17 May
2011
17 May
'11
6:11 a.m.
On Mon, May 16, 2011 at 8:08 AM, Michael Thompson
<michael.99.thompson at gmail.com> wrote:
One thing that puzzles me is that all these things seem to imply
powering the system's own power supply with a Variac rather than
bypassing it. Wouldn't it be possible to have connect a linear
transformer with the proper output voltages to a backplane and then
power that with the Variac to reform the caps only switching back to
the stock supply once the caps have been reformed?
Mike
From:?Geoffrey
Reed <geoffr at zipcon.net>
For general use on old personal computer systems, S100, Kaypro, etc....
Would a 5A variac be sufficient?
And is there any write ups anywhere on how to use the variac when powering
up a system that hasn?t been powered up in > 10 years?
There is lots of information available on reforming capactors. Some of
it is useful.
This is good introduction: http://www.nmr.mgh.harvard.edu/~reese/electrolytics/
A Variac can be used to reform caps in a linear power supply. Linear
power supplies were common in older computers. The RICM recently used
a Variac to reform capacitors during the resurection of a PDP-8/S. The
power supplies in the 8/S were linear ferroresonant design, common in
DEC equipment. With these power supplies the voltage on the capacitors
will slowly increase in relation to the Variac output until you reach
about 45 VAC. At that point the ferroresonant circuit kicks in and the
output goes to full voltage. The big electrolytic filter caps survived
this, but eventually the AC capacitors in the ferroresonant circuit
failed. Replacements for those are easy to find.
Newer systems, like the Kaypro, probably have a switching power
supply. Switching power supplies usually will not have any outout
until the Variac reaches the minimum acceptable input voltage for the
power supply, usually something like 85-90 VAC. At that point the
power supply turns on and tries to make the full output voltage. That
can be really tough on old electrolytic capacitors.
Eric Smith, as part of the PDP-1 restoration project at the CHM,
developed software to control an Agilent power supply and very
carefully reform capacitors. This is probably the best solution.
http://www.brouhaha.com/~eric/software/wrec/
--
Michael Thompson