PDP 11 gear finally moved

Johnny Billquist bqt at update.uu.se
Thu Jul 23 05:00:38 CDT 2015

On 2015-07-23 07:09, Tothwolf wrote:
> 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 then.

I've mostly tried to stay out of this thread, as I both am no expert on 
electronics, and also seems to have a very different view and experience 
than many around here.

But I thought I should atleast give a datapoint for you all.

I'm no museum type of person. I try to run my stuff all the time. And I 
normally do. And I do not replace things in running, working machines. 
And since I keep running the machines, they keep running on their 
original parts. And they just keep working.

So I have various PDP-8 systems that have passed 40 years now, and they 
are still running fine. The same is true of my PDP-11 and VAX systems, 
that are now past 30 years. Running just fine.

The biggest problems have always been with gear that have been sitting 
unused for a long time, and I don't think I have ever had to replace any 
large capacitors. A few small ones, yes. And lots of transistors in 
power supplies. Those are the thing most often causing problems in the 
+5V bricks in larger PDP-11s in my experience.
But then again, I also have plenty of spares, so most of the time I have 
not bothered repairing them, and most spares sitting around have been 
fine, so the machines keep running.

Biggest problems with PS for me was a couple of VAX 6400 machines, where 
the power supplies developed issues in running systems. The machines 
have been scrapped. Didn't like systems that develop problems like that. 
8650 on the other hand is much better. But the external Unibus box 
currently have a power supply issue. But that machine (unfortunately had 
to sit a couple of years powered down, at which point it developed the 

So I would recommend people to just get the machines in running order, 
and then keep running them. Leaving them off for extended periods are 
not good. Constantly turning them off and on is also not good.

Anyway, that's all I have to say. Keep machines running, and they seem 
to live a very long time. In spite of all papers and research saying 
that they should be dead.


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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