On Wed, 5 Sep 2012 10:52:48 +0000 (UTC), John
wrote:

Hi !
I'm tryin' to service an IBM Server 325 PSU, suddenly dead.
I checked all capacitors, diodes and other stuff for shorts.
I put a paper clip connecting the power on pin with gnd.
The rectified voltage (generated by a separate board) reaches a primary
stage, with discrete components and three ICs
TOP200YAI - Three-terminal Off-line PWM Switch
LM393P - as usual
UC3843AN - Current Mode PWM Controller
CQY80NG - Optocoupler
and two transformers.

Well, from this stage exits nothing, with or w/o the paper clip.
My experience on switching PSUs is very limited.
The PowerOn signal doesn't reach directly the first stage but a custom made
riser card, so the fault could be elsewhere.
Any hints on where to begin lookin'?

As a last resort, since the output voltages are the same of a standard ATX,
only with different connectors, I could swap them and use a "normal" ATX.

But I'd give a try, it could be a useful experience...
Usually the AC input is filtered, then goes to a bridge rectifier.
The output of the bridge is filtered by one or more caps - typical
values start at 100uF, voltages often run up to 400 volts. If it
isn't obvious, that can pack quite a wallop. Remember, this is a live
ground - voltage is present on all points relative to earth ground.

The first step is to check the output voltage of the bridge - it
should be 1.4 x your nominal AC voltage. Since that is present, make
sure it is correct, then check the three ICs. Using the negative
terminal of the bridge for the negative lead of the dmm, check the
voltages on each pin of them. One of them MUST have a voltage to
start the whole process working.

With the components you list, odds are the TOP200YAI is the controller
for the standby supply, while the UC3843AN is the controller for the
main supply. But it's not impossible the roles are reversed.

A common design uses a high value resistor to drop the AC input
voltage down to the 20 volts or so the UC3843AN can tolerate on pin 7.

One suggestion - datasheets and application notes are available for
both. Study the schematics they contain and compare them to what you
see.

PlainBill