PDP 11 gear finally moved

Tothwolf tothwolf at concentric.net
Thu Jul 23 00:09:34 CDT 2015

On Tue, 21 Jul 2015, Lyle Bickley wrote:
> On Mon, 20 Jul 2015 23:14:36 -0600 Eric Smith <spacewar at gmail.com> wrote:
>> On Mon, Jul 20, 2015 at 4:46 PM, Rich Alderson <RichA at livingcomputermuseum.org> wrote:
>>> industry white papers with tables of decay rates for the aluminum 
>>> electrolytics that indicate that, *no matter what*, they lose 
>>> capacitance over time, until c. 14 years from manufacturer date they 
>>> are at 10% of rating.
>> That's very interesting. I haven't seen those white papers, but the "no 
>> matter what" must in fact depend on something, since on the PDP-1 
>> Restoration Project we found that most of the 40 year old aluminum 
>> electrolytic capacitors still met their original specifications, 
>> including capacitance within rated tolerance. Of the few electrolytic 
>> capacitors that had failed, the problem was a catastrophic failure, not 
>> the capacitance being outside the rated tolerance.
>> In the PDP-1, we preferred to keep the original components as much as 
>> possible. Had there been a capacitor, the failure which would have 
>> caused extensive damage to other components, we would have given 
>> serious consideration to replacing it. However, that was not the case 
>> for any of the capacitors in the PDP-1.
>> Had our analysis indicated any expected benefit to replacing all of the 
>> electrolytic capacitors, we would have done so, and bagged and tagged 
>> the originals similar to what we did with failed components, so that 
>> they could be replaced if it ever was desired to return the artifact to 
>> its pre-restoration condition.
>> I'm not recommending against LCM's policy, but I also wouldn't 
>> necessarily encourage anyone to adopt it, nor to adopt the practices of 
>> the CHM PDP-1 Restoration Project, without studying the issue.
> As Eric, I'm a member of the PDP-1 Restoration Team. The PDP-1 
> restoration was completed in 2005 - and annually we check the power 
> supplies for voltage, ripple, etc. Not one of the re-formed capacitors 
> have failed in the ten years since the completion of the restoration.
> I also re-formed all P/S capacitors in my PDP-8/S in September, 2013. 
> Not one has failed since...

How often is CHM's PDP-1 powered up and operated?

If LCM's computers are going to be powered up and used routinely, it 
actually makes a lot of sense to go to the trouble to replace really old 
aluminum electrolytics, even if they seem to test good, since doing so is 
going to increase the reliability of frequently operated equipment.

This is the reason why /I/ replace aluminum electrolytics when I'm making 
major repairs or fully reconditioning electronic equipment...I want said 
electronic widget to be as reliable as possible because it is never good 
when something breaks down while you are using it, especially with gear 
which needs to run 24/7/365 for years and years at a time.

One example I can give are some Pentium P55C architecture (Socket 7) 
systems which I've been running with minimal downtime for ~15 years. The 
original power supplies with their original (and relatively low quality) 
capacitors lasted about 15 to 17 years (I think the manufacture date code 
stamped on the oldest one was 1998) before the systems began to develop 
stability issues, requiring me to rebuild the power supplies with new 
capacitors. I fully expect that the replacements would last even longer 
than 20 years, however I rather doubt I'll be running those computers by 

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