Switching PSU Theory of Operation / Repair Tips & Techniques

From: Geoff Roberts <geoffrob_at_stmarks.pp.catholic.edu.au>
Date: Mon Jul 24 19:28:41 2000

----- Original Message -----
From: "Douglas Quebbeman" <dhquebbeman_at_theestopinalgroup.com>
To: <classiccmp_at_classiccmp.org>
Sent: Tuesday, July 25, 2000 1:25 AM
Subject: Switching PSU Theory of Operation / Repair Tips & Techniques

> So, I'd like to eventually repair it. It's a monster, and
> potentially lethal. From poking around with a voltmeter,
> I can see that it chops the AC to DC and doubles the voltage
> to about 280 volts, one positive supply, one negative supply.

Ok, so it's a switch mode power supply that produces +280v and -280v
(wrtg) rails right?
Is this used as a rail to feed low voltage supplies, or are they
generated in the main supply.
Or do you REALLY mean that it produces a postive and a negative with
280v across them?
That is what I would expect to see.

> From there, I'm lost. Has anyone every written up a general
> description of troubleshooting and repair techniques?

Not that I know of. Others may know of something other than the usual
service manuals.


A circuit diagram would be useful here, but psu's aren't THAT
complicated really. Often the type of fault will indicate where the
problem lies.
I take it the 280v rails you mention are still present in it's non
functional condition?
That suggests that the mains rectifier is still functional, they don't
'chop' the ac mains.
Switch mode supplies generally rectify and filter the mains, and use the
resultant 250-350vdc to drive
the rest of it. If you have no low voltage stuff running, and no blown
fuses, it could be as simple as
a startup resistor being open circuit. Look for a high value (~300k -
1Meg) resistor from the + side
of the dc rail to the electronics of the switch mode supply. These go
open circuit at switch on fairly often,
and the supply doesn't get the initial starting pulse it needs.

These supplies are filtered by one or more largish reservoir capacitors
that can and do store a potentially dangerous amount of power, they are
DANGEROUS even when the unit is off and unplugged until they are
discharged. They may or may not have a bleed resistor across them. I
strongly recommend that you check with a meter after about a half hour
with it switched off to see if they have discharged before you stick
fingers in there or you might get a nasty surprise.

If you identify a likely resistor, you will find that you get ambiguous
readings unless you remove it from the circuit. Open circuit startup
resistors are a common fault in switch mode supplies.

> possible for devices that vary in design not only from
> manufacturer to manufacturer, but wven model to model?

Never fixed TV sets have you. :^) Emphatically yes.

Hope this helps.


Geoff Roberts
Computer Systems Manager
Saint Mark's College
Port Pirie,
South Australia
ICQ: 1970476
Received on Mon Jul 24 2000 - 19:28:41 BST

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