LG 42pt85 plasma TV turns on but has no picture

[Dannyx]

Re: LG 42pt85 plasma TV turns on but has no picture

Let's see: disconnecting the Y buffer doesn't change anything - already tried it.
I must admit I don't know where to measure VSC, VY and Vzbias...still some things I don't know, but that's the point, so thank you.
Output of Y sus and Z sus: for Y sus, you mean measure the connector to the Y buffer ? For Z sus, the sockets where the flex cables run into the panel
No scope sadly

[tom66]

Re: LG 42pt85 plasma TV turns on but has no picture

Figured it was about time I provided some pics. Here's the Y sustain which I believe is faulty.
Some more details: the PSU doesn't seem to want to turn on without a load attached to the Vs line: the relay closes, Vs goes up to around 206v then goes back down to 0 (despite the relay staying on). If I connect an incandescent lamp between Vs and GND, the PSU stays on and I get 60v on Va and around 180v on Vs and all of the other correct outputs, so I think we can safely say the PSU is fine.

This is OVP, the PSU cannot work without a small load on Vs so voltage goes up then it trips out.

Do you have an oscilloscope or access to one?

You should also check Vsc, -Vy and Vzbias and compare.

Use a multimeter to test output of Y-sustain and Z-sustain on DC volts. Also check across energy recovery caps (should be 1/2 of Vsus).

[R_J]

Re: LG 42pt85 plasma TV turns on but has no picture

Have you tried disconnecting the buffer boards from the y-sus board to see if it stays on?
If it stays on with the flat flex cable removed do you also have -Vy voltage on the Y-sus

[Dannyx]

Re: LG 42pt85 plasma TV turns on but has no picture

Figured it was about time I provided some pics. Here's the Y sustain which I believe is faulty.
Some more details: the PSU doesn't seem to want to turn on without a load attached to the Vs line: the relay closes, Vs goes up to around 206v then goes back down to 0 (despite the relay staying on). If I connect an incandescent lamp between Vs and GND, the PSU stays on and I get 60v on Va and around 180v on Vs and all of the other correct outputs, so I think we can safely say the PSU is fine.

However, if I leave everything connected, the relay clicks once, I briefly get Vs, then everything shuts down and the LED goes back to red.

If I remove the flat flex cable going between the control board and the Y-sus, ironically, everything stays on, even Vs, despite not wanting to turn on WITH the flat flex in place, so it's as if a control signal to the Y-sus is causing an overload of the PSU causing it to shut down, or a LACK of enough load to make it stay on...confusing AF, I know...

Control board seems OK: I get 3 lit LEDs, plus a flashing one the bottom corner.

I have thought of a trick but am not quite ready to pull it off: lamp between Vs and GND with Y-sus still connected, but I fear that might damage something if the Ysus is indeed busted and I force voltage into it....

Starting to think more and more that the Ysus is somehow faulty, despite not having any blown fuses or shorts, that's the main issue here: everything seems fine and it's downright impossible to track down.

[Dannyx]

Re: LG 42pt85 plasma TV turns on but has no picture

UPDATE no 3.....sorry guys, but I figure out things as I go I found the flat flex cable going from the T-con to the Y sustain board was disconnected, so someone before has been inside this thing. Now the led turns green, then back to red, which is slightly more normal for a faulty Y sustain IMO...NOW we're getting somewhere. Let's push on.

[Dannyx]

Re: LG 42pt85 plasma TV turns on but has no picture

Unfortunately no, soldering back that cap had no effect at all...

Also observed those LEDs on the main board more carefully: they all come on and remain steady after the TV has powered on (despite not having picture). I read somewhere that one of them should be flashing or something...

[Dannyx]

Re: LG 42pt85 plasma TV turns on but has no picture

I made another discovery: upon close inspection I noticed THIS on the Y-buffer: C18 was completely torn off and only hanging onto one pad, while the other one was up in the air (picture was taken after I removed it entirely)....I have no idea how that happened, but it's certainly not ok. I thought I somehow touched it when I removed the back panel, since it IS very sharp on the edges and could've easily done that sort of damage, but I dismissed the thought because the board is lower than the edge of the cabinet, so there's no way the edge could've scraped on that part, plus there are no scratch marks. I highly doubt it would cause the WHOLE picture to lack, but it shouldn't be like that to begin with, so I shall solder it back and see what happens. And no, neither the cap nor those pads are shorted.

[Dannyx]

Good day folks. Perhaps someone could help me with this plasma TV, since it's a pretty difficult thing to track down: the TV powers on and stays on (green light is on), but it's not displaying anything. I'm not sure if sound would play either, because it could be on a random source and without being able to see what I'm selecting, it's hard to tell where I am and what to plug in. To make a difficult project even more difficult, there are no signs of damage anywhere: all the fuses are OK and all voltages seem present and fine, 12v, 19v, Va, Vs, it's all there, I measured them, but there's no picture, not even a sparkle or something....nothing. Power going into the Y-sus and Z-sus doesn't necessarily mean those boards are operating properly, sure, since I measured at the connectors.

This could be ANYTHING really: a dead component that's too "small" to blow a fuse or cause a voltage to shut down yet enough to cause the behavior; a faulty main board; a corrupt firmware; bad panel.....not even sure where to start with this one :|

The service manual wasn't too helpful either, because I was mostly interested in two things: what the little service LEDs on the main board indicate (there's like 4 of them, green and red, lighting up) and what the Va and Vs voltages are supposed to be, but I found neither of that sadly.

Any help from some plasma gurus out there would be greatly appreciated and I can also provide pics if need be. Cheers and thanks.