Testing Power Supplies! Re: Norsk Data Nord-10/S restoration

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Mon Nov 15 18:03:52 2004

>
> Testing power supplies before powering up a system is just plain
> paranoid! (And I'm trying to be polite here!) There are a thousand parts

Nad anyone who doesn't do so is just plain clueless (and I am being
_very_ poilite) here. I've seen (and had to repair) the damage caused by
a wayward PSU...

Some PSUs have no overvoltage protection at all. Top of that list is the
HP9815, 9825, 9831, 9877, etc. If the 5V chopper shorts in one of those
(and it does quite often) you can bet you'll be replacing every chip in
the machine, just about. The 9815 is a particularly nasty one since the
PSU can't be disconnecte from everything else, but at least you can
remove the CPU and kayboard/display PCBs, the tape drive, and unplug the
printhead (which is a custom hybrid circuit). Then all that dies if the
PSU is faulty are a few TTL chips on the PSU/interface board.

Next on the list are the Bozchert 2-stage switchers. These have a
non-isolated switching regulator to bring the 350V (rectified mains) down
to about 150V, which then feeds a free-running oscillator driving the
chopper transformer. Regulation is applied to the first regulator stage.
If that first chopper shorts, the output rails all jump to about double
what they should be. You hope the crowbar fires, but then the overcurrent
trip on the primary side doesn't do anything (it works on the first
regulator, which is shorted), so you hope that the fuse or the second
choppers fail befor the crowbar SCR. Normally they do, but...

And I am sure I've come across a device (I don't think it was a computer
per se) where the PSU outputs lept to the unregulated input voltages
(less the saturation voltage of the pass transsitors) if the PSU control
PCB was removed. You're talking about a second-hand machine here, no idea
what board have been 'borrowed'...

> in modern computers that can cause just as much damage!!!!!!!! Just

Name one other failure that will take out every chip in the machine.

> location and every register. (Anybody have schematics for the HP
> F-series CPU?)

Trace them out yourself. I've done it for lots of HP devices...

> happened! The PSU started making a loud sizzling sound and poured out
> copious smoke! All that testing and prior running gave no hint of the
> impending failure! What's more, this is the SECOND time that I've had a
> HP 1000 PSU fail after running for over an hour. FWIW I pulled the PSU

That sounds like a PSU that needs to be run on a soak test fro quite a
time, then...

> out of another untested machine, put it in this one (without testing
> it!) and everythings working again.
>
> IF I had an absolutely unrepairable, irreplacceable computer and IF

IMHO _all_ classic computers are irreplaceable!

> I had all the PSU specs and pinouts it MIGHT justify the trouble of

Powering up machines without having read and understood the technical
manual/schematics is a second moan of mine, but...
 
> testing a PSU before using it to power up a computer but otherwise
> forget it!

It takes about 5 minutes to test a PUS on dummy load. No longer. It's
_always_ worth doing.

-tony
Received on Mon Nov 15 2004 - 18:03:52 GMT