(Maybe solved?) Why is my monitor power supply broken (WITH SCHEMATIC!)

[xavier60]

Did you also measure the voltage on the F2229L's VREF pin 10?

[LooseJunkHater]

At this point, I think I have given up on this. I've probed nearly all resistors near the 3 IC's, no shorts, capacitors test normal, no obvious faults, voltages are present, and all the transistors appear to be working (no shorts, test fine in diode mode).


___

And just when I was about to give up, I decided, for the hell of it, to remove CM801 and CM802, and you know what the problem was? CM801. Nobody had that on their bingo card.

Pulled CM801 out of circuit, it wasn't detected by my LCR meter. What oddly lead me to finally pull these two caps was that when measuring around F9222L, sometimes my multimeter would shut off from that brief 500v + spike, as well as that CM802 was at 50v on the good board but like 380v on the faulty PSU.

However the mystery isn't over yet. While I now have 5.4v on the output, my 24v rail is going from 16-9v, likely because I had broke a sot-23 QM801 while trying to test it.

Now I need to find a replacement for CM801 (model "Pilkor WK0727") and QM801 (model sot-23 "ZH2H").

After many days, I may actually fix this board.

[LooseJunkHater]

Alright so I can't find any datsheet on the Pilkor part, nor the sot-23. Thankfully with the schematic, I was able to use a C1815 to-92 as a replacement for the sot-23 (which seems to be working fine?). I tried to use a 1uf 630v capacitor as the replacement of CM801 but all I get on the 24v output is ~3v, so I obviously need to go much lower. The schematic says the original capacitor was 630v 100n but I'm not sure exactly what type of capacitor it is (film?)? Maybe someone can help me find a datasheet of the Pilkor capacitor (images attached).

[Kim Christensen]

Looks like it may be a Metallized Polypropylene (MKP) according to this datasheet.
The markings make it look like a 18nF 630V.  Hmmm...

[LooseJunkHater]

Looks like it may be a Metallized Polypropylene (MKP) according to this datasheet.
The markings make it look like a 18nF 630V.  Hmmm...

When I tested the capacitor from the working PSU, it was around 18nf, so possibly another datasheet error.

[LooseJunkHater]

This will likely be my final post reply here.

In conclusion, it was CM801. In the process of finding CM801 I actually damaged multiple SMD components and since I don't have the BOM and can't be bothered to purchase replacement components, this power supply will be left dead. It was a success story in finding the problem and learning a lot along the way, so thanks to all the contributors. However for the next device that I find that is faulty, I'll be able to repair it successfully. Just a few years ago, there was not a chance I would have been able to find this fault, even with significantly more assistance, so thank you.