DOS Days

Repairing an Osborne 1 - Part 3

14th April 2022


In Part 2 I used Dave Dunfield's excellent utilities to create some bootable CP/M disks using my Retro PC and an old 5.25" 360 KB PC floppy drive.

In Part 3, we are ready to test whether these will work on the Osborne 1!


Shiny new Osborne 1 disks!

So I have no idea whether the two floppy disk drives in the Osborne 1 even work, so to begin with, let's check them over. I noticed on initial power-up that both floppy drive LEDs momentarily light up, which is always a good sign.

The Osborne 1 came with two full-height, single-sided, single-density drives made by Siemens (model FDD 100-5), but you could get double-density drives fitted - these needed a small daughterboard to be installed on top of the main circuit board - the absence of this daughterboard tells me this computer is an original, unexpanded one.



  
Clockwise from top-left: top of Drive "A", underside with drive belt,
top with controller circuit removed and rear showing model details

The belt appears to be intact and gently pulling on it does rotate the motor and spindle, though it's a little bit stiff. I applied some oil under the smaller wheel and some grease to the worm shaft before exercising them - the shaft has to be turned by hand rather than attempting to slide the head carrier back and forth.

I have reconnected the drive, but left Drive B disconnected for now. Drive A is daisychained on the same ribbon cable used by Drive B. This Drive A has a terminator at RN3, which is just a resistor pack (you can see it in the top-left picture above, in the upper-left corner) - this is missing on Drive B of course, as it's not the last drive on the line.

Powering up with the CP/M 2.2 System disk in the drive now boots into CP/M !!!

  

It's pretty sporadic when it wants to boot - the keyboard appears to be fully working, but hitting 'RETURN' as per the system startup screen will not do anything most of the time, and then somewhat randomly it chooses to take the command and boot. I make the assumption the keyboard is working because the slight whine you hear from the computer changes when a keypress is made - if it was a duff key it's likely it wouldn't change the pitch of the whine.

Two new problems have emerged, however. Now when I start the computer, the startup screen asking you to insert a disk appears to flash continuously. After waiting several minutes the power supply seemed to behave really well (no buzzing), and the screen display was rock solid, as you see here:

This tells me the screen is operating 100% and it's the noise coming off the PSU that's interfering with the smooth analog signal being shown by the monitor.

When I do get it to start booting from the floppy disk, within seconds it displays garbled text on the screen:

Bad RAM perhaps? Video RAM?

Since this is not a DOS machine, I won't be hanging on to it - the power supply also provides mains voltage to the CRT, so finding a suitable drop-in replacement for it is going to be almost impossible, but replacing just one of the transformers will likely resolve any further PSU issues (if indeed there is one). This Osborne 1 will be going up on Ebay soon, so watch out for it and if you're interested, bid away!

More Disk Images

Also found on Dave's site was another ZIP file containing the following:-

  • OS1BASIC.IMD - CP/M 2.2 BASIC disk for Osborne 1 (single-density, 40 trks, 10 spt, 256 bytes/sector)
  • OS1DBASE.IMD - dBase II for Osborne 1 on sysgen'd disk (single-density, 40 trks, 10 spt, 256 bytes/sector)
  • OS1DIAS.IMD - CP/M 2.2 Diagnostics disk for Osborne 1 (single-density, 40 trks, 10 spt, 256 bytes/sector)
  • OS1MCAL.IMD - CP/M 2.2 Micro-Call communications disk for Osborne 1 (single-density, 40 trks, 10 spt, 256 bytes/sector)
  • OS1MDM7.IMD - Modem 740 communications for Osborne 1 (single-density, 40 trks, 10 spt, 256 bytes/sector)
  • OS1NUEVO.IMD - CP/M 2.2 System disk for Osborne 1 w/ Nuevo DD * 80 column mods (double-density, 40 trks, 5 spt, 1024 bytes/sector)
  • OS1SYSD.IMD - CP/M 2.2 System disk for Osborne 1 w/ DD drives (double-density, 40 trks, 5 spt, 1024 bytes/sector)
  • OS1SYSS.IMD - CP/M 2.2 System disk for Osborne 1 (single-density, 40 trks, 10 spt, 256 bytes/sector)
  • OS1UTLS.IMD - CP/M 2.2 Utilities disk for Osborne 1 (single-density, 40 trks, 10 spt, 256 bytes/sector)
  • OS1WRDST.IMD - CP/M 2.2 WordStar 3.0 disk for Osborne 1 (single-density, 40 trks, 10 spt, 256 bytes/sector)
  • OS1XUTLS.IMD - Extended Utilities for Osborne 1 w/80 column video card (single-density, 40 trks, 10 spt, 256 bytes/sector)