reforming capacitor tutorial/walkthrough

Tim Shoppa shoppa_classiccmp at
Mon Oct 8 15:46:19 CDT 2007

Al Kossow <aek at> wrote:
> "William Donzelli" <wdonzelli at> wrote:
>  > The caps from 1960s seem to be of little concern, and those from past
>  > 1970 are of no concern.  If the cap decides to die, its going to die.
> This was not the experience of the PDP-1 team during the restoration of
> that machine. Detailed records were kept of every cap that was reformed.
> This process took several months to perform.
> Blowing up a cap in the machine during restoration was not an option.
> Lyle, or one of the other hardware folks would know the details.
> I am disappointed at the 'let it just blow up' attitude that has been
> expressed on this list so far in the discussion.

Was that the PDP-1 at the Capacitor History Museum, Al? :-)

I am certainly the guilty party in advocating the position that
equipment expected to operate under adverse condition should be
margined to available extremes during the PM cycle. At my day
job margining the logic (be it relay, solid-state, SSI, or microprocessor)
up and down also happens to take care of
margining the capacitors up and down too. Margining isn't a procedure
to preserve components, it's a procedure to make sure that equipment
will work to specification.

That's not to say that margining has a goal of blowing bad parts up, either.
After 30 years we rewrote one margining procedure for track circuits
after we had a loss-of-shunt and two trains nearly collided at rush
hour in a tunnel under the Potomac. We found out that the procedure
could result in adjustment to pass the margining but not be safe. So
margining is to be done with thought and care towards important
function, not just to blow the bad parts up!

I don't say that babying the tenderest components cannot yield a working
result, just that it's not my personal style nor the standards of
the industries I've worked in, and being responsible
for the logic that runs a railroad 24x7 and has to run
even when it's -20F or 110F, it's more likely than possible that
my experience doesn't apply to something in a museum environment.


More information about the cctalk mailing list