TH-P50G15A - 6 Blinks- Bad TNPA4782

[tn245]

Re: TH-P50G15A - 6 Blinks- Bad TNPA4782

I'm jumpering SC50 and isolating buffers so hoping they are out of danger for the time being.

I've got 4.4KOhm on P15V line but weirdly about 6MOhms on P5V. Should be more like 5.6KOhms shouldn't it?

I'm about to try changing IC501 as it was the last on my list of possibles.

[tn245]

Re: TH-P50G15A - 6 Blinks- Bad TNPA4782

Changed IC501 and got the dreaded pop sound and lots of newly shorted bits. Damn!

[tn245]

Re: TH-P50G15A - 6 Blinks- Bad TNPA4782

So the 2PG011s all went and one of the 30G123s plus of course Q661, IC773 and also D865. Replaced those and most of the smd stuff again. I feel like I've tested or replaced everything that could cause another bang but I'm scared to test it again because I'm running out of bits and patience.

I've installed it with a 60W 240V bulb in series with SC2 to test, SC50 jumpered and buffers isolated. On powering up the bulb glows quite bright and there's a bit of a buzz from the board like something is oscillating. Then it trips out with 7 blinks. Nothing shorts out. Don't know if it trips out because of the bulb but according to tom66 on another thread it should run with the bulb there but dimly. But maybe not with SC50 bypass? Got only about 4V on MID. How to test further?

[tn245]

Re: TH-P50G15A - 6 Blinks- Bad TNPA4782

I went back to the other tnpa4782 I've had sitting around and I think I now have it in the same state as this one. It needed a replacement optocoupler PC461 among other things. Now both boards have pretty much identical resistance measurements on all components I've tested.

I'm testing with the 60W series bulb on SC2 and buffers isolated with SC50 bypass and both boards now do the same thing on switch on. Bulb glows fairly bright and a slight buzzing comes from the sc board for about 3 seconds, then the TV clicks off with a 7 blink SOS.

My question is about the bulb test. I think I need to go up in wattage as 60W may be limiting it too much, but if the bulb is glowing bright does that mean something is bad and drawing too much current, or would that be normal at switch on as it's trying to charge up capacitors? Are there any further tests I can do or do I just need to go for it and switch to a 100W bulb? I really don't want to blow anything and go back to square one on these if at all possible!

[tom66]

Re: TH-P50G15A - 6 Blinks- Bad TNPA4782

60W is too little, I have had boards not start with 60W, the problem is the bulb starves the DC-DC converters on the board so your -Vad and Vscn do not come up properly. This will trip SOS7. You need 100-150W bulb, minimum.

You might still have a fault, but you can test with a scope to see if the waveform output looks ok. Otherwise, you can go up in wattage and hope nothing goes pop, sadly it can be hard to tell with a bulb alone. The bulb however will limit the fault current and prevent destruction of many parts.

[tn245]

Re: TH-P50G15A - 6 Blinks- Bad TNPA4782

Thanks! I tried with a 150W on the first board way back when and still fried lots of igbts although I think some of the smaller bits were spared. As I still don't have a scope I will risk going up to 100W and see what happens.

I did just score what should be a working tnpa4782 in a p50g10 with a vertical black bar issue that I haven't looked at yet, so that should be useful for some comparisons.

[tn245]

Re: TH-P50G15A - 6 Blinks- Bad TNPA4782

We have a working board!

The one from the 46" was fixed as it turned out. After a lot of diode mode comparison with the working board I just got I tested it again in the TV. With 100W bulb it shut down SOS7 the same as with 60W, so I went to 150W and the little green LED lit up and the fans started spinning. Great! Plugged in buffers and TV works!

I'm not sure how much use the bulb thing is really given that when I used a 150W with a bad board most of the bits blew anyway. Possibly one or two were saved I don't know.

This was with mostly utsource bits btw. The ibgts appear suspiciously second-hand but at least have turned out to be the genuine article.

The older board doesn't quite match up yet on measurements and I need to order more of Q818 at the least to finish it up but I have high hopes of finally getting that one out of my garage!

Thanks for the input guys!

[tn245]

Re: TH-P50G15A - 6 Blinks- Bad TNPA4782

Working on the other board now. It's cutting out very quickly with 7 blinks. I am getting some voltage ~50V on MID and TPSC1, ~290V VSET.

I'm looking at the area of the circuit around IC724/725 where the sos7 is originating. On the input of those I should be getting 15V but I'm getting nothing (measured across C721). On the anode of D721 which supplies them I do have 15V measured with respect to VAD, but nothing on the cathode side with respect to VFG.

IC724/725 measure identical to the good board in diode/resistance modes so I think they are good and I can't find anything else amiss yet. Feel a bit stuck at this point.

[tn245]

Re: TH-P50G15A - 6 Blinks- Bad TNPA4782

Some progress, and it's a little bit embarrassing. I was going crazy measuring all over the place. Then went back to the longest list of possible bad components and went through one by one. Right at the end of the list was R707 and when I went to check it, it was.. missing! Just some nice clean pads where it should have been. Sure enough it was on my original list of parts found bad and then I found a new pack of 3.9K smd resistors unopened. Somehow 3 years ago when I worked on this I forgot to put back in R707. Oops!

So the resistor is finally in and now we get to the stage of fans spinning and then it trips out with 6 blinks.

Vset, Vad and Vscn are all spot on what they should be according to the service manual. MID is hovering around 63V. SOS6 triggers below 67V. So need to figure out what is causing low MID.

[tn245]

Re: TH-P50G15A - 6 Blinks- Bad TNPA4782

FIXED IT!! FINALLY!!

I recently decided it was time to get myself an oscilloscope and without it I'd probably have been working on this board till who knows when.

SOS6 was triggering super fast so I decided to risk jumpering SOS6 to ground and in that state it would fire up with fans spinning. Was testing with buffers isolated and SC50 jumpered so no idea what would have happened on screen. Noticeably a lot less buzz than a working board.

MID voltage would go to 60V or so on start-up and then drop down to about 9V where it would sit. Breaking out my scope I saw the sustain waveform looked about right. Scoping further I found that Q451 was missing its drive waveform. From another helpful post by tom66 explaining how MID works I figured out that Q451 charges MID and Q441 discharges MID. So Q441 which had its drive signal was discharging MID but Q451 was not doing its job of charging it.

Long story short I followed the signal back and eventually (really eventually!) found the culprit was a damaged trace under pin 12 (HIN) of IC521, hence the missing drive signal for Q451. Trace bridged and now the TV is alive and displaying an image!

Very happy to put the lid on this one finally and get it out of the garage. It has been with me too long and followed me through a house move. Shame it's probably worth less than I paid for it now but I was too stubborn to give up on it. Thanks for all the helpful posts! Now I've got a few more Panasonic SC boards to work on which I'm hoping will go a lot smoother! I do enjoy the challenge.