Samsung 50" plasma pn50b450b1d has poor white images after 2 hours, turns off after 6

[awilder1015]

As the title says, when I keep my TV on for longer than a couple hours, the whites turn somewhat grey, and high contrast borders, such as black text on a white background, have a white halo. The white halo is the correct brightness, but only located at the border. After the TV has been on substantially longer, between 4-6 hours, it will turn itself off for a minute, then turn back on for 5 more, then off again. It does this until I turn it off and let it cool down.

I've also noticed that if the image only has a small amount of white in it, like if I were using it as a computer monitor and had a white window occupying 1/4 of the screen with the rest black, that white portion looks fine. The issue only occurs when the majority of the image has a lot of white in it.

I'm guessing that it's a thermal issue, but I haven't done very thorough testing yet. The capacitors all look fine (none visibly blown or bulging), the few capacitors that I tested were within spec, and the power supply voltages are all exact, but I haven't checked the voltages after the TV starts acting up.

Ive already tried changing the x-main board, to no avail.

The attached image shows the white halos around black images that appear on a white background

Thanks for your help!

[tom66]

Re: Samsung 50" plasma pn50b450b1d has poor white images after 2 hours, turns off aft

May be worth checking if Vs remains in regulation as the TV warms up.

[awilder1015]

Re: Samsung 50" plasma pn50b450b1d has poor white images after 2 hours, turns off aft

I just took the back of the TV off, and checked all the voltages after the TV warmed up, and sure enough, Vs drops from 209 (spec) to 197.

I'm assuming that since the Vs test point is on the power supply board, that the problem lies there.

What could cause this? What can I do to fix it? Should I just trace Vs back from the test point to try to find some component that might be bad? Since this is clearly a thermal issue, could it be that one of the transistors mounted to a heatsink is underperforming, and either needs to be cooled better or replaced?

Thanks for your help