Panasonic TH-50PX75U top half of screen blank

[canadaboy25]

Hey guys, I was given a Panasonic plasma because the top half of the screen was black but the bottom half was still working normally. Sometimes the top half would flicker on and off. I tried reseating all the ribbons and cables thinking it was a loose connection but it made no difference. After I fooled around with it for a while it was working most of the time with only the odd flickering on the top half. There was no real correlation between my tests and it starting to work. I set it up and started using it and it was perfectly fine 98% of the time. I used it like this for a couple months with no real issue. However, the TV has been left off for 4 months and now when I turn it on the top half it always black. Percussive maintenance has no effect.

I found this thread where the solution ended up being a replacement upper buffer board: https://www.badcaps.net/forum/showthread.php?t=27128 However, in that thread it was stated that it is very easy to get a bad replacement board due to parts being pulled from cracked TV's. For this reason I would rather repair my board than chance buying a bad replacement. This should also be cheaper than buying a whole board.

I recently repaired a Hitachi plasma with help from you guys where the issue was a shorted buffer ic on the upper buffer board: https://www.badcaps.net/forum/showthread.php?t=75627 I did the same diode mode test on this board as I did on the Hitachi and found no shorted buffers. I suppose this makes sense as it is not bringing down the whole display like on the Hitachi.

So my question is, what else could the fault be? The only components on the buffer board are the buffers themselves and a few resistors/diodes/capacitors. Can the buffers just stop working even though they test fine in diode mode? I also can't see the part number of the buffers as they have a heatsink glued on top which I would rather not pry off if the chip is in fact fine.

Any ideas on what to look for next are appreciated.

[FrozenHaxor]

Re: Panasonic TH-50PX75U top half of screen blank

Most likely your problem are the high voltage driver ICs on the board responsible for supplying the side that has a problem on it. Try buying a board from scrapped TV. Most of those are out of circulation at this point due to high failure rate and general age so your options are rather limited. I know that at least in my country it was possible to still order new original parts from Panasonic but the cost was outrageous and my customer wanted me to scrap the TV.

[canadaboy25]

Re: Panasonic TH-50PX75U top half of screen blank

Most likely your problem are the high voltage driver ICs on the board responsible for supplying the side that has a problem on it.

That was my suspicion but they all seem to test the same so it is difficult to narrow down which one is causing the board to fail. I guess I will just have to replace them all.

Try buying a board from scrapped TV. Most of those are out of circulation at this point due to high failure rate and general age so your options are rather limited. I know that at least in my country it was possible to still order new original parts from Panasonic but the cost was outrageous and my customer wanted me to scrap the TV.

Buying a board from a bad TV is what I was trying to avoid due to the reasons listed earlier and also like you said, because they are all old by now and few left.

I suppose I will pry off one of the heatsinks and see what number the chips are. This tv is not worth spending much money on so expensive repair options are not feasible.

[FrozenHaxor]

Re: Panasonic TH-50PX75U top half of screen blank

Last time I worked on one of those the high voltage drivers had a nasty glue all around them bonding to PCB making it really difficult to replace. Perhaps you could try local heat from a hot air station on given chips and then look for a change on the screen, it just might work to narrow the rotten apple down. From experience heat works better than freeze spray. Bring it almost to soldering temperature but not too high. I'd remove the heatsink before heating though.

[canadaboy25]

Re: Panasonic TH-50PX75U top half of screen blank

Tried heating each chip but no results. Guess I'll have to pry one heatsink off and see if I can tell what the chip number is.

[R_J]

Re: Panasonic TH-50PX75U top half of screen blank

The buffer ic's should be AN16076A-VT, most of the drive data lines and voltages are common to both the SU & SD boards, it is very likely it is just the SU board, there are a number of resistors that you could check IE: R6902,2,4 (22Ω) or R6925,6 (0Ω)
When you checked the buffer board with your meter you were checking the buffer outputs, The whole top half is black so it is likely the buffer inputs have the problem, a shorted clock or data line on one ic would kill the signal to all of the upper ic's

[canadaboy25]

Re: Panasonic TH-50PX75U top half of screen blank

Well I spent some time probing the input signals to the SU and SD boards to no real result. On the connector going from the SC to SU board I get a clock on CLK and some data on OC1 and OC2. It looks to be the same on both the SU and SD boards. However, the SI pin which appears to be serial in and is then daisy chained through all the buffer chips, has what appears to be random pulses on it. The strange thing is there is no signal at all on the SI pin for SD which is the board that is actually working. I also have no idea what OC1 and OC2 actually stand for.

In order to do these measurements relative to the floating ground I had to float my scope using a ups. Yes, I know this is not recommended but I'm not going out to buy a $1000+ differential probe to fix an old plasma. I also don't think that having the ground pin on my scope live while probing the tv is any more dangerous than actually probing a running plasma tv.

None of these observations matter though because during my testing I forgot to unplug the ups from the wall to float the scope and the ground pin on my probe touched a large resistor on the VSUS rail which killed the TV and now I get 10 blinks. Amateur mistake but hindsight is 20/20. None of the fuses seem to be blown so I'm not sure where to look for what I blew. If it's not a simple fix to undo my mistake then I will probably have to admit defeat on this one.

[canadaboy25]

Re: Panasonic TH-50PX75U top half of screen blank

Well I went to start troubleshooting what I blew up and the tv powered up just like it did before I shorted the VSUS rail. I must've popped some resetable fuse or something that just took a while to cool off. So basically I'm back to troubleshooting the original problem.

[canadaboy25]

Re: Panasonic TH-50PX75U top half of screen blank

I finally had time to get back to this TV.

Just for the sake of anyone else who might encounter this thread by googling for the same problem, I figured I would post the solution.

I did some diode mode tests on the SU and SD board data connectors. All values were basically the same except for one pin. I then traced the problem on that data line down to a bad 0 ohm link R6925 on the SU board. Replaced the link and the top half of the TV is working again.

Reading back through this thread I can see that it was suggested to me to test this component but I seemed to have been distracted by accidentally shorting something while testing.

Oh well, I got there in the end.