LG 42PC1DA Plasma Sound but blank screen

[LesWoollam]

I have an LG 42PC1DA plasma TV with good sound but a completely blank screen (nothing at all even in a blacked-out room).
The Y-sus heatsink is warmish but the Z-sus heatsink is cold.
Voltage measurements are:
Y-sus VS 180.5v VA 60.1v VSC 120.1v -VY 199.9v 5vdc 4.9v
Z-sus VS 180.5v VA 60.1v Vzbias 100.2v
Any ideas??

[randtek]

Re: LG 42PC1DA Plasma Sound but blank screen

Here's a trick that may help you. It will immediately tell you if you have a main board problem, or a panel drive board failure.

1) Unplug the TV from AC.
2) Disconnect the power supply cable going to the main board at the main board end.
3)Disconnect the LVDS cable going from the main to the Digital board.
5) On the digital board find 2 test point holes, probably labeled "Ext. Gen.". Short these 2 points together.
6) Plug the unit in to AC.

This process will generate test patterns from the digital board. If you see the test patterns begin to cycle on the screen, you know everything is working properly except your main board. If you do not see the test patterns, you have an issue in the panel drive circuitry.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

Thanks for the tip.
Checked but no pattern on screen - still completely blank.
Any suggestions?

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

Check the fuses on the Y-sustain and Z-sustain boards.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

All fuses (including the 5v fuses) on both boards are good.

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

We have sometimes seen the hybrids fail on these TVs without shorting out. Do you have 193V both ends of the fuses?

Try without Z-sustain disconnected. You should get a dim, blurry picture if Y-sustain is still working.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

There's 186v5 at both ends of the fuses, I believe this is the VS supply from the smps. I've completely unplugged the Z-sus board but the screen remains completely blank.

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

It is possible the Z-sus is failed in such a way that means the plasma panel is not lighting. Try with the ribbons disconnected from the plasma panel on the Z-sus side. Otherwise, the Y-sus may also have a problem.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

I've removed the ribbon cables from the Z-sus board - no difference, screen still blank. Are there any more tests I can do on the boards?

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

Unless you have access to an oscilloscope, unfortunately, no...

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

I have a dual trace 60mhz scope.

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

Brilliant... do you have a 10X probe which is capable of 400V measurements? (Peak voltage on Y-sustain can be 350V+)

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

Yes. What is the test?

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

Connect the probe to the Vscw test point, set your scope to 2ms/div, 10V/div, and see if you get any waveform (set trigger to auto for now.)

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

Is that the "Waveform" test point on the Y-Drive board on the far left of the TV?

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

That might be it. I might be getting confused with the older generation LG plasmas.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

I'll scope it and post the results.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

Scope set to 10V/Div (probe x10) 0.2ms/Div.
Scope trace attached.

[tom66]

Re: LG 42PC1DA Plasma Sound but blank screen

Y sus is working, that looks normal. Check Z sus too.

[LesWoollam]

Re: LG 42PC1DA Plasma Sound but blank screen

Scope set to 5V/Div (probe x10) 0.2ms/Div.
Z-sus scope trace attached.