I have searched already but despite many many promising threads on this and similar power boards I have been unable to locate anything that has done the same in previous threads.
The board was presented to me with no power outputs whatsoever and a few clearly blown components:
QS802 and QS804 were shorted and cracked open.
RS833 and RS815 had likewise blown apart.
DS810 had also blown the top off its casing (and was open)
DS808 had no visible signs of damage but was measuring open with a diode test.
the main fuse had also blown
All the other components in this area tested as good with the tools I have (a basic DMM), I checked resistors, diodes and transistors and they all tested good.
So based on a few places that seem to have had a similar issue I went ahead and replaced these:
I replaced QS8020 and 804 with identical MOSFETS to those originally fitted (12N50T).
RS833 &815 were replaced with 47Ohm resistors as seems to be fitted in this spot based on a web search (the original markings were erased when they had exploded).
The original diodes "A4" BAV70 were replaced with "B3" 1SS184 diodes - the closest I had on hand, but they are very close specs (basically just +5V breakdown voltage) and I've seen these diodes on the 64" version of this board in that spot.
replaced the main fuse.
When I plugged it in with nothing else I get the standby power coming on as expected.
When I pull PS_on to ground I get the 5V and 15V rails coming on as expected.
HOWEVER when I pull VS_on high I get a power cycling of the main relay with VA/VS not really moving at all (there seems to be a few mV DC between Vs and ground coinciding with the very end of the turn on cycle but that is it).
I would wager that the power cycling is the MICOM detecting there is no VS/VA and shutting down the power board before restarting again, so i doubt its that side of the circuit and that its just there's absolutely no life from the VA/VS lines. so it really has to be something on the switching side as otherwise there would be *something* happening on the secondary side.
I tried swapping in a few components from another board that had the same ones, the capacitor CS825 - 68nF 1250V MMKP, and the controller - "25600" - I assume a UCC25600. Just got exactly the same result - just a clicking.
I have confirmed that the 25600 is good and that the FETS I'm using are good (by putting them into another board and it powers up and works OK).
Can anyone give any guidance as to what may be causing this issue? And how to go about diagnosing further?
I have access to a few other Samsung power boards with the same topology (slight component changes) to scavenge parts from if necessary.
I have attached some photos of the board overall, and some closeups of the area the failed components were located in. Its a relatively isolated part of the board, seems that the chip is turned on/off by cutting its VCC ~+15V supply, and the only other inputs are the PFC boosted ~400V and the feedback - which is difficult to test seeing that there is absolutely no life on the secondary!
Every repair kit I can find for this board seems to just replace the FETs in this area and nothing else (at least in this specific area) - it makes me wonder if this failed in a different way to have taken out the resistors and diodes aswell.
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The board was presented to me with no power outputs whatsoever and a few clearly blown components:
QS802 and QS804 were shorted and cracked open.
RS833 and RS815 had likewise blown apart.
DS810 had also blown the top off its casing (and was open)
DS808 had no visible signs of damage but was measuring open with a diode test.
the main fuse had also blown
All the other components in this area tested as good with the tools I have (a basic DMM), I checked resistors, diodes and transistors and they all tested good.
So based on a few places that seem to have had a similar issue I went ahead and replaced these:
I replaced QS8020 and 804 with identical MOSFETS to those originally fitted (12N50T).
RS833 &815 were replaced with 47Ohm resistors as seems to be fitted in this spot based on a web search (the original markings were erased when they had exploded).
The original diodes "A4" BAV70 were replaced with "B3" 1SS184 diodes - the closest I had on hand, but they are very close specs (basically just +5V breakdown voltage) and I've seen these diodes on the 64" version of this board in that spot.
replaced the main fuse.
When I plugged it in with nothing else I get the standby power coming on as expected.
When I pull PS_on to ground I get the 5V and 15V rails coming on as expected.
HOWEVER when I pull VS_on high I get a power cycling of the main relay with VA/VS not really moving at all (there seems to be a few mV DC between Vs and ground coinciding with the very end of the turn on cycle but that is it).
I would wager that the power cycling is the MICOM detecting there is no VS/VA and shutting down the power board before restarting again, so i doubt its that side of the circuit and that its just there's absolutely no life from the VA/VS lines. so it really has to be something on the switching side as otherwise there would be *something* happening on the secondary side.
I tried swapping in a few components from another board that had the same ones, the capacitor CS825 - 68nF 1250V MMKP, and the controller - "25600" - I assume a UCC25600. Just got exactly the same result - just a clicking.
I have confirmed that the 25600 is good and that the FETS I'm using are good (by putting them into another board and it powers up and works OK).
Can anyone give any guidance as to what may be causing this issue? And how to go about diagnosing further?
I have access to a few other Samsung power boards with the same topology (slight component changes) to scavenge parts from if necessary.
I have attached some photos of the board overall, and some closeups of the area the failed components were located in. Its a relatively isolated part of the board, seems that the chip is turned on/off by cutting its VCC ~+15V supply, and the only other inputs are the PFC boosted ~400V and the feedback - which is difficult to test seeing that there is absolutely no life on the secondary!
Every repair kit I can find for this board seems to just replace the FETs in this area and nothing else (at least in this specific area) - it makes me wonder if this failed in a different way to have taken out the resistors and diodes aswell.
),SMPS Circuits as a hobby and knowledge, i'M building up my LAB equipment ATM. 
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