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 <space...@gmail.com> wrote:
On Mon, Jul 20, 2015 at 4:46 PM, Rich Alderson
<ri...@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
problem).
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
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: b...@softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
