On Wed, 21 Sep 2011 13:31:46 -0700 (PDT), senator richards
wrote:

I am trying to troubleshoot a small SMPS that came from an A/V
switcher.

A photo of the PS would be helpful. Mostly, I'm looking for what
manner of regulator chip is being used and whether there's a
transformer or optoisolator involved.

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.

If you have +30VDC on an output circuit that is suppose to only
tolerate +15VDC, it's possible that the output caps are now dead,
especially if this PS has been run for quite some time. The output
caps are probably rated at 25WV, which doesn't apprecitate the 30V (or
more if there are voltage spikes on the output).

My experience with SMPS has
usually been shorted rectifiers or bad output filter caps

Yeah, that's the usual problem.

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,

Fluctuations like that are sometimes arcing in the xformer or spurious
oscillations in the regulator chip. High output voltage usually means
a failure in whatever is regulating or sensing the output voltage. As
others have suggested, get an isolation xformer before you kill
yourself. Then, attach a scope probe to the output lines, regulator
pins, and optoisolator, and see what's causing the fluctuations.
That's not normal.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558