LG 42PC1DA Plasma Sound but blank screen

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

That is weird, I do not expect that ramp type wave on the Z-sustain, is that with power connected to the Z-sustain?

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

Everything is connected to the Z-sus board.

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

I think somehow the Y-sus signal is getting to the Z-sus, and the Z-sus is not working.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

Could it be that the 15v supply voltage from the Y-sus is not getting to the z-sus board?

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

That could be a cause; did you check it?

BTW, I should have said I think the Y-sus signal is just being picked up on one side because the Z-sus is essentially open circuit and your scope doesn't represent signficant load.

[cliff_hootarski]

Re: LG 42PC1DA Plasma Sound but blank screen

Quick question I've been meaning to ask and this is related to this thread.

Does the O-scope need to be isolated through an isolation transformer?

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

No, that is only necessary if you are measuring the mains/primary side of the PSU.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

15v good coming out of the Y-sus board and going into the Z-sus board.

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

Is there a fuse on Z-sus for 15V?

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

There doesn't appear to be a fuse for the 15V supply.
I've noticed that only D14 temperature LED on the Control Board lights even from cold power-up. This does not appear to agree with the sequence suggested in the attached diagram. Could this have anything to do with the problem?

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

I've removed both the Y and Z-sus boards and performed diode tests on the IPMs.
The Z-sus IPM seems to check-out OK, but the Y-sus IPM reads an open circuit between pins VS & VER1 and VS & VER2. Does this indicate a fault which would cause a blank screen? What are Pins VER1 & VER2?

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

VER1 and VER2 must not be shorted together, so I think your Y-sus IPM is OK.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

According to the manual, there should be a diode reading between "VS and VER1" and also between "VS and VER2". I don't get a diode reading between those pins, instead it reads open circuit.

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

I assume you've tried both directions?

Your Y-waveform is correct with the sustaining and ramps. The Z-waveform should not have any ramps in it at all.

Can you try measuring the Z-waveform without the Y-sustain connected, and the Y-waveform without the Z-sustain connected? This should eliminate cross-contamination, as such.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

I'll measure those in the morning and verify the traces.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

Y-sus trace with Z-sus board removed.
Z-sus board will not power-up without 15v supply from Y-sus board (therefore no trace).
I've attached another Z-sus trace with all boards plugged-in.
Any more thoughts?

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

Which is Z-sus, and which is Y-sus?

I suspect your Z-sus is faulty.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

Z-sus tace is the one on the left.
Are there any more tests I can do on the Z-sus to confirm it's faulty?
Every component on the board remains cold but the voltages all check-out OK and the diode test readings on the IPM all look fine.

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

Can you post a clear picture of it?

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

Pic of Z-sus board attached.