cap replacement policies / was Re: VT101 8085 CPU Fault
js at cimmeri.com
js at cimmeri.com
Tue Feb 24 11:12:14 CST 2015
On 2/23/2015 11:58 PM, Brent Hilpert wrote:
> For solid-state consumer equipment
> like transistor radios, caps from the
> 1960s can be a problem but even then
> not inherently so. For solid-state
> digital equipment: I don't think I've
> ever done a blanket replacement of
> caps. The vast majority of my many
> dozens to hundreds of calculators,
> digital test equipment, computers,
> etc., most of which date from the
> 1960s and 1970s, are running with all
> their original caps - including
> specifically electrolytic filter caps.
My experience is different. 1940's
through 1960's -- I replace all AECs
because most are either bad or too far
on their way out. 1970's-1980's,
depends on too many variables to give
simplistic rules, but I will test and
reform all AECs. Some are found to be
bad, but most are still ok -- again,
depending on many factors. 1990's
onwards, I don't worry about as there's
rarely a failure (unless of those bad
> I have rarely, if ever, reformed a
> capacitor for a solid-state item.
** I do, all the time, as per above.
> Keeping in mind that a mildly leaky
> cap which could benefit from reforming
> will reform during normal operation.
** I don't concur with this statement
because the charge rate is too high.
Countless times, there's been caps I
couldn't reform with too high a charge
rate, but when it was slowed down, they
came back around. Many factors are at
play.. amount of leakiness to begin
with, level of operational voltage vs.
cap's rated voltage, etc. BUT, AECs
*do* benefit from and *do* require
periodic use... which keeps them from
losing their oxide layer to a certain
degree (within the circuit's applied
> (Also, those big screw-terminal filter
> caps from the linear-reg days were
> called "computer grade" for a reason.)
> So, in relation to computers and
> solid-state digital, I don't
> understand why people get all
> concerned about caps.
** Because they go bad... even those big
computer grade ones. I've got a whole
bucket here full of them.
> In my experience, blanket replacement
> and reforming just hasn't been warranted.
** Here, agreed.... *blanket*
replacement isn't.. but they should all
>> For those of us who are basically software people (or even pure digital),
>> this stuff can be a little daunting - not necessarily because it's actually
>> hard, it's just out of our comfort zone. I myself am certainly daunted by the
>> concept of replacing every electrolytic in all the power supplies of all the
>> vintage -11's I've got... (And I don't want to even think about all the filter
>> caps on all the boards! :-)
> Early (1970s era) switch-mode power supplies of significant capacity are a category one might be a little leery or cautious around, as the design and componentry may not always have been up to the task, as they hadn't benefitted from a couple of decades of experience and targetting.
> Perhaps more owners of DEC equipment from that era could add real-world experiences.
** 1980's DEC stuff still seems to be
OK, although I frequently find bad caps
in the 3rd party power supplies like
those from Astec (eg. BA123, BA23)...
but ALL the AECs benefit from reforming
-- you can see right then and there how
they do, how they perform before and
after. And you can tell whether a cap
does or not by how it RESPONDS during
the reforming process. 1970's DEC
PSUs like for an 11/34 also have bad
caps in them by now.... I just rebuilt 6
modules, and all had at least one bad
AEC... now, mind you, most not to the
point where the PSU had actually failed
completely, but to where it *would* have.
I don't replace caps but reform them if
their performance during the reforming
process indicates that they still have
plenty of life left in them. I reform
instead of replace when I can in order
to save money, and because going through
the process of finding the right sizes
and right models is a real major pain in
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