Switched power supplies

From: Tony Duell <ard_at_odin.phy.bris.ac.uk>
Date: Mon Oct 6 11:04:47 1997

On Sun, 5 Oct 1997, Marvin wrote:
> > I have several switched power supplies from IBM clones that I
> > would like
> > to try to repair, can anyone suggest a source of a book or schematics for
> > these things?
>
> Take a look at the IBM tech reference manual as one of them (don't recall
> which one, XT?) does has schematics for theirs. Also as far as trying to

Are you sure? _None_ of my IBM TechRefs (I have : PC, XT, AT (2 vols), O&A
(2 vols), Scientific, EGA, PC-jr) have any PSU schematics at all. Well,
the PC-jr PSU card is there, as is the schematic of the professional
monitor PSU (the CGA monitor PSU is missing...).

Horrowitz and Hill 'The Art of Electronics' has the Tandy 2000 PSU
schematic in the 2nd edition (silver cover), along with a circuit
description.

The best source I've found for SMPS operation/repair is 'Television'
magazine. This is a UK magazine for TV service engineers, and has a lot of
useful hints. Most SMPS's work the same way, so if you can fix the one in
a TV, you can do computer ones as well.

> repair them, the common failures I have found are the switching transistors
> going bad and the 10w or so power resistor opening up. The rest I haven't
> taken the time to fix :).

When the chopper fails, it's worth looking for other faults that caused
that. Things like :

Snubber capacitor gone O/C (VT100's are famous for this IMHO)
Shorted turns in the transformer. A 'ringing' tester is useful here.
Base resistor gone high, or its parallel capacitor gone O/C
Collector current similation resistor gone high (TDA4600-based PSUs are
famous for this - and they do turn up in some computers, like the Tatung
Einstein).

Other common faults are :
Mains rectifier diodes gone O/C or S/C (in the latter case the fuse will
blow)
Startup components (resistors/capacitors) failed (PSU won't start, but if
you can get it going, it will keep running)
Output capacitors gone O/C or with high ESR. This causes spikes (of a few
volts) on the output lines, which generally trips the crowbar. I've had
this one several times.

Boschert 2-stage PSUs have an unpleasant and common failure mode that
kills all 3 choppers, the control IC, several small transistors, some
passives, and the PCB tracks. I can provide details if anyone's
interested.

>
>

-tony
Received on Mon Oct 06 1997 - 11:04:47 BST