PDP 11 gear finally moved

Tothwolf tothwolf at concentric.net
Wed Jul 22 00:26:53 CDT 2015

On Tue, 21 Jul 2015, Mike Stein wrote:
> On Tue, 21 Jul 2015, Tothwolf wrote:
>> On Tue, 21 Jul 2015, Mike Stein wrote:
>>> FWIW I'm certainly not about to spend 100s of dollars, not to mention 
>>> time spent in sourcing and replacing, to replace the caps in systems
>> 100s? Where are you sourcing your components from? The typical board I 
>> rebuild has a component cost of about $20 or less. Smaller switchmode 
>> PSUs with a bunch of 10-18mm radials might be closer to $35-50. Larger 
>> PSUs /might/ cost closer to $100 if they have several large screw 
>> terminal capacitors in them. All things considered, that isn't very 
>> much money in today's dollars, and considering the full replacement 
>> cost of some of these boards (if they are even available), those 
>> preventive maintenance costs are an absolute bargain, /especially/ if 
>> you are doing the work yourself on your own time.
> Maybe it isn't much money in your world, especially when someone else is 
> paying.

Like anyone else, I have to buy the parts I use for my own equipment. If 
someone else wants to volunteer to buy them for me, I'm certainly not 
going to argue though ;)

> I just priced the main power supply caps in one of my Cromemco systems 
> and it comes to ~ $120 (and all special order of course); if I replaced 
> all the caps in all my (working) systems as you and a few others are 
> suggesting across the board regardless of the system, condition etc., it 
> would easily exceed $2000 if I could even find suitable replacements.

Without an actual list of components required and without knowing which 
vendors you are getting your price quotes from, I have no way to verify if 
your $120 total is representative of the norm.

If your Cromemco system is still functioning to your satisfaction, and you 
have zero interest in replacing aluminum electrolytic capacitors as part 
of preventative maintenance, why are you even bothering to price them?

I also seem to remember saying earlier in the thread: "In the odd case 
where a computer grade screw terminal capacitor is extremely expensive or 
completely unobtainable (those which I've purchased were under $20-30) I 
might be willing to leave an original part in place, *if* it can pass a 
leakage test."

> And what about those prone to explode tantalums while we're at it...

Well, if you want to bring those up and expand on the list of "bad caps" I 
mentioned, early SMD solid tantalums seem to be quite problematic in terms 
of spontaneously shorting out and going up in flames, even when operated 
at half their rated voltage (as specified by the capacitor manufacturers). 
I can't say I've seen a higher failure rate with newer SMD tantalums than 
say modern SMD multilayer ceramics, however after having to scrape the 
remains of many charred SMD tantalums off of (unobtainium) boards 
undergoing repair, I can't say I really trust them. YMMV.

> If you're recapping 20-year old or newer circuit boards for customers as 
> you apparently are then it does indeed often make sense to replace all 
> the aluminum electrolytics, especially if the board has problems or 
> there's visual evidence of failure, but let those of us with older, 
> well-working systems use our _judgement_ whether to replace or not. OK?
> To each his own...

I've previously done a great deal of commercial work (not now though), 
however I still do the very same work on my own equipment. I currently 
have somewhere north of 300 projects in my to-do queue (everything from 
modern stuff made a few years ago to test equipment and radios from the 
1950s and earlier) which I've already purchased and kitted up parts for 
(of which I'd say about 2/3 are aluminum electrolytic capacitors). I know 
/exactly/ what *I* spent on my parts (I have it all organized in 
spreadsheets, just like I did for commercial projects), and my own parts 
costs do not at all seem to match up with what you are describing.

I can also state from experience that the majority of capacitor failures 
(wear out; change in capacitance and increasing electrical leakage at 
working voltage) do not exhibit visual signs of failure or impending 
failure. The main exception are some of those really low quality far-east 
parts made in the last decade or so which manufactures use in consumer 
grade electronics.

>>> that are running perfectly "just in case"...
>> How do you -know- they are "running perfectly"? [Just because a widget 
>> itself is functioning, you have no way of knowing if that capacitor is 
>> working 100% properly /unless/ you actually remove it from circuit and 
>> run a full battery of tests on it. Simply measuring the capacitance 
>> with a DMM while a capacitor is in circuit isn't good enough.]
> They reliably do what they're supposed to do.

You didn't answer the question. How do you know those aluminum 
electrolytic capacitors are functioning just as good as they did when they 
were new? Unless you've tested them out of circuit, you simply cannot make 
that assertion.

Just like the NiCd and SLA batteries I mentioned, aluminum electrolytic 
capacitors by their very electrochemical nature degrade as they age and as 
they are used. You cannot claim that a 20-30 year aluminum electrolytic 
capacitor is going to function as good as it did when it was new. If the 
designer of an electronic widget designed in an adequate margin (which as 
I mentioned elsewhere in the thread, is a problem with modern made 
electronics), a 20-30 year old capacitor may still be functioning today. 
As it ages, how much longer that widget continues to function properly and 
at 100% reliably is going to be wholly dependent on how large of a margin 
the widget's original designer planned in.

As I also previously mentioned in the thread, my own experiences with 
repairing and servicing equipment (of all types), both for myself and 
others, seems to point to about a 20-30 year mark as being the far end of 
the bathtub curve for aluminum electrolytic capacitors. At 20-30 years, 
their failure rates seem to increase dramatically, and if increasing 
failure rates are a concern, then you replace those capacitors. [...and if 
you don't care, then why are you even reading this thread?] Other 
components such as resistors and semiconductors seem in general to be 
much, much more reliable, and do not follow the same curve as 
electrochemical components such as aluminum electrolytic capacitors.

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