Panasonic Plasma TH-50PX75U Test for bad panel?

[tucocaps]

I think my previous post wasn't viewed as much because I was missing the model number in the title. The full post is a bit down with the word "headache" in the title.

This set appears to be "eating" SD boards. Take SD board out, and you get just the top half of screen. Put new SD board in, and within hours it burns out, and throws a 7 blink power on code. I've heard this could be the panel, but I really want to make sure before I throw the entire set out. I can get the SU,SD,SC boards for this for a pretty good price if that might fix it, but I've got to know first if the panel is bad. How can I test the panel? I can spot nothing visually wrong with the screen, and it looks great when it works. The only other possibility is that I've gotten two weak SD boards off Ebay in a row, but my senses tell me this isn't the case.

Already got much time and a bit of money into this set. Was hoping to make it my "main" TV Thanks a lot for any help!

[tw2005]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

I'd love to help but I'm simply lost on this one, not sure if there is a way for checking the panel, Tom appears to be busy for a bit.

I think this generation may have had a 7 blink issue and firmware upgrade for it but not 100% about that.

Have you checked out the SD and done any resistance checks to confirm it has failed.

Could very well be a partial short on the panel but I can't talk from experience on that one.

I assume once it has worked, then 7 blink again you can no longer get it to fire up again at all?

I have this bulletin for 42", may also apply for 50 too but I just don't know.

This is a tricky fault by what your saying, not helping much here but I'm sure people are reading the post with a ??

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Thanks TW. The set won't fire up again until I remove the SD board that keeps burning out shortly after being installed. If I remove SD, I get the partial screen display only, but the TV powers on normally. That flowchart doesn't look good for prognosis on the panel being bad I guess I'll try to do the flow in the next day or two and see what I find. Again, not sure it is the same for this model?

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Also, no I have not done the resistance check on the SD. Wasn't sure on how to do that. Would there be any point when the TV works, then blinks 7 until the SD is taken out? Seems it's obviously burned out, but I'm not sure.

[tw2005]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

I always like evidence to confirm a suspicion but if you've replaced the SD twice and it's gone short twice ( which is an assumption since nothing has been measured) then I'd be sus on the panel or a hell of a coincidence.

The obvious dumb question, everything is bolted in and earthed, ribbon between the SU/SD connected?

Sorry I don't have better advice.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

tw: Not a dumb question, because I'm really new to this amateur TV repair

There are a few more things I think I should say that might help finalize this diagnosis. When I hooked up the first SD board the first time, I'm not exactly sure what happened. However, I think the board was somehow crooked in its mount (screw may have been loose). This was causing the bottom half controlled by SD to just have random wavy lines. When I removed board and reseated, problem went away ... however not sure if I damaged something else in that process.

The other note is that I've read about this "buzzing" noise from the rear, and also checking that the factory screws on the SC? board were tight. I did in fact hear that buzzing the entire time the TV ran for the first SD board. I thought it was normal. This was a light buzzing that you couldn't really hear with any sound turned up. It seemed to go away with the second SD board, but that may have just been coincidence. I still haven't checked any screws other than the ones I've messed with.

My main goal here is simple. I believe I can get the full combo of boards for this unit used on Ebay for about 75 if I shop it right, but of course that would be foolish if the panel is shorted. I'll take a look at that last pdf you sent. Why have I read on other forums about this unplugging a ribbon from one of the two buffer boards to confirm a bad panel? I thought they said if the set fired up with just one ribbon unplugged, then the panel is bad. Is there any truth to this test for this model or others? Thanks!

[tom66]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

If the picture is good, I can't see it being shorted, buffer failure should occur instantly.

To improve buffer lifespan you can drop Vad (**NOT** Vda) to the point of pixel misfire. On my 42" PZ80 the Vad was around -145V and I was able to go to -125V with minor misfires. (Below -120V causes 6 blink error, increase it back up to clear error.) I then increased Vsus to 209.5V from 204.5V and the misfires went away completely. Extra bonus of this is lowered black level and brighter whites, improving contrast ratio and picture quality. Alternatively, you can decrease Vsus by around 5V and increase Ve by about 30V or until pink/cyan maldischarge disappears, but you lose the lowered black level benefit.

The buffer boards seem to work as switches: they either connect the Vfg output to the panel electrode (the normally closed mode) where the panel is sustained, or they connect Vfg - Vad (which varies depending on Vfg) which allows the line to be selected. All voltages are relative to the floating ground, which is the sustain-scan board (SC) output.

If there is a scan-sustain short (SC thru buffer to SS) the appropriate line will not light and the buffer should immediately fail in my opinion. It will try conducting amps of current, leading to quick failure. So, the buffers may be dying because the panel has contamination, in which case reducing the stress on the buffers may make it a non issue.

In my opinion it is more likely they are failing because the buffers are removed from a cracked screen TV. I have found some outright dead buffers pulled from a cracked screen Samsung plasma, due to the cracked panel shorting electrodes. I would not be surprised if this also happened with some Panasonic buffers. In which case the solution may be to replace the buffer ICs.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

tom: wow! that was a lot of tech talk From what I gather, you seem to be thinking that (as crazy as it may seem), I could well have gotten two used buffer boards that were on the verge of failure. What you say about the cracked screen is also very likely , given that I bet most of these Ebay sellers pull these boards off cracked screen units.

How difficult is it to test and/or actually repaid the SD? I'm just wondering now that I have two of them in an apparent state of failure. Could I swap chips between to fix or something? Thanks.

[tom66]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Do you have a current limited power supply, capable of around 12V? Limit it at around 500mA. You can try applying voltage between VFG and VFG - VAD (not certain on exact labels, but the service manual should label them); if there's a buffer chip shorted, the appropriate chip will get hot and/or smoke, you can then swap it knowing it's the bad one.

Now, if you find both boards to have the same chip failed, it makes the bad panel theory a lot more plausible.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Tom: Given that I haven't heard of a current limited PSU, I don't think so. Can I rig such a thing from something like a universal laptop ps, wall wart, or other?

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Do you have a current limited power supply, capable of around 12V? Limit it at around 500mA. You can try applying voltage between VFG and VFG - VAD (not certain on exact labels, but the service manual should label them); if there's a buffer chip shorted, the appropriate chip will get hot and/or smoke, you can then swap it knowing it's the bad one.

Now, if you find both boards to have the same chip failed, it makes the bad panel theory a lot more plausible.

Where exactly should these "vfg" "vad" labels be? I have the SM, and the diagram up for the SD board, but don't see anything with any such label. Maybe I'm looking at the wrong diagram?

[tom66]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

On this one, I think between Vfo (positive) and Vf (negative.) Looks like Panasonic do it the other way around: their buffers switch between Vscn and Vfg, with Vscn being mildly positive (generally 30~40V) and Vfg being set at -Vad (-100V on this panel.)

You can try using a car headlamp bulb in series with a 12V battery or 12V power supply to test the buffer for shorts.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

tom,

thanks, but i'm lost on those labels. I don't see anything with a V label, except on the SC board listing for "test points". Where exactly should I be looking for these on the SD board? I'm assuming I can do this check with the sd board removed from the set?

[tom66]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

They're on the SD board schematic, do you have that? It is available in the service manual. Yes, check with board removed from set and disconnected from plasma panel.

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

They're on the SD board schematic, do you have that? It is available in the service manual. Yes, check with board removed from set and disconnected from plasma panel.

Yes I do, and I've looked at it several times. Perhaps I'm missing it. About where on the board are these points?

[tw2005]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

page 95 in SM, sheet labelled : 15.39. SC, SU and SD-Board Block Diagram, right hand side

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Sorry, I just can't see how I should do the polarity on this test. VDD and VDDH. Which one is positive and which negative? Does anyone know? Thanks!

[tucocaps]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Ah after looking further. i think it may be VDDH = POS (12 volts for test) and GND = NEG. right?

[tom66]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Vfo(Vscn out), positive to Vf(high voltage ground), negative

[Clydeuk]

Re: Panasonic Plasma TH-50PX75U Test for bad panel?

Don't know if there is a firmware update available for this model, I recall one being made available for another Pana to supposedly prevent SC failure. You can test which line of the SD has failed with a DMM, one lead to VFG, then check each output where the FPC's connect, one (or more) will be S/C if an i.c has failed. This may partially help identify the area of the panel this corresponds to. If you have more than one SD that has failed after being fitted to this set you may want to repeat the test on that, if it's the same output line that's S/C then the panel is a likely suspect. Can you see any evidence on the display of a burn-out when you examine the front of the screen carefully?