Vizio P50-HDM, no video

[Freezer]

I'm looking for some help with my latest project, a Vizio P50-HDM plasma monitor (not a true TV since it lacks a tuner). I've only worked on LCD monitors and LCD TVs before, so this beast is a lot more complex than I'm use to. And before I get started, I know someone has been into this set before me, I've noticed a few missing screws, plus most of the screws holding the back cover on were clearly the wrong size. Lastly, there is this weird soldering job between the Y-sustain board and the upper/lower boards that doesn't look factory. Maybe the set died a few years ago and this was a repair-tech's handy work.

Anyway, onto the diagnosing. The problem is the set has no video, and the state of the audio is unknown. I tried switching the inputs and turning up the volume but never got sound to come out. But without video to navigate the menu, maybe I'm doing something wrong.

The first thing I checked was the fuses. All the fuses are fine, although there's one surface mount fuse on the X-sustain board that's underneath the heat sink and I don't think I can get my probe leads under there to test it.

My next test is voltages. The main power supply outputs the following 5 voltages:

Name / Rated value / Actual Value when set it powered on
Vs / 192V / 191.9V
Va / 60V / 60.01V
Stdby / 5V / 4.965V
5Vsc / 5V / 5.035V
12Vsc / 12V / 11.96V

So the outputs from the power supply board look good. The Y-sustain boards has test points labeled "-Vy" and "Vsc" The -Vy tests as a solid 199.2V. However, the Vsc fluctuates. Most of the time the voltage reads 119.2V, but about every 2 or 3 seconds it drops briefly to about 70V. I'm guessing this fluctuation is not good, but I'm not sure where to go from here. I have checked the ESR of all the caps on the X and Y boards, although most of these caps are in parallel and I have not bothered to remove them for testing. So they still may be bad. There is no visual indication of problems on any of the boards.

One other thing I noticed, after the set has been running for about 5 or 10 minutes, you can here a very faint high-pitched squeal coming from the Y-sustain board. The sound is coming from the area of those two riser cards on the Y-sustain board. The high-pitched squeal has about a 1 hertz frequency to it (i.e. occurs about once every second). I don't know if this is part of the problem or normal operation.

Any suggestions on what to test next would be greatly appreciated.

[rayrod81]

Re: Vizio P50-HDM, no video

I have worked on a few of these and if the Vsc is jumping around it may be a problem with the buffers. Check for shorted FETs. You could also have a problem with the hybrid on the Y-sustain in addition to the buffers. That is where I would start and then if there is nothing obviously wrong there then I would concentrate on the main video board. I have seen a couple Vizio plasmas with bad main boards causing this problem.

[Freezer]

Re: Vizio P50-HDM, no video

I have worked on a few of these and if the Vsc is jumping around it may be a problem with the buffers. Check for shorted FETs.

Thanks for the reply. What or where are the buffers? I know the hybrid or IPMs are under the heat sinks on the X and Y boards, but I'm not familiar with the term "buffers" in the context of a plasma panel.

[cadiman]

Re: Vizio P50-HDM, no video

You can`t see the buffers because they are upside down. They are to the left of the y board (left of the power supply). They attach to the panel ribbon cables then to the y board.
Those are mounted to an aluminum heat sink and should be ok
The older ones were not and failed sometimes.

You should hear the ipms running and they should be warm.
On this set you may only have one running and one not. It will be cool
while the running ipms will be warm on y or z boards.

I would look at the z board on this model first.