cap replacement policies / was Re: VT101 8085 CPU Fault

Rod Smallwood rodsmallwood52 at
Tue Feb 24 12:35:58 CST 2015

I was working for DEC from 1973 onwards. I am mostly familiar with the 
manufacture of the VT and LA terminals.
I think they tested all of the active devices and batch tested the 

Once assembled the individual boards were given a resistive test (power 
off) before a full power up diagnostic test.
Failures went to manual rework and rejoined the queue at the resistive 
Third party and DEC branded (but subcontractor built) items where 100% 
tested on load.

In a lot of VT terminals the tube and its drive electronics (Mainly Ball 
Bros manufactured)  were tested and rejects went
back whence they came.

All of this meant the vast majority of terminals came alive on switch 
on.  But that wasn't the end of it.
With systems shipping world wide the cost of warranty and having the 
right people and parts in the right place  was a big issue.
They found out that the field failure rate was linked to the factory 
burn in time failure rate.
So by tweaking the burn in time they could control the field failure rate.

Yes DEC made reliable systems because it cost them less in the end.

On 24/02/2015 17:12, js at wrote:
> 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.

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