"Kripton" wrote in message ...
On 2011-09-23 02:56:37 +0200, "Arfa Daily" said:



"sci.electronics.repair" wrote in message
...
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

I've never seen a chip 3842 dead...
but the small cap 10uF beside it has really often been solving the case
!!!!!

--
----------
Kripton


No ? There are a number of chips in the 35 and 38 series pwm controllers,
which are all pretty similar in function, and whilst I'm not certain that I
have specifically had a 3842 faulty, I have certainly replaced many of those
series over the years, usually for a dead symptom, due to the internal
voltage reference having failed. I agree that the caps decoupling the supply
and reference voltages are by far the commonest problem though, and that was
what I first suggested to the OP as a possibility for his problem. The
symptoms that he has, are pretty weird for a switcher. I, like others on
here, would initially have suspected the filter cap for the rail that the
regulation feedback is derived from, but the fact that he replaced all of
the secondary caps right off, seems to knock that one on the head.

Arfa