A while back, I bought a cheap X68000 Compact that didn’t boot, and fixed it up well enough to be able to boot in 10Mhz mode, but not 16Mhz mode.
This involved both fixing issues with RAM accesses + bad logic IC’s related to audio. See earlier post here.
Considering it was bought broken and that repair took a lot of debugging, I was mostly happy with that outcome, but wanted to get back to fixing the 16Mhz mode too.
This summer, I finally went back to it and managed to fix it, so here’s a quick writeup.
Figuring out the crash
I started by probing the address lines of the X68000 until HALT was asserted, and noticed it crashed at the same spot as it did previously in 10Mhz mode before i fixed that mode.
It fails after a read of FF0636 which is a RTS instruction. The return address is incorrect, and things blow up.
Since that was what happened originally in 10Mhz mode, I could just assume that my previous fix was only _mostly_ correct.
Probing the RAM IC’s verified that only two of them were getting CS pulses in 16Mhz mode when LDS and UDS were asserted, while they all were in 10Mhz mode.
Looking back at the earlier “fix”
The previous fix I had for the RAM access was:
- IC84 Pin #13 -> IC84 Pin #4
- IC84 Pin #7 -> ASA Pin #3
- ASA Pin #3 lifted off pad
This was inspired by the schematics for the non-compact X68000 XVI (CZ-634C), since the schematics for compact was not available, and it appeared to be wired the same when inspecting signals.
Going back and reverting those fixes and probing things more carefully, I noticed that the compact does NOT have this exact wiring though.
When looking at UDS which were known to work, I can see that Pin#15 and Pin#6 are wired together, and continuity testing shows that this is also wired to the ASA UDS port.
On my machine, I could see that IC84 Pin #13 was NOT connected to Pin #4, which seemed weird, since I’d expect LDS to be have the same as UDS. Trace should be under IC, but assuming trace damage, the previous initial patch was correct.
What was wrong was the IC84 Pin #7 -> ASA Pin #3 part (which should be correct on the non-compact).
Simply reverting that fix, and cleaning up the bodge a bit made the machine boot in 16Mhz mode too.
Happy ending
Very glad to have this fixed. Machine now works 100%. Getting sidetracked a bit by messing up the wiring in the previous repair was awkward, but simply going back and probing things carefully made for a rather quick fix!
The best type of mistakes are ones that can be easily reverted.
