Hope you guys dont hate me for this, (starting plasma talk on the forum)..
But I have aquired this 42" Magnavox (Samsung guts) plasma. The Power supply had a blown voltage regulator IC for the Vscan circuit. I replaced it and a shorted zener diode in it's path. Now with Vscan restored it should normally operate at -190vdc. Im getting a whopping -305vdc and is not adjustable via the Vscan pot. I have checked the pot to make sure it is operating and it works. Any ideas? I am stumped.

Originally either the PS blew the y-sus and buffers or the buffer shorted, wreaked havoc on the sustain and then harmed the PS. Haven't gotten that far yet until I can restore the supply to norm operation.

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I've seen the problem in many cheap later model CRT sets with switching power supplies and the problem is generally one of the things that's mentioned above. Unfortunately, the extremely elevated B+ voltages usually fries other circuits in the TV. Many times, to the point where the TV isn't worth fixing. I suspect the same can happen to a plasma or LCD TV.

Absolutely. Im going to look at the optocoupler. I have seen these go bad. Good call guys! Thanks again

Is this close to what you are working on?

Gateway, it seems is also made by Samsung, and they post some of the service manuals for Plasma sets on their website. A Google Search turned up this one:

http://support.gateway.com/s/Manuals/HomeTheater/GTW-P42M203.pdf


LG, I could help greatly. With Samsung, it's always been about dead sets and 35V electrolytics - 470, 680 and 820uf, along with any others that are obviously bad.

Cheers,

This is not related to the Plasma TV. but I attended a seminar given by a company trying to design a solar cell inverter that will last 25 years(the same as the solar cell panel life). They said that they avoided two things in their design:
Optocouplers and Electrolytic capacitors.

Sounds like optocouplers, like electrolytics, are trouble!

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Which pins are the output on one optocoupler? The side with the dot denoting pins 1 and 2 or is it 3 and 4?

Somehow as I was testing caps last night I ran across a small non polarized picofarad cap that read nothing on my meter. It was labeled 104. Well replaced it with a 102 now I still have unmanageable voltage but it's lower at -220vdc. And surges.

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thanks for te tip Andy! Guess im lucky I didn't smoke something

Well Andy you were right! I shorted the two output pins on the opto and to my surprise the voltage dropped to roughly 1.2vdc. So the primart side is working. Since I have checked out the secondary side components minus the opto, im going to replace it. My question is the opto is labeled p421. I have some other plasma power supplies and im wondering if I can use it's optocoupler? Mine is tan colored and the others are black. They would be coming from the same principle on that board.

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Ok will do. When I get the power supply back to spec guess I get to see what lovely damage it did to the Ysus and buffers. Well I aready know the buffers are done for. The upper one smells great!

Well it certianly was the optocoupler, I found one off a junk ysus board out of an LG. Worked like a charm. Now im repairing all the damaged mosfets and IC's on the ysus from the havoc the power supply caused.

Oh and the upper buffer actually had all 6 IC's burn, and literaly burned the board. lol

These plasma's that have no internal cooling structure get so hot you could flip it face down in your counter and use it as a range top to fry eggs. I have a few junkers here, (I might actually try it and put it on YT). lol

I've heard of a few cases where these hot plasma TV's actually scorched the walls behind where they were hanging.

Good to know! Not sure which would be better.. A plasma that melts paint off the wall or a 60's GE that pulls nails outta the walls.. lol

i hope you didnt harm the COF ICs on the panel, if so, your toast.

I got one of those stupid units with bad buffers as well. problem is, i put another buffer on it, the picture just "flashes" and smokes a random buffer IC.

Changed the bad mosfet on the sustain card, but still does it.

gonna probably junk it.