Help me diagnose my lcd tv's power board

[kolyan2003]

Hello,
I got Viewsonic 32" lcd tv. was working fine for couple years,
then image started going out for short second and coming back, during this i hear spike-like high pitch sound coming from circuitry. It started to happen every once in a while, now happens a minute or so after tv is turned on. After couple of these blackouts tv locks up and does not respond to remote or power button. Backlight remains on, but its a dark screen.

I look at it closer, and appears sounds come from the power board. It doesnt have any puffed capacitors, or any visual deffects.

Does anyone know what causes it?
Thanks

 

[Resqueline]

Cap's may still be bad even thought they're not visibly affected, but this of course makes it a much harder fix.
If you can take a picture of it and post it I might point to the ones I'd suspect from experience could be bad.
Did you Google the TV model number? You could take out the board anyway and check it for bad solder joints.

 

[kolyan2003]

Thanks for reply!
I got power board out. Here are some pictures,
TV model is N3252W.
Board model is 4H.B0280.001 /E
This poweboard is available on ebay, but I want ot give it a shot before shelling out my dollars.

After I took heatsink/cover off, i found brown heat damage spot (circled), on other side was NKS 385GVG A5B29C55 (this may be off, since i can barely see the printing on the chip)

I'll try to find a replacement.

 

[kolyan2003]

did some googling - chip is NKS N3856VG
datasheet - http://www.niko-sem.com/in/ezcatfiles/nikosem/img/img/1875/N3856.pdf

 

[(*steve*)]

That's excellent photography. It makes it soooooo much easier to see what's going on.

I would certainly not assume any of those capacitors are bad, but if you have access to an esr meter, it may be useful to check them.

However that chip certainly looks like it may have seen better days.

While you're looking for a replacement it might be useful to try to determine why that IC got so hot.

 

[kolyan2003]

I cant find a replacement or compatible chip...
found a lot of info for this tv here: http://forum.eserviceinfo.com/viewtopic.php?p=118264
compatible psu, much cheaper than original on ebay: http://www.electronica-usa.com/Merc...en=PROD&Store_Code=EU&Product_Code=FSP2123F02

 

[Resqueline]

It's a little hard to tell from the pictures but it seems to me that the solder joints on the power transistors are almost completely defective, at least on the two near the hot IC.
If they are bad then I see no reason to do anything else but re-do them.
It the joints are all ok then I'd try to replace the four small caps on the primary side.
The circuit board seems to be of the kind that easily tans. I wouldn't worry too much about the brown spot & the IC.

 

[(*steve*)]

I dare say your experience way exceeds mine it this area.

I'm just a bit old fashioned in that I think that boards shouldn't do that.

Just in case it is this chip, these people seem to be selling them http://www.lcdpart.com/lcdpartsnet/N3856.html (not cheap)

 

[Mitchekj]

My favorite part of that chip's datasheet:
▲Solved heat dissipation.

 

[mypcbis$crewed]

Wow, that chip put out an impressive amount of heat before failing. Good luck with you repair.

 

[michael]

be carefull to choose 4 the replcement