Eternal Sunshine of the Spotless Memory (or not)

I ordered the parts necessary to build a pair of extender boards for the Imlac, so while I wait for those to arrive I'll be working on other parts of the system (and the Imlac Emulator I'm working on).  Last night's endeavor was to clean up the nicer of the two core memory assemblies and see what I'm up against there.  So, I put on some MST3K ("Space Travelers" -- Gene Hackman's good in anything) and got to work.

Rust = Bad

You may recall from my earlier overview that the memory system is in pretty sad shape.  Because the PDS-1D was stored in a damp environment for some period of time there's a fair amount of rust on the machine. Fortunately most of the circuitry was protected by the chassis.  Not so for the memory -- it is mounted to the outside of the rear panel, and the cover that would normally protect it is missing.  As you can see at the right, this lead to some serious corrosion on most of the socketed chips.  It also wore the identifications off of most of the chips.  As if that's not bad enough, this is the one component of the machine I do not have documentation or schematics for, so I have no references to help debug the hardware.

I believe that this entire assembly (the control logic + core plane) was manufactured by Dataram, even though the control board is labeled Imlac, with Imlac part numbers.  I suspect this because of this blog post that I stumbled on on Dataram's site; if you scroll down this picture, despite its blurry dithering, is a dead ringer for the memory assembly I have.  It has more chips (I suspect it's an 18-bit memory board where mine is 16-bit) but is otherwise identical.

I have sent Dataram a query to see if they still have any documentation for 40 year-old hardware but I'm not holding my breath.  Since I don't have a Dataram part number it's difficult to search for.

My initial plan was to remove each socketed chip and clean its legs and socket as well as possible.  The legs are carefully cleaned with an XActo knife (gentle scraping with the sharp knife edge does wonders) and the sockets are cleaned with a spritz of contact cleaner (that's about the best that can be done -- the sockets do look to have fared better than the chips, fortunately.)  This took a long time and by the end my fingers were pretty tired, but in general things cleaned up pretty well.  About a dozen chips had legs that were so corroded that they were falling off so I marked those for replacement.

The other memory board is in considerably worse shape.  I pulled a random selection of chips from their sockets and all of them had legs that were corroded through.  I don't feel great about getting the first memory board running, but this one will take a miracle.

This afternoon I catalogued the chips on the boards.  The identifying numbers are very worn on both boards, but after cleaning the surface of the chip, under just the right light (and with a magnifying glass) I managed to get them all identified (see the picture at right).  Some of these chips are going to be difficult to find (in particular the Q2T3725 multi-transistor packages; there's an NTE equivalent but even those are hard to find and cost $7-$10 each -- there are 26 of them between the two boards.  Ouch.  The rest of it is standard 7400-series logic and I don't think those should be hard to find, though they're still not cheap ($3-$5).  If it weren't for the cost, I'd consider just replacing all of the chips pre-emptively, it'd probably be easier.  Maybe I'll do that anyway...

As an alternative, Tom Uban managed to replace his core memory with a small board containing modern semiconductor memory, it might make sense to do something similar.  It would definitely make debugging the CPU easier.

Decisions, decisions...

Update:  Just about 20 minutes after posting the above I actually stumbled on the manual/schematic set for the core memory!  As I suspected, it was manufactured by Dataram; it's a Dataram DR101 variant.  I've put a copy of the schematic (a whopping 64mb) here, if you care to look.  Maybe I stand a fighting chance now.