LG 42PC1RV - Powers up, sound ok but no picture

[andyc]

Hello!
I have this LG 42PC1RV Plasma (display type 42V8) that went bad a few days ago. The TV turns on and the sound is working fine but there is no picture on the display. Checked the SCART out and the video signal output is there, channels change, volume control works etc so i'm guessing the main board is OK.
I took it apart and found that all the fuses for the Ysus and Zsus boards are ok and so are all the voltages from the power supply.
I checked Va, Vs, Vsc and Vzb and they are all fine, showing very close or exactly the same voltages as labeled on the panel. The only one that is wrong is -Vy. The Y-sus board emits a strange buzz/hiss every second or so and the -Vy voltage goes from 0 or -1 to -2.8V in sync with the buzzing. This also happens when the YDRV board is disconnected. I checked the YDRV input lines and there are no shorts. I think the YDRV board is ok.
The Va and Vs voltages jump up a few volts when the Y-sus board is disconnected. Is this normal?
Could this be an IPM related issue? I can't find a schematic for this Y-SUS board and i have no idea where the -Vy supply is generated from and where it goes to (the PCB is multi-layered and i can't trace the connections properly). The IPM looks somewhat different from what i've seen in other people's threads as the parts under it are not exposed. I didn't desloder it so i couldn't see what's written on it (if anything).
I looked carefully and could not see any sign of blown components or leaked/vented capacitors on any board.
After being on for about 2-3 minutes the IPM, MOSFETs on the YSUS and ICs on the YDRV board get warm. I can't find anything excessively hot.

The boards are in this TV are:
YSUS Model 42V8&X3, part no. 6870QYH005B;
YDRV Model 42V8, part no. 6870QKE010A;
ZSUS Model 42V8&X3, part no. 6870QZH004B;
X-L Model 42V8, part no. 6870QME015A;
X-R Model 42V8, part no. 6870QSE017A;
CTRL Model 42V8&X3, part no. 6870QCH0C6C;
and PSU Sanken 1H328W, part no. 6709900023A.

This is my first plasma screen repair so any help is much appreciated.

Thanks,
Andy.

[andyc]

Re: LG 42PC1RV - Powers up, sound ok but no picture

Today i removed the flat cable that goes from the YSUS to the Control board and now i get proper -180V -Vy and the YSUS does not make the weird regular buzzing. Could this be a control board issue? Maybe it's not sending the right signals to start the YSUS?
The control board temperature LEDs do not light up with the flat cable removed so i'm assuming it's powered by 5V from the YSUS.
I recorded the sound it makes and the varying -Vy voltage measured accross R36:
https://www.youtube.com/watch?v=oy7bYYbWvVM
Hope you can hear it, youtube compression dulled the audio a little.

I think the noise comes from one of the little transformers on either the "setup" or "setdn" daughterboards but i can't tell wich one (the blue and yellow ones in the attached picture).

[dcapper]

Re: LG 42PC1RV - Powers up, sound ok but no picture

I repaired 2 LG's with those symptoms. Both were the YSUS board. Not saying that is your problem but here is a question. Leave the tv running for a bit and touch the big heatsinks on the YSUS and ZSUS. I see you checked them but were any cold?

[charles frost]

Re: LG 42PC1RV - Powers up, sound ok but no picture

Yea bud bad Y sus. Be advised there could be bad buffers also. Not all the time. At least check buffers, top and bottom for any burned ICs or small holes in them.

[andyc]

Re: LG 42PC1RV - Powers up, sound ok but no picture

@dcapper I let it run for about ten minutes and both the YSUS and ZSUS IPM heatsinks become warm. I don't know how hot they should get during normal operation...

@charles frost I checked the buffers board (there is only one large board with 6 buffer ICs in this TV) and there are no shorts between the input and output pins and the inputs (two of them, one for 3 buffer chips) show up as diodes like i read on the forums that they should.

I also put a scope on the pin labeled "waveform" on the buffer board and this is what it shows when the tv powers up:
https://www.youtube.com/watch?v=Su8TgXIwdhY

[andyc]

Re: LG 42PC1RV - Powers up, sound ok but no picture

Hello again!
I had some extra time over the weekend and i did some further research and tracing of the Y-SUS. Turns out the two daugtherboards are the power supplies in charge of generating -Vy and Vsc and they take power directly from the Vs line. The left one (blue transformer) is for Vsc (even though the resistor across which we measure -Vy is right below it) and the one with the yellow transformer is for -Vy. The yellow one was buzzing. So since they are powered by Vs and that voltage was nice and stable, i concluded that te problem must be something shorted on the output of the -Vy module. I traced the problem down to one of the MOSFETs on the smaller heatsink. There were 3 of them in parallel that showed some strange readouts on the meter so i removed them for proper testing. The defective transistor was not shorted but showed a weird small resistance across two of it's pins. I've attached a picture of the culprit.
I bought a replacement and now the TV came to life and works great. Good picture too! The panel seems to have some life left in it despite the age of the TV.

Thanks for helping!
Regards,
Andy.