replacing +15v regulator on BA-11?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Thu Mar 25 22:38:53 2004

>
>
> Tony Duell wrote:
> >Please don't module-swap classic
> >computers (this rant has been here before, you won't change my views on
> >it, I've seen the unpleasant reuslts all too often!).
>
> Interesting point. And the first time I've heard it. I'll check the
> archives... I'm don't disagree.
>
> (having said that, I am fan of having spares, getting the unit up and
> then going back and fixing the problem; I'd be curious to hear if that
> is seen as bad form or bad idea).

There are many reasons why module swapping is a bad idea. The main one is
that in a lot of cases you don't _know_ what the fault was, so you don't
know you've fixed it. This particularly applies to intermittant faults.

Then of course there's the issue of what happens if the module was
damaged by a fault elsewhere in the computer (e.g. a short circuit on one
part is killing a PSU regulator under particular circumstances).

And the issue of revision levels (do you know the replacement module will
work correctly with all others in the machine?) (Spent hours tracing a
'fault' caused by this -- it turned out that the later revision module
needed a couple of extra wires on the backplane...)

Perhaps the most nasty situation is that : You have some signal between
Module A and Module B. There's a fault on A, so that the timing of this
signal gets out of spec. For some reason you replace B first, and the
replacement is more tolerant of bad timing. The machine works again. But
the timing continues to drift, and eventually the machine fails again. I
have actually seen this one happen.

> I've had good luck swapping H744's and fixing them later, replenishing my
> "spares" box ;-)

Thing is, they may not end up in the machine they were originally used in
(this is certainly the case round here, I've got half a dozen machines
using that brick!) and thus you've changed more than you have to in that
machine...

> >Asjustments rarely, if ever, drift. If some voltage is out of spec then
> >there's a reason. If it's something you did -- like adding an extra load
> >-- then yes, you use the adjustment. But if it's just gone down while the
> >machine is in use, you need to find out why. A dried up capacitor in the
> >brick probably.
>
> Excellent points. I don't know if the voltages have changed; I just
> noticed that when I first checked I only got 13v. Probably due to dry
> caps, as you pointed out.

What I'd do here is tweak it back to 15V (Assuming this is possible and
that the preset is not hard against the stop) and then keep an eye on
that voltage. If it starts getting low again, you know there's a problem..

>
> (time for that ESR meter :-)

Indeed.

-tony
Received on Thu Mar 25 2004 - 22:38:53 GMT