Dead PSU

From: <(>
Date: Sun Jul 5 21:23:40 1998

> Date: Mon, 6 Jul 1998 00:52:46 +0100 (BST)
> Reply-to:
> From: (Tony Duell)
> To: "Discussion re-collecting of classic computers" <>
> Subject: Re: Dead PSU

> >
> >
> > I'm a novice at soldering - I've made at most 10 connections in my life.
> Well, then, changing all those capacitors is good practice :-)

Nodding... True...Max, practise on junkers first.
Oh, bit about the fan jump a bit with power removed. I'm 100%
certain something is shorted in the secondary outputs and by the way
most of time usually has none of scr or clamp downs to block
overvoltage, the PWM ic usually '494 and LM339 monitors the output

Remove the main transformer and check all diodes for dead shorts and
leaky capacitors ONLY in the output side, about 6 or 8 diodes and few
large 470uF or 330uf or so in 12V part and larger ones for the 5V

The fan jumping was the switcher ic trying to spin the transformer
into one of the short circuit but switcher also have self-protection
circuits kept it from spinning until main power is removed.

Don't be fooled, at least 2 diode pairs are in two "transistors"
casings, but there's markings that looks like two diodes back to
back. Both of those "trannies" are always mounted on heatsink. One
is large for 5V and one looks like TO-220 case for 12V and few
discrate diodes for negative voltages rated at very low currents,
sometimes taps off the -12VDC via a regulator on tiny heatsink of all
itself to drive the -5VDC. I'm certain this PSU of yours has no
"Power Good" wire, that is controlled by same LM339 that monitors all
4 voltages or usually 2 voltage levels and come up on power good wire
in delayed fashion after all the voltages is up.

> Seriously, Please, Please read the FAQs before
> you get started. That's . There are
> dangerous voltages in there. At least read the safety notes and the notes
> on SMPSUs.

Excellent beginner info to surf. Go there. :)
But this does not beat the info if you really trace out where all the
outputs out on that PSU and pick out most likely components to check
with your meter and catch the baddie red-handed. Pure joy when that
happens! :)

Before I got the ESR, I had to play the game of who's is bad on
either caps or semis or both. Sometimes I just go out and swap all
the outputs caps right there and there. :) But now that I got ESR
meter, the worry is over and this is a MUST to have because
the cost is so cheap. Get one from "Bob Parker's" DSE's ESR meter
kit from under and to the left without a second thought.

> > And, there is the looking for replacements, possibly buying them.
> Which takes the same time for 20 as for 1 :-). I sometimes forget that we're
> lucky in the UK in that there are a number of companies who sell
> components across the counter. Here there's a Maplin about 30 minutes
> away by public transport, another one about an hour away, Grandata (TV
> spares) aren't too far, etc. And having bought components many times in
> the past I know where to look for them.

Bragging rights/Boasting aside, I had to use my friends contacts to
obtain very specific components and rely heavily on subs. :-(
Boo hiss. :)

> -tony

Jason D.
Jason D.
Received on Sun Jul 05 1998 - 21:23:40 BST

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