LG 42pt85 plasma TV turns on but has no picture

[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.

[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]

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

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

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.

[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

[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).

[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

[Dannyx]

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

Oh, is VSC like the "t-con" ? If yes, then I believe that's OK: there's a couple of regulators like 5v, 3.3v and 1.8v on it and they all checked fine. Even tried powering it with an external 5v adapter (disconnected from the TV of course) and it turned on, generated those voltages and began flashing the green LED in the bottom corner of the board, so I BELIEVE it's fine. One thing I noticed is that there's a row of 4 LEDs slightly off-center and only 3 of those come on. Without a manual to tell me what they represent, it's hard to tell if that's an error or it's normal. Can't believe the most crucial information is not in that manual I got....
UPDATE: ok, no VSC is not that.....but at least now I checked another thing off the list FAAAAAIL !

[Dannyx]

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

Very good material here, albeit for a different model. I'm mostly interested in learning as much "plasma lingo" as possible so you guys don't have to struggle and waste time explaining it to me. VSC and VY are both somewhere on the Y sustain apparently. Given that Vs shuts off, even with the Y sus in place, I doubt I'll get either of those.

[tom66]

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

Vsc, -Vy are marked on the Y- and Vzbias on the Z-sustain
You should be able to see them

[Dannyx]

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

I think I recall seeing Vy across a large resistor on the Y-sus, but like I said, I can't convince the TV to stay on, except by disconnecting that ribbon between the control board (which I mistakenly call a T-con) and the Y-sus.

[freakaftr8]

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

You may have a short on the IPM module.. Very common.

[Dannyx]

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

You may have a short on the IPM module.. Very common.

I did have that fault in a different TV (think it was called Platinum or something like that....anyway), that had a dead IPM but at least was very obvious: fuse was blown on Y-sus and the Vs pins of the IPM were shorted to GND. Not this one though: fuse is ok and there are no shorts, unless IPM somehow failed OPEN or something's messing up inside....

[tom66]

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

You can try checking it before it shuts down

[freakaftr8]

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

I did have that fault in a different TV (think it was called Platinum or something like that....anyway), that had a dead IPM but at least was very obvious: fuse was blown on Y-sus and the Vs pins of the IPM were shorted to GND. Not this one though: fuse is ok and there are no shorts, unless IPM somehow failed OPEN or something's messing up inside....

Its happened to me before.. Not quite as common, however it happens.. Ive had DC regulator issues within an IPM module fail as well causing this with no blown fuse.

[Dannyx]

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

The meter doesn't have time to refresh fast enough to capture it - I tried it The only way I can try it is to remove the ribbon like I said, which makes it stay on...this is what's actually puzzling the hell out of me. There's probably an error signal or something that's sent by the Y-sus via that flex cable that causes it to shut down.

[Dannyx]

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

Removing the IPM module alone makes me sick. Combine that with the fact that it's cast in gel and with the probability it's not the issue and it makes me hesitant to even struggle to pull that IPM out...probably easier to go with a replacement Y-sus, though that's also a gamble since the issue could also be somewhere else. I'm leaning towards a faulty Y-sus myself TBH. Fixing it would be awesome tho I HAVE fixed some before, but there were obvious shorts and dead components.

[freakaftr8]

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

Just to recap, you have check all the diodes on the board? Resistors too? Those flat I believe 100 or 200 style resistors like to open.. Usually they are paired

[Dannyx]

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

Just to recap, you have check all the diodes on the board? Resistors too? Those flat I believe 100 or 200 style resistors like to open.. Usually they are paired

Diodes yes, resistors no, at least not extensively. I did check some of the larger SMD ones, since it was easier to do, but haven't gone all out to check every last bloody one on there