On Wed, 26 Sep 2012 14:59:08 -0500, Jon Elson wrote:

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

The PSU was repaired, with the aid of a friend technician.
The PWM was faulty, together with a couple of resistors.
Then it started and went down again.
All caps were desoldered, finding that many of them had leaked (the
PCB under them was wet).
Checked with the ESR meters, they looked good.
Changed all caps resolved the remaining troubles.

Thanks to you all !