John wrote:

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.

OK, the classic circuit here is a rectifier feeding the main input cap with
a resistor. When the cap is charged to some value, a circuit tickles the
power transistor and starts the (usually) flyback converter. A winding on
the transformer turns on a triac to shunt the charging resistor. Usually,
if the converter starts without turning on the triac, the resistor burns up
in a couple seconds. If the converter starts to power up, then the problem
is likely the rectifier, etc. that triggers the triac. If the converter
doesn't start, then the problem could be the PWM chip, the power transistor,
or maybe the rectifiers, etc. on the output side.

Jon