Subject: reforming PS caps w/primary-side components

From: Bill Yakowenko <[email-masked]>
Date: Mon Jun 2 22:17:00 2003

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.

---

From: "Mike van Bokhoven" <[email-masked]>
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.

---

From: Ethan Dicks <[email-masked]>
Date: Tue Jun 3 10:52:00 2003

--- Mike van Bokhoven <[email-masked]> wrote:
> ...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'.

I had an SMT filter cap blow in a 200MB SCSI drive... it was surprisingly
loud.  I powered on the PC, heard the PSU click, then, before I could shut
it off... *BANG*.  The machine proceded to power up as if everything was
normal, including the cloud of smoke hanging over it.

-ethan

---

From: [email-masked] (Tony Duell)
Date: Tue Jun 3 14:02:00 2003

> to reform instead of, say, exploding in my face.

Light bulbs will limit the maximum mains current in the event of a 
catestrophic failure on the mains side of the PSU (this is particularly 
valuable when repairing SMPSUs), but I doubt they'll have much effect 
here. You really need to remoce the capacitors and connect them in series 
with a fairly high value resistor across an adjustable DC power supply. 
And then increase the voltage while keeping the current fairly low until 
the capacitor will stand the rated working voltage.

> caps themselves on the secondary side of the

Yes

> getting less voltage than they were designed for?

Switch-mode PSUs for one. These things appear as a constant _power_ load, 
and draw more current as the input voltage decreases. If you're unlucky, 
this will kill the chopper transistor, etc.

> the beam hit something it shouldn't?  Anything

CRts are an interesting case. A low supply voltage will almost certainly 
result in a low EHT, which will mean a less stiff electron beam. So the 
deflection angle might actually increase!. However, I think the main risk 
would be damage to the cathode by trying to draw too high a beam current 
from it while it's too cold (the heater voltage would be low too).

What are the devices in question? My experience suggests that most stuff 
from the late 1970s and 1980s is fine -- the capacitors are still good 
(OK, they'll be the odd failure, but..).  This particularly applies to 
computers and test gear which used good quality components in the first 
place. Older, valved, stuff, needs to have the capacitors removed, 
checked and re-formed. More modern stuff, particularly consumer-grade 
stuff, may well have capacitor problems too.

Incidentally, all my HP desktop calculators, my Tekky 555 'scope, and 
most of my minicomputers still all the original capacitors!

-tony

---

From: Vintage Computer Festival <[email-masked]>
Date: Tue Jun 3 18:46:00 2003

> startup - what is it with Apple and AC filter capacitors? Both the machine

I've never had an Apple ][ power supply do this.

For what that's worth.

-- 

Sellam Ismail                                        Vintage Computer Festival
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