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

[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.

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 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.

Buffer to panel.

[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.

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.

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.

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 rechecked the regulator on the main board. I either misprobed it or didn't remember it right because it has 5v at the center pin now.

I think it's gotta be the panel. I just unplugged the ysus and powered it on and there is absolutely no buzzing before the speakers come on. Did you watch the video I made? Coul you hear the buzzing?

[tom66]

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

I can barely here any buzzing, but it's almost always continuous and very loud. All plasmas emit a very slight buzzing in operation.

[johnboy1313]

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

Looks like this one has me beat. I'm not going to throw good money after bad in an effort to fix an antique anyhow. So with one last effort to hit it out of the park, I'll post this.



The top pin of connector P800 labeled 3.4V_ON shows 4.75V when on and 0V when off. Does that mean anything to you? I will be parting this one out with the assumption that the panel is bad if I haven't figured anything out by this weekend.

[tom66]

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

3.4V_ON just turns on the 3.4V output, so it's a logic signal around 0V off, 2~5V on.

[rayrod81]

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

If the logic board has been worked on then that could be the fault. The oscillator is what produces the signal to turn on the panel drivers. If that is not present then you would get the no picture fault. It should output a 100MHz sine wave. If that is not present then that is your problem.

Edit: It looks as though that might be the Main Board but it is hard to tell with out seeing the whole board. Either way I would check the oscillator on the logic board.

[tom66]

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

There should still be pre-discharge from the panel if it's getting the Vsetup ramps though. And the Y-sus and Z-sus won't generate the right outputs without correct timing.

[johnboy1313]

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

If the logic board has been worked on then that could be the fault. The oscillator is what produces the signal to turn on the panel drivers. If that is not present then you would get the no picture fault. It should output a 100MHz sine wave. If that is not present then that is your problem.

Edit: It looks as though that might be the Main Board but it is hard to tell with out seeing the whole board. Either way I would check the oscillator on the logic board.

Yeah, it's the main board. What's this oscillator look like?

There should still be pre-discharge from the panel if it's getting the Vsetup ramps though. And the Y-sus and Z-sus won't generate the right outputs without correct timing.

Is this directed at me? I haven't any idea what it means. Sorry

[tom66]

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

It was directed at both.

The Vsetup ramp is that rising voltage captured on the scope.

All other timings are correct.

As far as I am concerned the control board is sending all the right signals and the Y-sus and Z-sus are properly interpreting them.

Check the oscillator on the control board -- it's usually underneath the main board on this chassis.

[johnboy1313]

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

What's the oscillator look like? I had to bring the scope back this morning do I need to get it again?

[tom66]

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

Typical appearance (left), bottom of device (right):
http://www.sz-wholesaler.com/userimg...llator-701.jpg

[johnboy1313]

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

What am I checking it for? How am I checking it?

[tom66]

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

Depends -- if you find it, post a picture. I can highlight the oscillator output pin.