I'm currently resurrecting an Apple III. It's a machine which was broken and (nearly) dumped almost 30 years ago. I litterally found and pulled it from a hardware garbage pile when I was young. Back then, I was too young and too inexperienced though, so I could not get it to work. It's been sitting in a box in my basement ever since - travelling through time (and space) :-) - awaiting a happy day, where it would finally be saved... But I never found the time and had almost forgotten about it. Well, until now that is... I had a working Apple IIe ever since (my first computer). Thought it would be nice if the Apple III finally joined.
The Apple III Service Manual is absolutely great - certainly one of the best and most detailed service manuals I have ever seen. And the mainboard is so incredibly servicable - almost all the ICs are in sockets and easy to swap.
I already managed to fix two major issues:
- First, it would not boot at all (a "NO BOOT" issue as described in the service manual, where the machine would just do nothing at all when switched on - and the screen stays blank). I've traced this issue to D13/SN74374, the driver for the memory board's CAS signals. Without this chip, no memory cell will work - so the boot ROM locks up immediately when running its initial zero-page memory test (which is about the first thing the ROM does, before even enabling the video output). According to the manual D13 was the most likely cause for the "NO BOOT" issue anyway. The manual was certainly right in this case. With a fresh replacement 74374, it finally booted - for the very first time after almost 30 years of sleep. Yay! :-)
- Then, it always entered "diagnostics mode". The diagnostics did not show any errors, but the keyboard didn't work. Luckily, this was also easy to fix. Traced it to a broken H12/74257 - another standard TTL device. Replaced the chip - and the keyboard works again. Now it no longer enters "diagnostics mode" when booting, and defaults to boot from disk instead. Yay again!
But unfortunately, it's still not fully fixed. Currently I'm stuck with issue #3:
- The internal disk drive isn't working. When booting, the machine enables and resets the drive, but aborts immediately, writes "RETRY" and endlessly repeats this all over. I have investigated this and currently believe the issue to be K11/P6A (the PROM with part number 341-0028). I can trace the disk's read data signal all the way to K11 (entering A4/pin 5 at K11). But it's output pin D2 (pin8) is always low. This is where the decoded data signal should be fed to the 8bit shift register (K10). Since it's output signal is always low, the boot ROM does not see any changing disk I/O data - hence believes the drive's motor wasn't even spinning.
Seems conclusive to me - but now I'm stuck: that's a custom Apple logic device. And I've been unable to find any offers on ebay or elsewhere.
Long story short: any suggestions where to find a suitable replacement/spare part for the Apple III's disk drive's logic PROM (341-0028)? Also, I noticed the service manual contains detailed descriptions of all logic PROM devices (essentially these devices seem to be simple 256 x 8bit PROMs with containing custom programming). However, the manual has no such description of the logic contained in this specific device for the "disk conditioning" logic.
Any suggestions/hints are highly welcome...
PS: Sorry, somehow I have created a duplicate/empty forum topic. Not sure what I did there...