I have (7) Switching Systems Inc SQM150-1433-6 (L3) Power supplies
5v,24v, +/- 12v that have all failed as a result of the Balck-out in
the NorthEast...... I was able to easily fix two by replacing faulty
caps. The other five have proven to be much more challenging. I was
hoping with these that I would just change the bad caps as well and
all would be good.... Wrong....
What happens is when power is applied, they will fire off for
anywhere from 5 seconds to a minute (Voltages are all dead-on and
clean) and then shut down. I found that a thermistor connected through
a current limiting resistor to the collector of a RCA9202C NPN is
heating up and going open which in-turn shuts down the supply as it's
designed to do. I measured the current through the thermistor PTC C884
and it's ~ 70 ma (@330v) and from what I can find out, the thermistor
is rated for ~30 ma.
I originally thought maybe the thermistors are bad but from what
I've read they rarely go bad and then once I measured the current it's
obvious there doing what they were designed to do and they don't
appear defective.
I have gone through all the devices with a Huntron tester and have
found nothing obvious wrong and have also checked and rechecked
all the caps for both ESR and DCR and still no luck.
Out of frustration, I separated the supply in half electrically by
removing the coupling transformer so now I have the high voltage
section completely isolated from the low voltage section. Even with
this done, the thermistor still overheats and shuts down. Arghhhhh....
Well, at least I'm pretty sure the back-end of the supply is not an
issue......
The RCA9202C is no longer made along with many other parts used in
the supply but I was able to find and SK part that was a sub so I
replaced it. Still No Good....... Same exact behavior.... I had
checked the SK part against the RCA 9202C on the Huntron and their
signatures appeared to be the same so I had a feeling I was going to
strike out but I think it was worth a shot......There are 3 diodes
associated with the 9202 and are marked DT RP158 (x2)
and the other is marked C160304. Again, the signatures don't show
anything that I consider unusual but I have heard some devices will
test fine and fail in actual use and maybe that's what is happening
here. I suspect that these devices are either house marked devices or
have long been discontinued as I can't find any details on them. If I
could replace these devices, I surely would.
Each one of these supplies is different internally . Some use a NE
556 as a controller, some use a custom circuit on a dip header, and
the one I've been focusing on uses a NE5560 switched-mode power supply
circuit. I downloaded the data sheet on this which has been some help
but I'm still at a loss. SSI is out of business (not that they would
help me anyway) and we could get another brand supply but mounting
constraints would make that a bear.....

I'm determined to fix these as if I can do one, they're all
fixed..... But it's been a rough ride... I have read Sam Goldwassers
excellent repair FAQ as well as info on several other sites w/o any
luck so I figured I'd try here.....Just maybe, someone here has worked
on a similar supply and maybe can shed some light on what's going on.
Thanks for any help or insight you may offer.... Bill