Switch-Mode PSU Failure Modes, Repairs, & Parts Substitutions

From: Douglas Quebbeman <dhquebbeman_at_theestopinalgroup.com>
Date: Mon Jul 31 09:57:55 2000

Good Morning,

I hope everyone had a great weekend, I sure did (not
Classic-Computer related so I won't bore anyone with
the details)...

Subsequent to my first post about my Prime PSU, Don
Maslin sent me a howto on repairing switchers, that
was written by Keith Lofstrom. The howto is fairly
comprehensive, but does assume that most repairs
are taking place due to bad PSU design.

I'm not sure mine failed due to bad design. It was
dusty, and after I removed it, I found that the
2455 cabinet has an interestingly-design set of
airflow guides that's above the cage; it's basically
a sheet of thick plastic with holes in it, so that
the dual-squirrel-cage blower dumps air into the
cage through these holes. While there was some
airflow, once the PSU was out, I could see that
many of the holes were clogged up with dust and
lint, and thus I think the PSU simply overheated
inside the cage.

One additional data point that I thought I'd
mentioned, but may not have, is that: The system
was up and running fine, no evidence of any problems.
I shut the system down that night, and then the next
day when I turned it on, I got nothing. I did not
see any evidence that the PSU briefly came on and
then died; it simply never came on.

>From what I've gathered from the comments posted
about switcher operation, and from Keith's howto,
it seems like something failed that is part of
getting the secondary supplies "jump-started" to
an up-and-running state. So, as long as it was
powered up, it would continue to operate. But once
I turned it off, the portion of the PSU (something
between the primary and secondary supplies? Something
like that big power resistor, maybe?) that gets the
secondaries going, couldn't get them going.

So, has anyone else been in a similar position? Do
I have a fair chance of simply replacing the parts
I've found that failed, reinstalling it, and being
successful? There is a 5v adjustment that needs to
be made before bringing the system up all the way,
and while I hate to use the system itself as a dummy
load, I don't have the Prime bus pinout, so I can't
easily determine what's what.

One of the things Keith discusses in the howto is
about parts substitutions. Most of his comments are
w/r/t transistors and the like, and he suggests
using as replacements devices that have voltage and
current capacities that are increased over the part
that failed. He goes on to say you need to test each
such substition with a curve tracer before finalizing
each such substition.

However, I don't think any semiconductors failed for
me. He says that in a good design, there are some
power resistors protecting the switcher transistors,
and from my examination to date, it appears that Prime
did in fact put these in the design. I'm going to remove
the other 3 of the 4 that are in that area to be sure
whether any did or did not go open-circuit, as there
is significant thermal discoloration of the underside
of the PCB in that area.

How reasonable a course of action is this? What's likely
to happen should a problem still be present? I'm thinking
that worst case, the parts that failed will just quickly
fail again, putting me back at square one.

As always, thanks in advance for any help y'all can give me!

-doug quebbeman
Received on Mon Jul 31 2000 - 09:57:55 BST

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