On Wed, 21 Sep 2011 13:31:46 -0700 (PDT), senator richards
put finger to keyboard and composed:

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem ...

Just to add to what Arfa said, the supply to the PWM chip often starts
with a simple resistor feed from the +170V rectfied mains voltage, and
is then augmented by a regenerated supply from a tap on the switchmode
transformer. If this is the case, then there will be a small
electrolytic filter capacitor (~10uF) that often develops high ESR.

- Franc Zabkar
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