Switching PSU Theory of Operation / Repair Tips & Techniques

From: Douglas Quebbeman <dhquebbeman_at_theestopinalgroup.com>
Date: Thu Jul 27 12:37:07 2000

> > If we have a 300vdc source, positive with respect to ground, and we
> > have a 300vdc source, negative with respect to ground, don't we have
> > a 600vdc potential between the two sources (as long as they share a
> > common ground)???
>
> Absolutely. All voltages are relative to another point, and 2 300V
> sources in series as you describe would, indeed, give you 600V.
>
> Bu what puzzles me, as I'll explain below, is where the 300V
> sources are coming from.

Nowhere.

Ok, to make a long story short, I'm now seeing precisely the
voltages you say I should see. With nothing connected to the
outputs of the rectifer, I get 105vdc across the + & -.

With the rectifer reconnected and the electrolytics back in
place, I get just a hair under 160vdc between the caps'
terminals, and just a hair under 320vdc measuring between
what we're calling the +ve and -ve terminals.

So, either I did in fact misread the meter, or...

Well, the entire large PCB containing the secondaries
is completely disconnected, removed, and set aside.
In addition to the cable carrying +ve and -ve to that
board, there are a few (two? I should have written that
down) other cables that connect the two boards. So my
only other theory is that some of what's on that board
is involved in some kind of feedback process with the
primary to step it up even higher.

My original measurements were done with the entire
PSU assembled and connected, except that it was not
installed in the backplane (and thus had no load on
it). It's just not going to be possible to debug it
while installed in the backplane. Not in this Prime.

I still intend on shooting some shots of this stuff,
but yesterday someone ran off with the PCMCIA adapter
for the digital film chips, as well as the only chip
that didn't have important pics on it, so that has
yet to take place.

> > The components that I've shown as {MCI 307138} are what appear to be
> > either large disk ceramic caps, or, judging by appearance, more likely
> > MOVs. The one on the + side is discolored and cracked open along its
>
> They're thermistors. Their resistance falls when they warm up. The idea
> is to limit the current that flows into the PSU via the bridge rectifier
> when the supply is turned on and the capacitors are totally discharged.
> Once they've charged up a bit, then the thermistors warm up, and their
> resistance falls.

Are you sure? My Alled Electronics catalog has thermistors, but only a
few in the catalog is as large as these are- about 1 inch diameter.

With everything disconnected, the on in the + circuit has
1.2 ohms resistance, the one in the - curcuit has 1.0 ohms
resistance.

Can you help me cross-reference that part? Any idea what
manufacturer uses the initials "MCI"?

Additionally, the Allied catalog doesn't carry any of those
old-styled power resistors (the long, square-profile ceramic
things, this one with PCB-standoff feet)... in the needed
range, all they have are the newer jobs that come with a
built-in heat sink. It was 15k ohms, 5%, 20 watts, made by
Dale. I can get the exact part # from it tonight if needed.

Wel, that's all I got at the moment. Boy, this sure is fun!

regards all,
-doug quebbeman
Received on Thu Jul 27 2000 - 12:37:07 BST