Vsetup voltage in a Plasma Question

[ghiotom]

Does the peak of the Vsetup voltage equal the Vsetup voltage PLUS the Vs voltage? I'm getting both proper voltages to the Y sus board ( Vs 200V & Vsetup 195 V). However, when I hook up the Oscope to view the output, the whole waveform peaks at 200 volts. I don't see the setup portion at all. I am experiencing some image retention and I'm wondering if this is the issue.

[tom66]

Re: Vsetup voltage in a Plasma Question

On LG plasmas, I believe it's just relative to ground.

[ghiotom]

Re: Vsetup voltage in a Plasma Question

Thank you tom66 for answering my questions. This one is a Philips (Samsung). I would think that if the Vsetup voltage were missing in the waveform, I would be experiencing something worse than some image retention, what do you think?

[tom66]

Re: Vsetup voltage in a Plasma Question

Hmm, the Vsetup ramp should appear before the first subfield (and on older plasmas, before each subfield) of each frame, how are you setting up the trigger? Have you got a slope trigger on your scope?

Your lack of Vsetup will easily cause image retention.

Oh, and you also should see a negative ramp after the positive ramp. Do you see any negative ramp?

[ghiotom]

Re: Vsetup voltage in a Plasma Question

Yes, I see the negative ramp and I can adjust it with the pot on the Y sus board. You can see the the waveform change when I turn the pot. However, the pot controlling the ramp up voltage has no effect on the waveform.

[tom66]

Re: Vsetup voltage in a Plasma Question

I think this is your problem then, can you post a picture of the Y-sustain?

[ghiotom]

Re: Vsetup voltage in a Plasma Question

The board itself or the waveform?

[tom66]

Re: Vsetup voltage in a Plasma Question

Can I ask for both?

[ghiotom]

Re: Vsetup voltage in a Plasma Question

I couldn't get a good shot of the actual board in the tv, but here is a shot of the exact same board along with a screenshot of the waveform.

[tom66]

Re: Vsetup voltage in a Plasma Question

Ah, nice Rigol - I've got one of those.

You can actually set up a slope trigger in the trigger menu; set the two thresholds and then set the "greater than" and "ramp negative" modes. This will trigger on the negative ramp, which makes plasma diagnosis so much easier.

Also, go into the Acquire menu and turn long memory on if you haven't already. Then, if you press RUN/STOP, you will be able to zoom into the individual subfields to inspect them individually.

Vsc is the negative voltage on this TV? I've only ever worked on two newer Samsung plasmas.

[ghiotom]

Re: Vsetup voltage in a Plasma Question

Yes, the Rigol seems to be a very nice scope for the price ($315 US). I actually have it triggered on the AC and it's fairly stable, I guess the field or frame rate is some multiple of the 60Hz AC we have in the US. Yes, the negative voltage is Vsc and it's -175V for this set.
What do you make of the waveform, do you see the ramp up? Since the voltages going to the Y sus board are spot on, I guess it's replacement or possible repair of the Y sus board.

[tom66]

Re: Vsetup voltage in a Plasma Question

It "looks" reasonable but I'm not that familiar with Samsung plasmas. Whenever I have a Samsung plasma with image retention (and two out of two Samsung plasmas I have repaired have had a form of it) I always tweak the Yrr and Yfr potentiometers on the board until the picture looks a little better. If you go too far it will get worse, so reverse the direction and see when it looks better. If it still looks poor, I drop Vs by about 5-10V.

It is probably a 600 Hz sub-field plasma hence why it syncs with 60 Hz mains, although it will run at a separate frequency (not actually line sync'd), it will just appear close. The marketing advertising plasmas as "600 Hz" is exactly what all plasmas have had for a very long time - 10 sub fields giving in theory 1024 levels of intensity (in practice lower because they use uneven width subfields to reduce false contouring.)

[ghiotom]

Re: Vsetup voltage in a Plasma Question

UPDATE: For those who might come upon this thread in a future search, the Vsetup (Vrr) voltage was indeed missing. The first pic shows the original waveform with the Vsetup missing and the second pic shows the proper waveform. According to the Philips training manual on plasmas, if the Vsetup voltage is weak or missing, image retention will occur which is the problem I was having. Now, the 195V Vsetup was present at the Y sustain board, however, the drive signal was missing from the logic board. An smd resistor array on the logic board had one open resistor preventing the drive signal from making it to the Y sustain board. In addition, there was an smd zener diode across the gate-source junction of the Vsetup output MOSFET that was shorted. I'm happy to report that this 7 year old plasma is working normal again.

[tom66]

Re: Vsetup voltage in a Plasma Question

I am glad you fixed it. Thanks for posting the fix.