cap replacement policies / was Re: VT101 8085 CPU Fault

js at js at
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 
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.

** 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.

More information about the cctech mailing list