Continuing Adventures in RetroComputing (Repost)

(Originally posted on LinkedIn May 18th, 2020)

As I have posted here before, my current hobby is fixing up old computers. Some of this involves working with components and tools not easily available at your local walk-in electronics store. Some of these can be covered by the “usual suspects” such as RS Components and Element 14, (My preferred suppliers) but some of these can only be purchased from obscure suppliers, often in China.

As you can imagine, recent global events have slowed the supply chain down dramatically, but things are finally starting to trickle through.

As a result, I was finally able to tackle my Apple II+ recently, having finally received my 4116 DRAM chips. The Apple II used 3 banks of 8 of these to generate the first 48KB of memory, with an additional bank of 8 on a “Language Card” (So called because they were needed to run additional languages such as PASCAL) to bring it up to a compliment of 64KB.

The whole point of this was to get the whole shebang ready to run a serial link back to a PC so I can copy across floppy disk images.

Unfortunately I still have an intermittent issue with memory corruption, and it’s really annoying. I am in a “chicken and egg” situation insomuch that, if I had a working system, I could get diagnostic disks made which would help me to resolve the issue, but as my system doesn’t work, I can’t get the disks, which means I can’t find the problem…

(Insert joke about holes in bitbuckets)

I’m not giving up, however. My current plan is to pull it all out of the case and check key busses with an oscilloscope. See if I can isolate out where things are “going bad. I’m also tempted to get a EPROM programmer, as many of them will also test 74 series logic. I may also see if someone on the various Apple forums are willing to burn me a couple of Diagnostics disks.

If that fails, I might just have to buy another Apple II to use to help me fix this one!

Here’s a list of what’s already been found and fixed:

1. Many chips with significant oxidation on the legs. They looked like they had been steeped in strong tea with a browny black patina. Cleaned with a sand eraser and their sockets cleaned with electronic contact cleaner. This improved things a lot, and got me to the point where it’s semi-stable.
2. Custom cable constructed to connect from the Floppy controller to the Floppy drive. I had to customise both the cable and a connector. Apple used a 19(!) pin D connector, which I worked around by getting a 25 pin connector and pulling 6 pins out with pliers.
3. On the Language card, replaced ALL the 74 series logic chips with new ones. Also replaced the ribbon connector between the card and the motherboard. This was, once again, a custom made part.
4. Identified and replaced all 4116 memory chips. I believe all the ones I was using are good components as I “swap tested” them, but it was cheap to swap out all of them, so now they’re identical. I did find several bad ones.