How scarce (valuable) is core for the PDP-8?

From: Chuck McManis <cmcmanis_at_mcmanis.com>
Date: Mon Apr 19 11:23:49 1999

At 04:58 AM 4/19/99 -0700, Ethan Dicks wrote:
>So... for those people who have been trying to get core over the past year
>or two, what's it costing? I'm trying to decide if I want to grab this
>stack to put into my -8/i and bring it up to 8kW, an entirely optional
>project (I have all the other parts I would need for the upgrade from a
>PDP-8/L that I got in 1982 that was sold as parts-only, bad core, and most
>of the I/O and part of the CPU missing).

As Allison pointed out, its called a "stack" for a reason as I've
discovered over the last 20 months.

Core memory consists of three major elements, the select logic, the planes
themselves, and a set of current source/sense amplifiers that are used to
drive it. Depending on the design these can be somewhat generic or not at
all generic. The absolute best value is to get a complete set of core plane
boards that came out of the DEC factory working. The second best is to get
a set that DEC had refurbished, and everything else loses value from there.

When I joined this list, I did so because I was looking for some memory for
a PDP-8/M that I had. The system was complete except for memory. Later, I
discovered that 4K wasn't sufficient really to run much more than the paper
tape stuff so I really needed 8K.

I have searched fairly widely, both through the internet and through the
"unofficial" DEC channels. This is what I've discovered.

DEC spares still carries core for the 8/e/f/m, 4k is $1,725 per set.
Ideally you need 12K for OS/8 and that means you really want an 8/E dual
backplane or an expansion box.

Many of the DEC "wholesalers" have a very small selection of OMNIBUS
boards, I discovered Mitch over at Keyways and bought from him a G104,
G227, and H220 board (one 4K stack), but they were not a matched set.
(total cost $275) While Mitch would have been happy to take them back, I
chose to keep them on the assumption that getting a complete set working
would be easier than finding a complete set.

Every now and then one of the wholesalers has a complete PDP-8 for sale. I
found one in LA for $800. I had a friend check it out and it was a dead
8/f. Memory was third party (DATARAM) and couldn't be verified. Pass.

Two months ago I found an 8/M at auction at SRI with a 8K stack and a 4K
stack in it. I bid $206 and won it. Amazingly it actually worked so I
either managed to get memory for my non-working 8/M or I still have an 8/M
with no memory and an extra 8/M. :-)

The bottom line is that real honest to god core that works is pretty hard
to come by. I've never seen any core for the 8/I or 8/L (except the ebay
one, and the 8/L that wsa offered for trade on this list).

There is an excellent theory of operation on core planes in the PDP-8/a
miniprocessor users handbook. Between that, my current probe, and my scope
I believe I can get the dead 4K stack I've got running again. And I'm
currently building a 32KW semiconductor board so that I'll have a backup.

--Chuck
Received on Mon Apr 19 1999 - 11:23:49 BST

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