Samsung PN60F8500 Plasma TV power board

[VertigoHorizon]

Had one of two 15amp 250VP ceramic fuses blow on the power board. Checked over things visually for bad caps or burn marks and saw nothing. I replaced both of the fuses with new ones and tried plugging it back in and both new fuses blew immediately.

Next I pulled the power board to have a look at the back side and noticed an arc mark on the metal panel behind the board. Located the spot on the board and took a couple of pictures of the front and back. Not sure if the heat sink is shorting across the PCB where the contacts are? There is continuity across the screws holding it to the clips as well.

 

[cjdelphi]

Have you tested for shorts across those transistors? Not sure what they are, could be a shottkey rectifier...

But you need to test them

 

[(*steve*)]

Rapid and immediate blowing of fuses in a SMPS is mostly caused by the chopper transistor failing short circuit. Another possibility is the bridge rectifier failing short circuit.

In either case I'd replace the rectifiers as they've been highly stressed.

And remember to discharge any high voltage caps before you work on the board. Whilst with this fault it's unlikely they will still have any charge on them, discharging them through yourself is extremely unpleasant (and potentially fatal for you), and discharging them through your meter if potentially fatal for your meter.

After removing the rectifier(s) check to see if there is a short across the DC (input) rail. There is a very small chance this could be something other than the transistor, so re-measure it after you remove the transistor. You should note see the obvious signs of your meter charging up the capacitor across the input high voltage DC rail.

Assuming it is the transistor, read the markings and get yourself a new one. When replacing the transistor, attach it to the heatsink before you solder it in place to ensure to don't put stress on the leads.

After replacing it (and the fuses), power up the power supply through a 100W light globe without any load connected to the power supply. It should flash bright for a moment then go dimmer. If it stays at full brightness you still have a problem.

 

[VertigoHorizon]

Bridge rectifier failed testing and seems to have shorted. Also one of the transistors on the heat sink in the picture seems to have shorted as well. Not sure how I can remove all the GS the put around the transistors to remove it after I de-solder the heat sink?

 

[(*steve*)]

Try gently prying that gunk away with a small screwdriver. Depending on its texture you may also be able to cut it with a craft knife to make this step easier. Be very careful not to damage things as you do this.