Monroe OC-8820 repair project (part 1)

Hi everyone! Good afternoon!

I have decided to start with the repair of my old Monroe OC-8820. I have tried information about it here (Trying to find a Monroe computers list) with a lot of interesting comments and good information. Thank guys!

Now, It’s my turn to do something with the machine. Sadly I couldn’t be able to get any manual/schematics/software of it, but I want to start with something basic while I’m still trying to find any helpful information.

My first step has been a complete disassembly of the unit. I have found a lot of dust, broke ics and several 4116 memories that seem that are bad. Them turn a bit hot, I will change them in the next step. Another ICs really suspicious are two 74LS283 (4 bits adder) that are really hot, so I will need to buy several new pieces before continuing with the repair.

Some pictures:

a lot of memories to check:

improvised lab:

I hope to have time to share my findings, any question or comments are welcome!
Also, if anyone has the same machine and need to check something with my unit, please let me know and I will try to help.


I found one reference to the Monroe OC 8820 in Dr Dobb’s Journal, May 1985, in a table of disk formats (as used by Disk Maker I). According to this, the format is Single Sided Double Density (SSDD), 96 tracks per inch, 306KB, if this is of any help.

(Download Dr. Dobb’s Journal, Vol.6, 1985, at

BTW, the manufacturing seems to be of outstanding quality: clean layout, no bodge wires, a self-contained CRT unit, quite impressive…

P.S.: A pretty good image of the outside of the machine, for reference:


My usual steps are to check the PSU first … make sure that the voltage rails are stable and within a known range. a 5V rail running at 6V will make alot of things get warm quickly. (Use a scope to verify this as a multi-meter may average out high noise spikes)

After that its a power up, check for heating (you are already on that one!).

Then the nice stuff can start - is the CPU reset being de-asserted, does the clock run; any acticity on the bus etc etc…


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Hi @NoLand, thanks very much!

It’s really helpful thanks! luckily seems easy to get some disks to test the floppy drives. The controller used bu the board it’s a FD1793, after check that no one chip it’s abnormally heat and a successful initial test I want to try to connect the units with a PC. It would be great to be able to make a bootable disk with CP/M.

Hi @RichardP, thanks for the comment!

I have tested the unit as you have mentioned, the PSU seems that it’s working fine, and could be replaced by the PSU used by the JAMMA Arcades: it provides +5 +12 and -12 volts.

After removing the hot chips, and some solders more I have been able to get a boot screen more promising

After some security checks, I have attached the floppy drive and now I’m receiving the following message

The interesting thing it’s that the machine still running without several chips:

  • An adder 74LS283 (that I suspect that it’s part of memory mapping)
  • An 4116 memory
  • two 74LS240 that seems that are related with the floppy controller
  • two 74LS241 that seems that are related with the parallel port

The next part will be to replace those chips and check the floppy drives :smiley: