Didn't take any pictures or measurements, and not worth like writing up anything. Just thought this was odd enough to share.
PC shut down suddenly.
Power button doesn't seem to do anything.
Standby power still on (LED on motherboard).
Oh that's interesting, standby still on... wait, it's being back-powered from my USB hub? Oh that's... probably not good.
Unplug that, standby confirmed still good, goes up and down (down very slowly of course) with PSU.
Now pressing power button makes the fans twitch just a little bit. Hmm. Cycle PSU power then get one press again. Seems like a latched fault?
Main power maybe borked, that's not great.
Take PSU out, test on bench.
Pulling PWEN# indeed makes the outputs light up momentarily.
Probing the outputs, 5 and 12V look good, 3V3 is shooting up to 4.2V peak though, yikes. Okay, so the voltage monitor is doing its job, that's... good.
Further probing, the 3V3 regulator (a TL431) is clearly getting Vsense, the divider measures right, and it's even turning on as voltage rises above 3.3V.
And the transistor it's driving (a PMBT2907A), seems fine on diode test, and all the connected resistors measure as labeled.
And yet, probing across the actual transistor during startup, there's 1, almost 2V peak across E-B.
Replace it with a MMBT4403. Starts up just fine.
So, freak failure of a SOT-23 PNP in the voltage regulator circuit. No apparent cracked solder joints, or open or short or blown components. Weird.
For those curious, it's a EVGA 80PLUS 500W supply. Quite old at this point, but still going strong, it seems. Not sure how much power I run at normally but it's hardly stressed, the fan barely runs. Does have a Capxon (post PFC / main bulk cap) and Teapo (everything else), none of which are bulging or leaking. They're probably rated alright for the design (3 year warranty). Curiously I've had to repair it twice before, one more or less as expected (replace the shitty fan), the other a freak incident (blown fuse, nothing else!?).
Tim
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