On 9/23/2011 7:22 PM, Jamie wrote:
Jim Yanik wrote:

"Arfa Daily" wrote in
:


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

On Sep 21, 8:08 pm, "Arfa Daily" wrote:

"senator richards" wrote in message


m...

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.


IIRC,the values on the DSE meter are MAX values,so IMO,if a cap is
reading what's listed,it's marginal.


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



usually,the startup resistor doesn't supply enough current to keep the
IC running,just enough to start,that's why they have the housekeeping
supply from the XFMR. Otherwise,you get a "burst" or "chirp" mode of
operation;the PS starts,then dies because the IC isn't getting enough
power.
Often,it's the housekeeping supply electrolytic cap that's gone bad,as
you said.
I've also seen where the start resistor goes high in value,even
open,preventing starting.(but not this guy's problem)

Talking about switching supplies. Today I decided to look at a supply
removed from an allen Bradley Panel computer, this is the second time
the supply has quick, we were able to buy exact replacement supplies so it
was cheaper to do that at the time.

This time I decided to investigate the issue on one of the broken
supplies. I found the main cap on the HV side to be shorted, it was
something like 230Uf 450VDC. This cap did not cause any components to
open that I could see other than a varistor in series to it for the
inrush currents. Any way, I could not get that damn cap out of the
board, I don't know what type of solder they used on that cap but the
only thing I had that would melt the solder was a 140 watt soldering
pistol, and that didn't do me a lot of good since I needed to get to the
other side of it.

My regular station set to 850F would not melt this solder but items
around it had no issues breaking down with that temp.. I guess If I
really want to fix this thing, I will have to use my air iron and some
added heat with another iron on the area to blow the solder out the back
side.

Oh well.

Jamie


I find adding fresh solder to difficult melts and sucking it out
repetitively eventually will fill the joint with regular solder. Of
course if theres a heavy plane of copper, its a problem.
JC