24 Feb
2015
24 Feb
'15
9:12 a.m.
On 2/23/2015 11:58 PM, Brent Hilpert wrote:
** 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
the ass.
- J.
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
Chinese variety).
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
voltage range).
(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
be checked.
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.