Hi ! I gave up on the PCServer 325 PSU, since at 99% was damaged a custom board. It was a standard ATX with different connector, so it was enough to adapt the failing PSU connectors to a "standard" (I chose 285W instead of 230W, just to be sure) and it worked.
Now, there's a another small PSU which I can't substitute, belonging to a DECServer 700. I followed your suggestions and hints. The only ICs are UC3844N PWM SFH601-3 Opto IRFPE40 AVS10 kit (AVS10CB, AVS1AC) Automatic voltage switch
I checked voltages (after the usual short-circuit hunt), and around PWM and Opto there's nothing or some mV. Voltage stops around the AVS10 kit. The Triac and the surrounding high valued resistances are really hot. I checked the datasheet of the AVS10CB controller against measured values, finding something odd.
I haven't figured well, yet, AVS10 kit behaviour. By the way, according to the datasheet typical app, Vm (main) is connected through a partitor to the AC main. Following the circuit is not easy, due to plane changes :( So, Vm should be an AC value, around 4 V. I found 2.8V. DC values are around 150 V. I checked R1 (exactly 1MOhm) and I changed, just to be sure again. Can't identify still R2, the scheme differs. Removing R2, I should have no Vm: I found 4V, so the PSU circuit differs from the datasheet typical one. Since on the two big caps after the rectifier bridge I have 150V, I should assume the AVS kit is working, anyway, even if the associated Triac is really
*very* hot. On the IRFPE40 pins I found the whole rectified voltage, over 300V. Then I lose track of the signals. Next step would be to follow 3844 supply line, in order to see where it comes from. Any hint is welcome !