Wire Wrapper needed for PDP-7

From: William Maddox <wmaddox_at_pacbell.net>
Date: Sun Feb 8 07:01:10 2004


If I understand you correctly, you are saying that major subsystems were
added to the CPU and wired into the backplane by a former user, who removed
these components without regard to its effect on the operability of the
machine before you came into possession of it. That wouldn't surprise me at
all. Many academic and lab shops did extensive custom modifications, like
the famous BBN paging box on the Tenex KA-10s.

If this is the case, then clearly you've got your work cut out for you. I
would caution you against assuming that the maintenance manual is complete
and accurate for your machine. These machines went through multiple
revisions and field ECOs during their lifetime. There was a set of field
maintenance prints issued for the specific revision that a customer had,
which was then kept up to date with the machine. (At least this is true of
the Straight-8 of similar vintage.) It is even possible that you will not
have exactly the same cards as documented in the manual, so certainly check
this before pulling any wires loose! Consider an ECO that requires an extra
gate or two, for example.

I think the most prudent course of action would be to leave the existing
wiring in place for now, but to carefully trace out what modifications were
made and note what wiring has been added and removed. The pictures of the
machine show some wiring of a different color -- frequently new wire added
during customer modifications or ECOs would be a different color precisely
to make it easier to identify, so you might want to see if that theory
checks out for your machine. Once you understand the nature of the
modifications in detail, then you can undo them. Wire-wrapping is tedious,
so you might as well avoid as much of it as you can. A young guy like you
might be able to trace out the wiring by eye, with a little help from a
continuity tester (cards out for this unless you have a "zero current"
tester) and a probe to displace wiring that is obscuring your view. A
better bet might be a "fox and hound" type device such as described on Aaron
Nabil's "Tools and Tips" page: http://pdp-8.org/tools.html If you have any
contact with the original owner, it would definitely be worth tracking down
any information that you can dig up on the nature of the modifications, even
if the detailed wiring changes are not documented, just to help you make
sense of them and sort out what were customer changes and what might simply
have been manufacturing differences and ECOs relative to the maintenance
manual that you have.

As mentioned by others, it would indeed be a good idea to pull all of the
cards and clean the connectors. I would recommend labelling all of the
cards first with their locations, and not assuming that cards of the same
type are interchangeable. Some of the flip-chip cards may have trimpots or
jumpers that are adjusted for their particular role.

Good luck!

Received on Sun Feb 08 2004 - 07:01:10 GMT

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