supervinx wrote in message
...
Hi to everyone, since it's the first time I write in this NG

I've little experience in switching PSUs ...
I've one of them (coming from an Amiga 2000) which shuts off at the first
load.
With no load at all, it delivered 5.9V out of 5 nominal.
I reduced it (with the trimmer) to about 5 with no load.
Inserting a little dummy load (47 Ohm) now it doesn't shut anymore, but
with more serios loads (e.g. an HD), it shuts again.
I checked in-circuit capacitors for shorts and ESR increase, tested
diodes and transistors.
It seems an aging problem (the trimmers were still sealed, so no
tampering), but who's bad ?


As it happens I will try fathoming a dedicated SMPS today that as it stands
is in tick-tick mode, 1 ultrasonic cycle per second.
Putting a dropped LED on each output shows life on all rails.
Plan of campaign will be
Run up on variac to find lowest point and will still run of sorts, tick-tick
Put a x10 larger timing capacitor over the one on the 555 that feeds the
UC3842 PWM controller. Then make up an exerciser to feed into the ,isolated,
LED of the optoisolator, over-riding the error feed. This will be a
monostable of variable period from 1mS to a second or so with push switch,
hopefully to elicit which area is playing up by gradually ramping up .
All active, big Rs seem ok and caps ESR-check ok. I suspect false error
signal.
Anyone else any ideas/advice on a generic plan of action for these sorts of
situations.?