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Cromemco System Three restoration - initial questions


Experienced Member
Apr 3, 2008
I'm at the early stages of attempting to restore a Cromemco System Three. This thing is built like a tank. It's the second-heaviest S100 machine that I own.

This one is fitted with a ZPU / 64KZ / TU-ART / 16FDC (fitted with RDOS 2.01 PROM) and a single PerSci 299 drive.

It came to me with about 10 boxes of hard-sectored 8" floppy drives. I don't now for certain whether they belong to this system. They were stored in close proximity to it (but also with many other vintage computers). Most of the disk labels identify a typesetting program, font names etc. None of them explicitly mention Cromemco, CDOS, CP/M, etc.

Sadly its been subject to rough storage and handling, so all rear panel fuse holders and Drive 1's front panel eject button have been sheared off. I'm currently fixing this damage, rejuvenating power supply capacitors (and replacing with that has suffered physical leakage), etc.

I've found some helpful documents on Bitsavers and John's site.

So here are my initial questions:

1. Does the 16FDC/RDOS 2.01 ROM support soft-sectored 8" disks in the PerSci 299? If so, presumably I can download a CDOS 8" IMD image and use that to boot the CS3 via the PerSci 299 drive? If not, what are my options for downloading a bootable 8" hard-sectored CDOS disk? I am setup with ImageDisk, but haven't worked with hard-sectored 8" disks before.

2. If I can't get the PerSci 299 to work, what are my other options - hookup a single-sided 8" drive (per the article by Martin Eberhard) hopefully to use with soft-sectored disks, or perhaps use a single-sided 5" floppy drive instead (I see that the 16FDC was intended to work with the TM100).

3. The power supply circuitry on the rear panel has some interesting complexity to it. There's wiring that provides power (to or through a relay) that only get power when the rear-panel mains switch is in the off position. Any idea what this is for? I figured perhaps its part of a voicecoil retraction circuit, but haven't found anything to support that. John's site has a manual for the CS3, but it's for the later version that supports TM848 drives, so doesn't have any information about the power supply or drives in my CS3.

I appreciate any help or info about the 16FDC and PerSci drives. Thanks.



Veteran Member
Dec 23, 2005
Toronto ON Canada
It's only the second-heaviest machine?!? You need to install a second 299! ;-)

Your 16FDC and CDOS 2.xx should support DS/DD 5" and 8" disks as well as an IMI hard disk, but I don't think you'll be able to use hard-sector diskettes. Not every PC will be able to create a diskette from an image due to the first track being single-density FM; Dave's site has a list of which FD controllers are known to work.

The Perscis are great when they work but notoriously unreliable; you should be able to use a 5 1/4 DS/DD drive; I think the 16FDC can also handle a 'normal' 8" drive as well as a 5 1/4 HD drive emulating an 8" one.

Are you sure about powered circuits with the main switch off? AFAIR when the main switch is off everything is off; it powers a small 24V supply which in turn picks the relay controlling the main power supply.

Have fun!



Experienced Member
May 13, 2018
Melbourne, Australia
My friends and I at high school in country Victoria spent a lot of time between classes packed into a storeroom playing games and writing BASIC programs on a Crememco System 3 with four floppy drives that the school had retired from school administration duties and made available to students. I will be following the progress of your restoration with interest.

Good luck!