@Kai Robinson It would be really great to use I2C capabilities of the AVR to handle serial communications, that would hopefully speed up serial communications processing enough that a 32.768kHz system clock wouldn't be an issue any longer. Now I'm looking through the Universal Serial Interface (USI) and it looks promising so far... looks like we need to swap two pins around to get SDA in the right place, unfortunately. Adapter board and ATTiny87, that's the way to go I guess.
Regarding the 1-second interrupt accuracy, 15 minutes of time drift a month (approx. 30.5 days) is 15/43920 = 0.03415% error, I'm guessing 10 times worse would still be acceptable for the interrupts system. We simply need to know what tolerance range the RTC is required to operate within.
Good to see the info from the Hardware Reference, I was looking for that online but couldn't find it. Everything in the implementation thus far checks out with it, and it adds a few new ideas for writing test cases.
I have nothing to contribute here, other than my sheer astonishment and delight that a WalkMac is alive and running in 2020. I have vivid memories of reading about this thing in some Mac-centric magazine when I was a kid (maybe a mention or write-up in MacWorld or another magazine of that era), and just being blown away at the idea of using Mac software on a crazy-cool orange plasma display.
I'm so happy that this machine has been saved from whatever sad fate presumably befell most of the other WalkMacs that would've existed back in the day. Please keep sharing any further developments or discoveries about this machine!
Read through cheaper 9 of this book. You'll find most of your answers there. It's likely you need to replace the opto coupler.
What version of AB do you have? CP1 should be available from any major electronics dealer.
For sound my CII originally had scratchy sound then lost it all together. After recapping the AB and I think desoldering the sound chip and cleaning and putting it back on it stared working fine. One of the caps on the logic board directly affects the sound. If you pull that trace and don't repair it will have issues as well.