Samsung 2493HM and damn BN44-00195A PSU

[sushy74]

Hi everyone, sorry if I open a new thread on this power supply board, but after reading the present ones I was unable to solve the problem. A few days ago I turned on the monitor and hear a small bang, the monitor does not turn on and the blue LED flashes.
I disassemble the monitor and try to understand what the problem is with the power supply board, I immediately notice the RM801 (0.22ohm) resistor burnt. I replace it and try to turn it back on, of course it burns again, I put a new one and check with my digital multimeter the 2 fet QM802 and QM803 that are burnt, I replace them and, for safety I also replace the capacitors CM851, CM852, CB853 and also CM812 ( the blue 12nf 630V capacitor) also remove the brown glue from the board. I try to reassemble the board and turn on the monitor, this time nothing burns, the blue LED is on and does not flash but the monitor does not work. I disassemble the power supply board again and test the voltages 5.3v and 24v with a 3.3k resistance between S_B and PSON. Instead of 5.3v I have 0.45v and instead of 24v I have 0.05v. On the large central 82mf capacitor I have a voltage of 320V in standby and 390V in operation. Looking at the board from both sides I don't notice any other burnt components.
I attach 2 pics of the PSU, I hope you can help me solve.
Thanks
Francesco

[sushy74]

Re: Samsung 2493HM and damn BN44-00195A PSU

I would like to test TM801 transformer with my DMM, but i don't know how
if someone could explain would be great
tnx a lot in advance

[R_J]

Re: Samsung 2493HM and damn BN44-00195A PSU

Remove the transformer and check it out of circuit, The readings will be near 0Ω. With the transformer removed check the diodes and resistors in the gate circuit.

[sushy74]

Re: Samsung 2493HM and damn BN44-00195A PSU

Tnx a lot for reply. sorry for stupid question, but witch part exactly you mean talking about "gate circuit"?

[R_J]

Re: Samsung 2493HM and damn BN44-00195A PSU

Mosfets have three leads, Source, Drain and... Gate. The gate is the control lead of the mosfet

[sushy74]

Re: Samsung 2493HM and damn BN44-00195A PSU

Thanks a lot dear friend....
I unsoldered and tested TM801 and found a disconnected pin...it was for me very difficult to repair but i was able to sold pin to wire coming out from yellow tape, now there is continuity and testing pins with DMM at 200ohm scale i can read 0.06 in primary and both secondary. As you advised i made a test of gate circuit....all resistors are ok, about DZM802/3/4/5 with DMM set to diode test i can read a value of 0.712/15 for all, reversing DMM leads circuit is open. Now, talking about DM803 and DM806 i read a value of 0.21 in both ways i put DMM leads...is that normal?
is there something more i can test?
tnx a lot

[sushy74]

Re: Samsung 2493HM and damn BN44-00195A PSU

I haven't resoldered TM801 yet, so not yet tested PSU ...would like first to be sure about DM803 and DM806 test and if i have to test some more components without TM801 on pcb
Tnx

[sushy74]

Re: Samsung 2493HM and damn BN44-00195A PSU

Ok, sorry for my stupid mistake...i notice just now that two 22ohm resistors are in parallel with dm803 and dm806, that's why i have 22omh on both sides of diodes..i imagine the only way to test those diodes is desoldering them or at least desolder resistors in parallel. I'm a little afraid about desoldering smd components, i think i will waith some more help or trying to power on psu hoping nothing will explode

[R_J]

Re: Samsung 2493HM and damn BN44-00195A PSU

The diodes are likely ok, they usually short if bad so the resistance would then be 0Ω and not 22Ω.

[sushy74]

Re: Samsung 2493HM and damn BN44-00195A PSU

Fortunally i met this fantastic forum...repaired TN801 mounted on the board and my Samsung monitor is back to life!
Thanks a lot R_J for your valuable help....i hope this thread could help someone else with same problem.
regards
Francesco from Italy

[R_J]

Re: Samsung 2493HM and damn BN44-00195A PSU

Glad to here you have it working, thanks for the follow up.