12vdc is supplied by switched transistor on the logic board). Notes: T-CON board is mounted under the
metal cover shield.
Common failures: 
Scrambled pictures, Lines, multiple pictures, color smears/blotches, rolling pictures after warm up.
Blown surface mounted fuse.
No pictures, very dark screen almost like there is no backlights.

5) LCD panel assembly to display pictures. The LCD pixels will block or unblock the back lights
from the back of the panel to the front of the panel. This is the most expensive part of the TV, it
will be more than the TV is worth.


As you can see, you must have all the above Voltages and signals for the TV to work properly. This may
help you narrowing down the bad modules instead of buying the wrong module just to find out later that it
did not solve the problem. Hope this will help you out.

Sound problem:
Let verify first to narrow down to where the problem can be. First, check the on screen sound set up
menu to make sure the speakers are not set to OFF. If the setting is OK, then attach the powered PC
speakers to the Audio Line out RCA jacks to see if you can get the sound there, if you do, then the
problem can be in the power amp circuits, if you do not get the sound at RCA jacks, then the problem can be in the sound processor circuit.

To get the sound repair, it can be from $150 and up, so if you can put up with using the good set of Powered PC speakers which actually give better sound than the tiny speakers inside the TV, I would keep using it that way, you may want to find out if the on screen sound setup will allow you to set the audio line out to be VARIABLE which means the sound output feeding the PC speakers can be varied using the TV remote control, then all you have to do is to set the PC volume to about half way up and set the TV volume to what ever you want.




 

 


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