Back to Monroe OC8820 repairs

Hi everyone, good night!

First al all, a big hug to all, I have followed the group but I couldn’t participate too much due to time problems… a lot of work (luckily) and a big lockdown here have limited me in several aspects but I’m organizing myself to try to return to the things that I love. Between them I have my retro projects :smiley:

I have returned to the repair project of an old Monroe OC8820. This isn’t the first post about it.

At this point, I have achieved the following points:

  • I have been able to use the Micropolis drive with a common PC, and prepare a floppy with the CP/M version that (I think) should work with the computer.


  • I dumped the BIOS, but sadly I couldn’t find an emulator to test it.

After the last point, I have thought that would be great to have a memory map of the computer, but I found that it could be more difficult than I imagined.

This is the current state of the board.


I have tried to find other machines of the same period based on Z80 with a memory mapper based on two adders (74LS283), but I couldn’t find anything. I will remove all memory chips (4116) in order to check why the multiplexers have failed, and if it’s possible try to find a more modern replacement.

My main goal with this topic was to share my findings and let you know that I don’t have forgotten the group. :smiley:

I will try to keep the topic updated, any question, comment or additional information is welcome.

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