"Geo" wrote in message
...
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. 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, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

I don't see that a fluctuating supply would cause a polarity reversal
on the -15v rail.


No, you're right, it wouldn't, which is why Franc and I elsewhere in the
thread, have now entered into the discussion of how exactly the OP is taking
his measurements i.e. what point he is using as his ground, for a start.

Arfa