Panasonic TH-37PV7P (TNPA4268 PSU repair help)

[Dannyx]

Good day folks. A colleague of mine scrapped his plasma TV after it quit on them, so I decided to dig it out and have a look inside myself to see if it's something that can be fixed relatively easy. I attached the schematic to save some time.

The problem: shorted PFC transistor Q406 (page 65), shorted PFC diodes D417-420, open R448/R449, open R439/R493, open PR402, open R438, so yeah - quite a lot of damage. My colleague knows a little bit of electronics himself and tried replacing the FET himself but it blew again instantly, since he didn't know what else to check around it and with so many dead parts, it's no surprise. He also failed to add a thermal pad between the FET and the heatsink, essentially shorting the drain directly to GND. Not wanting to take any chances, I replaced all the parts I mentioned, including Q404 and IC406. I just used a piece of wire to bridge across PR402 to send VCC to IC406.

The testing: initially, I wanted to see if the rest of the board is still OK and at least gives me STBY, so with the FET removed and PR402 still open, I plugged it into the mains. On page 64 are all the connectors for the other boards and I do get 5v on pin 9 of P25. Getting the board to start without the main board is a bit of a challenge and I'm not sure I'm doing it right: I tied 9 to 11 on P25 and 8 to 10 on P12. I hear some relays clicking and get 327v on the main cap, so now I have power going into my board.

The failure: after replacing the parts which measured bad, or I ASSUMED are bad, I tried another power-on the same way. I got the same two clicks, but heard a pop immediately after and the breaker tripped at the wall ! The FET is now dead again, but other than that, there doesn't seem to be any other dead parts so far, apart from the IC itself which I'm not sure how I can test without an oscilloscope. Given R493/R439 still read OK, I think the IC may have survived.

Thoughts: I took the liberty of installing all three PFC diodes, instead of just the two the board came with (I added P420 as well), thinking it's be an improvement, but looking back now, I'm not sure if this was a wise thing to do. The diodes I used are MUR460.
I also screwed up and instead of 12ohms for R439, I installed a 10ohm res...., though would 2 ohms difference really make the difference between life and death of that FET ???
I took me a while to finally install Q404, simply because I wasn't sure which way it should go. The datasheet shows a SMD package, so it wasn't much help. I had to use my meter to work out which is the base and which is the emitter, so that concerned me a bit, but after removing the Q404 again, I'm 99% sure I got it right, so either the FET was a fake and couldn't take it, or the slightly lower value of R439 indeed let too much base current flow and killed Q406....

I'm not sure whether I should grab another FET and go again or just give up and scrap it. I haven't identified any other bad parts after the pop, so the FET would be the only thing that'd be different. I guess I could also maybe remove one of the PFC diodes and replace R439 with the right value....

This video shows the same exact board, but the guy was very lucky and had very little damage and was able to fix it. He has 3 diodes in there as well.

[Dannyx]

Re: Panasonic TH-37PV7P (TNPA4268 PSU repair help)

I made another discovery: open R411. Looks to me like an inrush current resistor. If this is open, as soon as RL402 closed, it sent line voltage full-blast to the main cap, which probably contributed to the FET's demise....

[Th3_uN1Qu3]

Re: Panasonic TH-37PV7P (TNPA4268 PSU repair help)

And that, folks, is why you use the series lightbulb.

[Dannyx]

Re: Panasonic TH-37PV7P (TNPA4268 PSU repair help)

And that, folks, is why you use the series lightbulb.

Yeeeeeahhh.....here's the thing though: I DID use the bulb when I first tried it without the PFC part and since I replaced what I THOUGHT was just about every busted component after that, I thought I should omit the bulb entirely and just go straight to live testing.

I also discovered over the years that PFC-enabled supplies often light up the bulb even if they're working fine, like a false-alarm, so even if the supply is indeed faulty, you'd just think it's normal behavior caused by the PFC circuit, so it's a big gamble, or at least that's how it's been for me whenever dealing with PFC supplies: after I'm certain the supply's fine, I just take a deep breath and plug it in...sometimes it doesn't work, as you can see

The wattage of the bulb probably has something to do with it too, so I always kept it at around 25-45w. Any lower and it might not fire up at all and any higher would let more current flow, increasing the chances of failure of a large semiconductor....

I'm pretty confident I can still get it going, especially after the discovery of the incorrect value resistor and the open one, so I'm willing to go again. Might even replace the control IC again, since it's fairly inexpensive if memory serves.

[Th3_uN1Qu3]

Re: Panasonic TH-37PV7P (TNPA4268 PSU repair help)

I also discovered over the years that PFC-enabled supplies often light up the bulb even if they're working fine, like a false-alarm, so even if the supply is indeed faulty, you'd just think it's normal behavior caused by the PFC circuit, so it's a big gamble, or at least that's how it's been for me whenever dealing with PFC supplies

You just need a bigger bulb. 100W one will power up a PFC-enabled power supply normally and is still limited enough to not make parts explode.

[Dannyx]

Re: Panasonic TH-37PV7P (TNPA4268 PSU repair help)

Ok, good news: after it's been on hold for a long time due to lack of time, I had another go at it today and I think it's working. I replaced all the dead components again and this time it thankfully didn't go pop and I got 400v on the main cap, so now it's time to test it on the TV itself under load Cheers.