88-16MCD restoration

[spiette]

TLDR- Anyone have one of the buss bars/strips (Part #101620) used in the RAM array available.

Back when I was at Pertec, I bought my Attache for home from spare parts and it came with the basic Attache board set, 8800b CPU, Turnkey Board, Video, and two memory boards a 16MCS and a 16MCD. The 16k Static RAM board has worked flawlessly up until this year but the 16k Dynamic RAM never worked and has sat in a anti-static bag ever since. I'd heard the horror stories of MITS Dynamic RAM cards and never bothered to really look at it.

The time has come as part of going through all my S-100 boards and systems to see if I can fix it and get it running. I've just finished building one of Martin Eberhard's 88-2SIOJP to replace the turnkey board, one of John Monahan's S-100 Bus Display boards to help debug, and fixed one of my 8800b CPU's along with the 16MCS that finally broke.

If your familiar with the 16MCD card, it has bus strips above and below each RAM column for I believe the three voltage rails (+5 -5 and +12 or +15 depending on the later vs earlier rev board). At some point the top most strip broke off from handling the board and will need a replacement.

Anybody have a spare, perhaps from a card they have salvaged?
Any insights as to issues I might have?

Mine is the later revision using TMS 4027 DRAMs, the +12v rail, and the 74LS00 for IC D rather than the 74LS20 and the MK4096-15 DRAMs.

 

[spiette]

At one point I borrowed the original 0.1 caps to fix something else and trashed all the Tantalum caps so they didn't have a chance to short. The Panasonic FC electrolytics should work equally as well from what I've read to replace the Tantalum in the voltage regulator circuit and replaced dipped ceramic's.

You can see the missing bus strip above the A column of DRAM sockets. I not sure but I believe these are two different strips insulated from each other and then combined into on. I know at least one is connected to ground but I haven't had a chance to trace the other holes yet.

 

[atariquest]

Based on your photo, my guess is the top strip is for VBB and VSS. If these bus strips are simply point to point connectors, I would make my own.

You can use paper clips for the through hole pins and solder them to a thin strip of galvanized sheet metal (the bus). Cover the sheet metal with electrical tape. You will need to make two of these, one for VBB and the other for VSS.

Next, cover both strips with thin cardboard or thick paper (something close to the original) and glue with Elmers. The result will be 2 bus bars in a single package that will look almost like the original.

Remember, the purpose of the bus bar is to provide a low impedence, high current path. Make sure to clean metal well and apply flux before soldering. Also make sure to avoid cold solder joints.

Hope this helps.

Marvin

 

[spiette]

Thank you. Something to consider.

If I have to, I was thinking copper or brass strips covered in adhesive vinyl. Once on the sides that would face, and a larger piece folded over the two to hold them together staggered like the originals. I'll either make a punch to cut the legs/pins from the strip, use a nibbler, or solder on leads to the strip before covering them.

I'd really like to find someone who cannibalized an old board that might still have an original part like I did at one point to this one before I learned that out of all the MITS dynamic ram boards this one tended to actually work.

The before: