Re: PDP-8 Straight 8 restoration - failure modes

[Ethan Dicks via cctalk]

On Mon, Mar 30, 2020 at 3:43 PM Bob Rosenbloom via cctalk
 wrote:
> In restoring my PDP-8/s, diodes were the problem. Turns out they have
> steel leads that rust. Easy to see with a microscope.
> Badly rusted ones broke the glass body.

Good to know.  My PDP-8/S was stored indoors but my Straight-8 was in
a storage locker for years before I got it.

I have a stereo inspection microscope.  I'll definitely peer at the
diodes as part of the cleaning/evaluation process.

But I also have to line up a bunch of spare bulbs.  The leads on more
than half of the ones on my -8/S are fractured where they enter the
glass.

-ethan

Re: PDP-8 Straight 8 restoration - failure modes

[Bob Rosenbloom via cctalk]

On 3/30/2020 12:02 PM, Josh Dersch via cctech wrote:

On Sun, Mar 29, 2020 at 1:48 PM Brendan McNeill via cctech <
cct...@classiccmp.org> wrote:


Here in NZ and around the world many of us are in lockdown and spending
more time on our computers, if that were possible.  I have just completed
the restoration of a PDP-8 Straight 8 which I believe is the only one in
New Zealand.  You can view the restoration story and find appropriate
resources here:  https://pdp-8.nz 


Very nicely done, and an excellent write-up.  That core memory repair was
amazing. It's interesting to read about a PDP-8 with such a high failure
rate -- I've personally worked on two straight-8 systems (one at LCM+L, and
one in my personal collection) where relatively few component failures were
found.  I wonder what accounts for the difference -- batches of diodes more
prone to failure, the environment the machines were stored in, or the
number of years of service...



While it plays Chess, it would be great if someone wanted to write (say) a
Prime Number Generator, or some other application and email it to me off
list.  I have Focal-69 and can probably source other languages for this
wonderful old machine with 4K of memory.


There's a 4K LISP as well, though it's a pseudo-LISP-1.5 dialect so it's a
bit different.

- Josh


In restoring my PDP-8/s, diodes were the problem. Turns out they have 
steel leads that rust. Easy to see with a microscope.

Badly rusted ones broke the glass body.

I had to replace more than 200. Most turned into ~600 Ohm resistors, 
some open circuit. I'm sure I'm going to have to
replace all of them over time. So how (and where) the machine was stored 
probably plays a big part in it's reliability.


Bob

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