Samsung QE 75 Q 7 FGMTZG Power Supply dead

[kevin_allein]

Hi, have this Samsung QE 75 Q 7 FGMTZG TV with no function. First analysis shows the power supply board is dead and fuse blown. Power Supply has the number BN94-11439A. I did short circuit test transistor and diodes and those do not appear to have shorts. I must admit I did not look at every single data sheet.
In the TV was some black dust residue, so probably one of the electrolytic caps blew, but they do not show short either.

Does anyone have schematics for that board ?
Any good tip, what could be the issue ?

[Diah]

BN94-11439A suffer always from crack soldering at the Transformers Primary side.

[kevin_allein]

I found in the meantime two N-Channel MOSFETs are dead. Those two I could not measure before, since they were completely covered in that white goop.

How sensitive are the Samsung power supplies if I replace them with some other N-Channel MOSFET with different parameters ?

[EazyBone]

You need to change the 2-3 blue caps there also. We have no pics so couldn't tell you. Very common death for Samsung PSU.

I've changed different n channel parameters but not too significant and it works fine

[kevin_allein]

You are talking about the ceramic capacitors visible in those pictures ?

Two more questions: the MOSFETs were blown, but the Diode next to them measures OK. Should that be replaced ?

There are two large electrolytic capacitors 500 V 100 µF. I took them out and they measured within tolerance 9x µF and ~0.4 Ohm ESR, so I soldered them back in. Should those be replaced anyway ?

[EazyBone]

R221 2kv caps yes they need replacing. Even if they test good. You can obviously always take the chance not to but every repair kit I've ever seen comes with them. Generally one of them explodes, it's rare that one didn't.

Up to you

[kevin_allein]

I am having trouble to find 220 pF ceramic capacitors with 2 kV. 500 V are easy to find. As the voltage there is probably max 325V, shouldn't 500 V capacitors be good enough ?

[lotas]

I am having trouble to find 220 pF ceramic capacitors with 2 kV. 500 V are easy to find. As the voltage there is probably max 325V, shouldn't 500 V capacitors be good enough ?

Don’t forget about the pulse voltage, power supplies are also often set to 1kV, they very often burn and take a bunch of components with them....