First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

[tom66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Everything should be there for the plasma panel but it still doesn't seem to light.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

I tried the set in the basement at night. The only light was the faint glow of the streetlight a block down coming through the window across the room.

[tom66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

The top leg and middle bottom leg need to measure the same, check your probing.
Could also be factory rework.

Have you tried in a COMPLETELY dark room for the slightest glow... it may be very faint....

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Well, I got the scope and found a wave at the very first point. However, I also bought some reading glasses today and found this when I put them on. I'm not sure how I missed it before. Obviously, someone's been here before me.



I probed the pins and found.

Top leg
5.0V
Bottom 5 legs from left to right
6.0V
3.2V
0.0V
0.0V
0.0V

Would a bad main board produce a similar "no picture" fault?

Also because I'm inexperienced with plasmas sets, I decided to make this video. There is buzzing when I power on the unit but not like what was in the video posted earlier. You be the judge.

http://s177.beta.photobucket.com/use...69E0F.mp4.html

[tom66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Buffer to panel.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

You just need to check a few, with the scope. If they are all missing, the buffer is at fault (possibly damaged by a bad panel but I've -never- heard of this on an LG.) But if even one is present you should see a glow from the panel, and the panel integrity might need questioning...

I have never done this test myself but you should definitely see the ramp and sustain waveforms for all buffer outputs, each should look almost the same as the other lines.

Thanks, I kind of expected that. I'll try to get the scope and report back this afternoon.

Again, am I checking the pins at the SUS to the buffer or the buffer to the panel? I would guess the buffer to the panel.

[tom66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

You just need to check a few, with the scope. If they are all missing, the buffer is at fault (possibly damaged by a bad panel but I've -never- heard of this on an LG.) But if even one is present you should see a glow from the panel, and the panel integrity might need questioning...

I have never done this test myself but you should definitely see the ramp and sustain waveforms for all buffer outputs, each should look almost the same as the other lines.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

You can check to see if each buffer line is being driven (there are test points next to each line), but be VERY careful not to short them out, or the buffer will be blown up.

Check with the oscilloscope? Am I just looking to see that they all look similar? Am I checking at the connections from the YSUS to the buffer or from the buffer to the panel?

[tom66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

The copper traces are all connected, so they are all supposed to be grounded.
You can check to see if each buffer line is being driven (there are test points next to each line), but be VERY careful not to short them out, or the buffer will be blown up.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Tom66, if I can get the scope again tonight, what am I going to scope?

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

First picture, these are the pins on the IPM from the back of the Ysus board. The pins in the yellow circle are the same pins in the yellow circle in the second pic. Of the pins in the yellow circle, they all show to be connected to ground though only the one says ground, I suspect that they are all supposed to be grounded. As for the rest of the pins, unless they say GND right next to them, there were none shorted to ground.





However, this thing, the grey can with the red vertical stripe, whatever it is, labeled L202 on the back of the board, is shorted to ground. I have no idea if it's supposed to be or not. It's counterpart, L203 just behind it and to the right, is not shorted to ground.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

I just read through this again. It took a second to set in. No, I didn't check the IPM for shorts. I will and will report back.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Did you check the IPM on the Ysus board for shorts? The fact you aren't even getting a dim image I would think would point to the Y sus board.

There are no blown fuses on either y or z sus. The boards both get warm when powered on and I scoped both boards and was told that the waves looked fine. Everything, except the lack of glow, suggests the ysus is working fine.

[ghiotom]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Did you check the IPM on the Ysus board for shorts? The fact you aren't even getting a dim image I would think would point to the Y sus board.

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

No, I took it back but I asked him if I could borrow it again if needed and he said that was fine.

[tom66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Do you still have the scope?

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

I started a second thread regarding scoping the ysus and zsus. For the sake of those who might have this same issue in the future, I'm coming back to this original thread. Here's the other thread.

https://www.badcaps.net/forum/showthread.php?t=24715

At this point, I am suspecting the y buffer board again. I know someone said that a bad y buffer would cause different symptoms, but hear me out.

I just unplugged the y buffer board from the y sus and powered on the unit, which resulted in the exact issue as when the board was plugged in. Now I know that this doesn't confirm a bad y buffer. However, my thought is that it does confirm that the set will display the same symptoms with the buffer removed. If I had removed the board and the unit. Would no longer power up, that would suggest, at least to me, that the buffer was not part of the problem. This is promising, is it not?

[johnboy1313]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

I found a point on the y buffer that says waveform. The problem is that I have next to no idea what I'm supposed to do. I know it makes me sound really ignorant. I do have experience with a scope. I took two years of part time EET classes at the local junior college but that was at least three years ago. What am I setting the scope to? Other than the waveform point, where do I touch for the other point? CH 1 to waveform, EXT TRIG to to ground? Seriously, I can do this, but not without some guidance.





Are any of you able to post pics to your profile? I can't even find my profile album. It's like I've been blocked or something.

[Cosmo66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

The IPM's don't always fail short to ground. But as someone stated earlier, this is by far the most common cause of failure in the LG plasmas. The IPM's are garbage. When they fail, you get sound with no picture.

[Cosmo66]

Re: First plasma, LG 42PC5D. Sound but no picture. Help me make sense of this.

Wouldn't there be blown fuses if something was shorted to ground? Both y and z sus boards get warm to the touch when the unit is powered on.

Probably but not always