Phillips 42PF7321D/37 Plasma No Video

[mike123333]

Hi -I acquired a Phillips plasma 42PF7321D/37 that has no video, but according to the individual that gave it to me stated that it does have audio. So far after numerous hours of troubleshooting I have found the following:
the upper and lower buffers are shorted. A matter of fact there was a pop and smell of an electical short. Upon metering it was definitely shorted.
I am trying to troubleshoot the Ysus board samsung LJ92-1200A. I do not want to purchase new buffers without having the Ysus board repaired. The obvious is not wrong with it. No capacitors shorted, bulging, leaking, etc.. the 7 Mosfets test out ok and are not shorted. While i was testing the board under power with no buffers attached over a minute or two the 2 cermaic resistors R5098 R5099 would get extremely hot and the sanyo STK795-518 IPM module woud start to hum. Could this be the culprit?. All thej voltages coming from the PSU are checkign OK, some voltages will need a little tweaking.
Anyone familiar with this board have a voltage checklist on the board?, Any known issues in specific areas of the board?
I am at a lost here!! the TV did not see much use based on the person that gave it to me. It was sitting in her storage for 2 yrs broken.
From what I see inside the TV, this is a Samsung with a Phillips Labels.
BTW -There are o error codes or red LED. the unit powers up fine and the L2 board has a solid green and the other LED is blinking. I don't know what the blinking is for.
Any input would be greatly appreciated.

Thanks!!!

[PlainBill]

Re: Phillips 42PF7321D/37 Plasma No Video

Hi -I acquired a Phillips plasma 42PF7321D/37 that has no video, but according to the individual that gave it to me stated that it does have audio. So far after numerous hours of troubleshooting I have found the following:
the upper and lower buffers are shorted. A matter of fact there was a pop and smell of an electical short. Upon metering it was definitely shorted.
I am trying to troubleshoot the Ysus board samsung LJ92-1200A. I do not want to purchase new buffers without having the Ysus board repaired. The obvious is not wrong with it. No capacitors shorted, bulging, leaking, etc.. the 7 Mosfets test out ok and are not shorted. While i was testing the board under power with no buffers attached over a minute or two the 2 cermaic resistors R5098 R5099 would get extremely hot and the sanyo STK795-518 IPM module woud start to hum. Could this be the culprit?. All thej voltages coming from the PSU are checkign OK, some voltages will need a little tweaking.
Anyone familiar with this board have a voltage checklist on the board?, Any known issues in specific areas of the board?
I am at a lost here!! the TV did not see much use based on the person that gave it to me. It was sitting in her storage for 2 yrs broken.
From what I see inside the TV, this is a Samsung with a Phillips Labels.
BTW -There are o error codes or red LED. the unit powers up fine and the L2 board has a solid green and the other LED is blinking. I don't know what the blinking is for.
Any input would be greatly appreciated.

Thanks!!!

This is a common problem; it's repairable, but IMHO, not worth doing.

You diagnosis is correct as far as you have gone; here's the final details. There are two heat sinks next to the connectors to the buffers. There are 5 or 7 power FETs in parallel under that heat sink, one is shorted.

The failure progresses like this - There are 6 buffer ICs, usually each with it's own heat sink on the buffer board. For reasons that aren't clear, one of them shorts. Because of the design, that overloads the drive FETS on the Y-Sustain, which further stresses the rest of the buffer ICs.

The solution is simple - replace all of the buffer ICs, replace the shorted FET, check for any other component on the Y-sustain that may be bad. The parts are readily available on eBay. If I recall correctly, the buffer ICs are about $10 each, the fets two for $14. The total cost would be about $140 and however long it takes you to replace a dozen 100-pin ICs.

Or you can let someone with more experience do the work. The LJ92-01200A, LJ92-01202A, LJ92-01203A are available on eBay, the complete set will cost about $200.

If you Google 'Coppell TV repair' and go through the blog on that site you will find complete details on troubleshooting the problem.

PlainBill

[mike123333]

Re: Phillips 42PF7321D/37 Plasma No Video

Thanks for the reply. I performed most of the troubleshooting using Copell's testing. I actually lifted all teh source and gates of the mosfets that you are referring to. I tested as follows: I used the common probe on teh source and the other ont eh gate. then tested from source to drain there as continuity. then proceeded to short the drain and gate then the source to drain tested open. which is the correct way of testing as I have been told and read. There were no shorts betwen the drain to gate. Based on all that test I assumed that the mosfets are good. unless I am not testing correctly? could they be failing under a load?

As for the buffer boards I will rebuild them. But I want to clear all problems with the Ysus board first.

I think the STK795-518 IPM is good, I will need to replace that as I damaged it taken it out of the board.

Is there points of interest on the board in reference to voltage and problematic areas?


Thanks!

[PlainBill]

Re: Phillips 42PF7321D/37 Plasma No Video

Thanks for the reply. I performed most of the troubleshooting using Copell's testing. I actually lifted all teh source and gates of the mosfets that you are referring to. I tested as follows: I used the common probe on teh source and the other ont eh gate. then tested from source to drain there as continuity. then proceeded to short the drain and gate then the source to drain tested open. which is the correct way of testing as I have been told and read. There were no shorts betwen the drain to gate. Based on all that test I assumed that the mosfets are good. unless I am not testing correctly? could they be failing under a load?

As for the buffer boards I will rebuild them. But I want to clear all problems with the Ysus board first.

I think the STK795-518 IPM is good, I will need to replace that as I damaged it taken it out of the board.

Is there points of interest on the board in reference to voltage and problematic areas?


Thanks!

None that I am aware of, but I am far from an expert on them. Before giving the Y-sustain a clean bill of health, hook it up WITHOUT BUFFERS!! And check the voltages and drive waveforms.

PlainBill

[mike123333]

Re: Phillips 42PF7321D/37 Plasma No Video

That is how I am testing the board no buffers. Once I get the new IPM installed I can do further tests.

I will keep you posted on my findings.

Thanks.

[mike123333]

Re: Phillips 42PF7321D/37 Plasma No Video

As I continue to check for shorts and failed components, I came across soemthing odd when probing the voltages. As I have the common lead on the board ground I am getting areas og th eboard in which the ground is not continuos across the board. I do not get continuity from one part of the board to others where a component will have a ground pin. I found this odd, its as though its behaving like a hot and cold ground, buts its not the power supply but Ysus board. Any ideas what I am seeing here? This board is multipl layered very difficult to trace the circuit.

Thanks.

[PlainBill]

Re: Phillips 42PF7321D/37 Plasma No Video

No idea.

PlainBill

[mike123333]

Re: Phillips 42PF7321D/37 Plasma No Video

Hello again - I ended up purchasing a used Y suatian board that supposibly came from working TV that ended up having a crack screen. Long story short I found a open fuse that supplies the 10v and one of the 6 mosfets shorted. I did the repairs and purchased brand new upper and lower buffers. Installed evertyhing and was checking for voltaeg and stability of. I did get to about 130v on the voltage supply to the te buffer boards. not 1 minute goes by as I was trying to troubleshoot, then I heaer a pop and needless to say the upper board shorted out. Luckly the bottom did not fry.

After that, I only installed the bottom buffer and powered the TV up. I would get a snowy bottom half picture that would fade into a sparkle and a red array toward the center. I then disconnected the Z sus and powered back on, then I woulkd get a black screen with soem sparkles. My question is, does this sound like the Y sustain board being the problem. I will have it rebuilt if that is the case. I don't want to dump a lot of money to come to findout that the panel is shot. Any input would be greatly appreciated. Thanks. See photo.

[RobPhotoPro]

Re: Phillips 42PF7321D/37 Plasma No Video

Hey Mike! What's the status of your TV repair? I'm working on a TV with the same internals (Samsung HP-R4252). So far I replaced one shorted mosfet under the heatsink and got a replacement lower Y-buffer, which had two ICs shorted.
So now, after powering up the TV, still no picture .... Nothing's blowing up either.
Any idea, how to proceed?
Also, what open fuse did you replace on the Y-sustain (the one supplying 10V)?
Thanks!