After plugging in a mobility scooter and hearing a "crack", the 24V output PSU no longer works - dead as a doornail. Might have to just buy a replacement, but can't resist attempting a repair.
I unscrewed it and was pleased to see that it is, in principle at least, quite repairable - nothing encased in epoxy or anything like that. No signs of anything major, no black marks or obviously damaged components. It is an offline regulator design based upon the UC3842B "high performance current mode PWM controller" IC. The IC appears to be powered by running rectified and smoothed 240V mains through a 300k resistor and then into the IC which internally incorporates a 34V Zener across its power pins. Seems a bit funky, but who am I to say.
After a bit of probing around with the multimeter, it seems that both the IC and the power N-MOSFET (AP09N70P, probably obsolete) were roasted on the primary side of the device (the half shown in the attached image). The FET's gate is shorted to source. I removed both the IC and the FET.
I've probed around the primary side some more and it seems like most of the components are showing sane values, bar R12, seen best in the close up. With the IC removed, it is only connected at one end and is reading 8.3k. It is connected to the "OUTPUT" pin of the IC and runs through a one-time thermal fuse to the gate of the FET.
The value of R12 is somewhat obscured. Actually, it was a little better before, but there was a raised bit that appeared to be flux residue or something and so I dutifully scraped it off - together with more of the white silk. Muppet me. That said, the middle digit was never really legible.
So after the TLDR, the question is: what is the likely value of R12?
I think the last digit may have been a 6 (as oriented in the picture), but that would make the resistor a very high value for driving a gate and would make the FET toasty warm. With the alternate orientation, the last digit would be 5, but likewise is a very high value. So maybe the middle digit is an R? That would make the value maybe 5R6, but that seems very low. What would the purpose be for such a low gate resistor?