21 Jan
2012
21 Jan
'12
7:46 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org
[mailto:cctalk-bounces at classiccmp.org] On Behalf Of Eric Smith
Sent: 21 January 2012 19:01
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: PDP-8/I at the RICM
On 01/20/2012 01:47 PM, Tony Duell wrote:
This really suprises me. I guess that the caps Radio Hams use may run at
higher voltages than those in vintage PSUs but the process has as I was
taught it was always to ramp up the voltage whilst checking the leakage.
Some interesting circuits here:-
http://www.vmars.org.uk/capacitor_reforming.htm
I was under the impression tha the oxide
dielectric film was
originally produced by a process that's much the same as
the reforming
process.
It is.
So I wonder why the reformed capactiros
didn't last. Was theres
somethign else wrogn with them?
I would think so.
If you do the reforming by simply throwing the capacitor on a
current-limited supply and not measuring anything (as seems to be the
usual method), then a completely defective capacitor may improve a
little bit but still be fundamentally broken, and still fail either
immediately or shortly after being put into service.
The process we used on the PDP-1 project was to start
with the power
supply set to 0.5V, watch the current until it dropped below
specified
leakage, ramp to 1.0V, watch the leakage, and so on up to the rated
voltage. At each step the current limit of the power supply
is set such
that the product of the voltage and the current limit were
less than the
maximum rated power dissipation of the capacitor. (I think there may
have been an additional constraint to avoid having unreasonably high
current limit settings at low voltages, but I don't recall
the details
of that.) Due to the current limit, for a capacitor that needs
reformation the power supply will be in current limiting, with the
voltage slowly ramping to the voltage setting as the reformation
proceeds. If the process takes more than a few hours at a
voltage step,
the capacitor is probably not going to reform, and should be replaced.
If you don't do at least some monitoring of the process (not
necessarily
as thorough as we did), you won't really know whether the
reformation is
successful.
In the PDP-1 restoration, we encountered a few capacitors that had
obviously gone bad and did not successfully complete the restoration
process. Most of the capacitors completed the restoration
process, and
at the completion, performed well within the factory specifications.
Although there are other wearout mechanisms, we expect that
most of the
reformed capacitors will enjoy a reasonable remaining service life.
Eric