"sci.electronics.repair" wrote in message
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On Sep 21, 8:08 pm, "Arfa Daily" wrote:
"senator richards" wrote in message

...

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.

Randy

The cap that filters the supply to the pwm chip on the primary side,
maybe ?
It's pretty common on most designs of switcher. Work on the thing on an
isolation transformer if at all possible. They are potentially very very
dangerous if you are not fully competent with them

Arfa

Thanks for the help. I borrowed an ESR meter and checked the cap that
supplies the pwm chip. The cap is a 47uf 50v and esr measures .5 which
appears to be about right.

I don't have an isolation transformer but will look into getting one.

Thanks for your help.

-R

A brand new cap might be a little better than that, but certainly right ball
park at 0.5 ohms, and would not be an issue at that figure. About the only
other thing that you could try, assuming that it uses a startup resistor
from the 170v, is to disconnect the self-feed diode from the switching
transformer. That way, you will force it to run from the startup supply
only, just in case the self-run voltage is fluctuating, and interfering with
your reading on the pwm chip's supply. Bear in mind though, that you can't
run it for too long on the startup resistor, as it will get quite hot, being
normally intended to supply current to the chip, only for as long as it
takes the supply to fully start up and settle. If the voltage supply to the
chip still jiggles around when it is only being fed by the resistor, then
after the 47uF cap, which may yet be faulty, but not in an ESR way, the next
prime suspect would have to be the chip itself.

Arfa