Panasonic TC-P50C2

[tom66]

Re: Panasonic TC-P50C2

Vscan seems fine, normally 140V above -Vad. -Vad should be around -180V.

[Mark W]

Re: Panasonic TC-P50C2

Update - now back to 6 blinks.

You're not going to beleive this but I had forgotten where the LED is located. The TV is lying face down on my bed so that I have easy access to all of the boards. Problem is that this faces the power switch and the LED down into the blanket on top of the bed. I have been using a stick to poke under the edge of the TV to operate the switch and was then looking at the back of the switch to see the LED which is ACTUALLY located a few inches to the left. Yes I know it seems impossible that I could count six blinks twice and then an hour later not see any. If any of you reading this are over the age of 70, you may understand. The short term memory gets shorter and shorter with each passing year.

So now back to trying to figure out why the voltage at MID only gets to +15.68 volts. I have been reading this with a Fluke 87 DVM but now think I should look with the HP 501 scope.

[Mark W]

Re: Panasonic TC-P50C2

tom,

Did not see your post about Vscan being OK. Thanks, that is one less gremlin to chase.

This is first forum I have ever used so am still learning how to navigate. Your post was first one on page 2 and I did not realize that page 1 was full.

[tom66]

Re: Panasonic TC-P50C2

Could you have a short between MID and 15V?

[Mark W]

Re: Panasonic TC-P50C2

Just measured 23.18k with + lead on +15 and -lead on MID
Reversing the leads gives 71.4k.

Readings are repeatable and take a couple of seconds to settle to the
final value. Acts like capacitors charging. I suspect it is a FET or IGBT
that is causing the problem. May learn more after I look with the scope.

[Mark W]

Re: Panasonic TC-P50C2

Here is a picture showing MID on the top trace and +15V on the bottom.

Sensitivity is 5V/div for both traces. Time base is 1 sec/div

Sure looks like MID is being clamped to the +15V.

[Mark W]

Re: Panasonic TC-P50C2

Back again. Have not given up on this TV. Taking a closer look at the schematic, it looks like a short between MID and +15V would result in smoke when Q16001 and Q16002 on the SS card turn on and drive MID to Vsus.

Since there was no smoke, I have assumed that Q16001 and Q16002 are not being turned on. The drive signal for these two transistors (AA_UMH) originates on the A board and is buffered on the C2 and SS boards. The input to buffer IC17203 on the C2 card measures 680 ohms to gnd on all three buffered signals. This agrees with the value of R17217 and is a pretty good indication that the inputs are not shorted.

Using the oscilloscope set to trigger on the rising edge in triggered mode, there is no signal on pins 2,4, or 5 when the TV is turned on.

I suspect that the A board may be defective but I don't know enough to say this with certainty. There could be something else that the A board is looking at that needs to be in a certain state before it will generate the UMH/UML signals.

Here are some voltage measurements taken with the DVM before the TV shuts down after two or three seconds: VSUS 191V, VSCN –39V, VAD –190V, MID 15.11V.

I sent an email to the TV company that I bought the A card from on ebay with the above information but have not gotten a reply.

Any suggestions would be appreciated.

Mark W
327

[Mark W]

Re: Panasonic TC-P50C2

Just about sure (99.9%) that the A board is bad. Talked to the guy that sold it to me and he is willing to refund my money if I return the board. He got it out of a TV with a cracked screen but I think maybe because of the craked screen, he never tried to power it up and assumed that the cards were good. I should have asked him about that but didn't think of it at the time.

Meanwhile, I have purchased a second A board which should arrive in a day or two. Hope this one works.

Mark W
418

[Mark W]

Re: Panasonic TC-P50C2

Second A board arrived today from Baltimore. After allowing a couple of hours for it to warm up to room temperature, I installed it in the TV. Connected the DVM to measure the voltage “MID” on connector SS3 on the SS board, held my breath, and hit the power-on button. Oh no, DVM reads 15.11 volts just like before. However, about half a second later, the reading changed to 120V. Problem solved.

I am a little disappointed with the guy that sold me the first A board. He committed one of the cardinal sins of electronics ( and other disciplines as well). He ASSUMED the card he was selling was working but obviously did not test it. So he wasted a lot of my time and energy and probably the return postage since he wants the card back. I hope he doesn't sell it again.

The Panasonic service manual could use some improvements. Had I strictly followed the troubleshooting guides, I would have replaced almost every card in the TV and it still would not have resulted in a working TV. In my case, the A card was reporting errors that were the result of a failure on the A card. A more comprehensive self-test would monitor the signals that drive the other cards rather than ASSUMING that those signals are present.

On the positive side, I learned a lot about how a plasma TV works, derived some satisfaction from saving this one, discovered this Forum, and did a tiny bit to help preserve planet earth.

Mark W
463