cap replacement policies / was Re: VT101 8085 CPU Fault

[Brent Hilpert]

On 2015-Feb-23, at 3:31 PM, Noel Chiappa wrote:
>> From: Tothwolf
> 
>> I've seen pushback from some people in the vintage computing community
>> to wholesale replacement of aluminum electrolytics which are long past
>> their life expectancy and I just don't get it.
> 
> Well, notice you didn't cause a big debate on the topic to break out here, as
> we often get on some of these things (e.g. crimping versus soldering, etc).

> My _guess_ is that vintage computer people aren't very 'religious' about it,
> and to the extent that they _don't_ do it, it's as much because they don't
> have the parts on hand (or the ability to find them easily), plus the work
> involved, as anything else.


Well, don't be misled - there are varying opinions on the matter. 
You're probably not seeing a big debate because we've had the debate/discussion on the list on assorted occasions in the past.

In answer to Tothwolf:

As matter of routine, I will do blanket replacement of paper and electrolytic caps in vacuum-tube equipment, because the caps there are older in age, they're an older technology of cap, and they're generally working at higher voltage. With that said, I also have higher-end tube equipment running with the original 'lytic filter caps from the 1950s.

Caps after somewhere around the 1950s, and in solid-state equipment, are another matter.

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. I have rarely, if ever, reformed a capacitor for a solid-state item. Keeping in mind that a mildly leaky cap which could benefit from reforming will reform during normal operation. (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.
In my experience, blanket replacement and reforming just hasn't been warranted.

There are always exceptions - I well understand why the PDP-1 restoration team would choose to be careful with the large filter caps there - age, historic significance, large amounts of energy involved. If you're doing this as a business such as the mentioned arcade-machine refurbishment, someone might choose to do blanket replacement as a pragmatic measure of insurance to help avoid call-backs, etc.

Blanket replacement (and even reforming) have an element of self-perpetuating myth to them, some people do it and recommend it, but instances of it being done don't necessarily add to a body of evidence as to whether it actually mattered.

Some of it is a matter of personal preference/values: for example some people are happy to do a blanket replacement on the probability of it encompassing a fault, others (such as myself) prefer to diagnose a fault down to the individual component level.


> 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.