reforming PS caps w/primary-side components

From: Mike van Bokhoven <mike_at_ambientdesign.com>
Date: Tue Jun 3 08:10:41 2003

Hi Bill (and others...)

This is just my opinion, but... the way I'd do this is remove the caps, and
charge them gradually externally. They should be marked with a maximum
voltage; I'd work my way up to that gradually, using a series resistor
(maybe a light bulb, probably something with higher resistance though) to
allow it to slowly approach peak voltage each time. e.g. for a 100V cap, I
might gradually charge it to 20V first, then 50, then 75, then 100.

Of course, to do that you'd probably have to remove the caps, which might
not be easy. But at least if they do go BANG, they won't do any damage to
the machine they're from. I've seen how much damage an electrolytic
capacitor 'can' does to items it collides with, when the cap suffers sudden
catastrophic failure.

Is it an option to just replace the old caps with new ones?

On a side-note - I just powered up two unknown-condition Apple //es and a
//e monitor. One of the //es and the monitor went BANG and let out smoke on
startup - what is it with Apple and AC filter capacitors? Both the machine
and the monitor continued to run fine after the filter caps died, of course.
I probably should have been a bit more careful about starting the machines
up, but generally I find that on that sort of gear, the only caps that
suffer catastrophic failure are those filter caps, and they do no damage and
are easy to replace. So my filter cap troubleshooting procedure has degraded
to 'plug it in, if the caps go bang, they're bad and need to be replaced'.
What fun!


Mike.



----- Original Message -----
From: "Bill Yakowenko" <yakowenk_at_yahoo.com>
To: <cctech_at_classiccmp.org>
Sent: Tuesday, June 03, 2003 3:16 PM
Subject: reforming PS caps w/primary-side components


> Anybody want to offer advice on reforming filter
> caps? I'm planning to power up some stuff that
> hasn't seen voltage in a long time. My plan is
> to put several low-wattage light-bulbs in series
> with the thing, to limit the amount of current it
> will get, and hopefully allow those old caps
> to reform instead of, say, exploding in my face.
>
> It would be better to put the light bulbs (or
> whatever kind of resistor) in series with the
> caps themselves on the secondary side of the
> transformer, right? But if I start digging into
> the thing's innards like that, there is a real
> risk that I'll damage it more than an exploding
> cap would. (In one item, there is a picture
> tube nearby, and I'm sometimes clumsy...)
>
> Now the questions:
>
> Could putting the light-bulbs (or whatever) in
> series on the primary side of the transformer
> actually produce the desired effect? IIRC, a
> step-down transformer divides the voltage but
> multiplies the current. So I'd have to limit the
> current on the primary side that much more to
> keep the secondary current down, right?
>
> Also, what kinds of things could be damaged by
> getting less voltage than they were designed for?
> I could imagine hard disks spinning too little to
> lift the heads from the surface, for instance.
> (This is moot, because there are no hard disks
> in any of the equipment I'm looking at.) How
> about CRT's? Could too little deflection make
> the beam hit something it shouldn't? Anything
> else?
>
> Thanks,
> Bill.
Received on Tue Jun 03 2003 - 08:10:41 BST