This is part of a series of posts for the Retro Challenge 2018/04.  See my index page for the other posts.

Today’s job was to service the PSU and disk drives of my second Osborne, prior to turning it on and attempting to boot it.

First off, let’s open her up, and see what condition she’s in…

Removing the front cover was slightly different from the first Osborne as this one has a composite video output on the front.  So the video lead needs to be disconnected to fully remove the panel.  It seems this lead has a nick in the insulation (which I repaired with electrical tape):

Round the back, the power input panel has much better insulation than the other model:

The motherboard looks very similar, but has the Screen-Pac video board installed (the L-shaped board below and to the left of the double-density floppy board):

The double-density floppy board is also installed, and this has plastic spacers to keep the board from moving around too much (the other Osborne doesn’t have these spacers):

However the floppy cables are a bit messy, with some quite unnecessary twists in them.  I didn’t attempt to correct them – if they still work I’m not going to touch them!

I won’t go into too much detail about the actual servicing, as it was much the same as the first computer.

The capacitors I removed were in the best condition of any I’ve ever seen in one of these PSUs.  But I replaced them anyway to give many more years of life:

Shiny new caps:

I figured out a way to clean the disk drive heads without removing the circuit board.  I closed the drive door to lower the clamp at the front, then used a plastic biro to lift the pressure head a little while cleaning the read/write head with an alcohol-soaked cotton bud.  There was just enough clearance beneath the circuit board.  A head-torch helped me see what I was doing.

And now to try booting the computer with one of the supplied CPM disks…

Success!  The A drive is a bit louder than the B drive, but both work well.

Next to make a copy of the CPM boot disk before anything goes wrong.  I dug out my second-hand DD floppy disks to use as blanks.

Intriguingly I found a PC format CPM-86 disk, and tried popping that into the B drive.  The Osborne can read it!  (Although it couldn’t boot from it.)  Later, when reading the service manual, I found that the double-density expansion board allows the Osborne to read certain non-Osborne formats as well, including CPM-86 for PCs.

So I re-used an MS-DOS install disk instead as my ‘blank’.

Osborne provided a nice utility program to format and copy disks without having to remember obscure CPM commands :-)

Having tested the new disk, it’s now time to return to the first Osborne computer and see if that will boot up from a floppy drive.

Sadly it failed to boot in both A and B drives:

Disappointing, but maybe that indicates I was doing nothing wrong when I tried to get it to boot from a USB drive?  Perhaps there’s something more seriously wrong with that computer?

To further tie down the boot problems of the first Osborne, let’s try repeating the USB boot process with the second Osborne…

And it boots!  So my strategy was correct all along, there’s just a problem with the first Osborne.  I don’t know whether to be happy that I was vindicated, or sad that the first Osborne is still broken.

Interestingly I needed to set the USB drive as a B drive to get it recognised as the Osborne A drive.  If you recall, I swapped the A and B drive-select lines in the cable as the documentation suggested they were reversed in the Osborne.  It seems that was wrong (or maybe they changed in in different revisions of the computer?)

So what’s wrong with the first Osborne?  Well it wasn’t the USB drive, the cable, or the disk image, as they work with the second Osborne.  It could be the double-density card (although I did remove it and try with a single-density image).  It could be some generic problem with the motherboard (although it does get as far as displaying the ‘insert disk’ screen, and responding to keyboard input, and spinning up the drives).

For me, the most suspicious component is the Fujitsu MB8877 floppy controller chip.  In the first Osborne it shows signs of soldering on the pins at each end, along with a possibly damaged socket:

I was also getting strange voltage levels from the drive controller on the floppy cable.

Compare it with the pristine chip in the second Osborne:

So I think it’s worth trying to swap that out.  I could remove the known good one from the second Osborne, but I’m reluctant to touch it now I’ve got it working.  Instead I’ll see if I can get a replacement chip.

I found this post that says the MB8877A is backwardly compatible with the MB8877.  I was able to order a MB8877A for £5.74 including shipping.  That should arrive next week, so let’s see if that makes any difference.

Back to the second Osborne, I tested the Screen-Pac display card by booting up the Extended Utility disk and modifying my boot disk copy to enable the high-resolution modes.

The 80 column mode is somewhat ‘compact’ on the built-in 5 inch screen:

And the 104 column mode is even worse! (In real life it’s harder to see than this close-up photo implies!)

I’m guessing the extra modes are more useful for the external monitor connection.  I’ll try that another day.

As a final step for today, I noticed that the screen display on the second Osborne is over-scanning a little (and made worse by the bezel obscuring the viewing angle from above):

I managed to find the schematics for the built-in monitor.

That indicated the vertical size adjustment pot is the white plastic one in the top right of the monitor circuit board:

I’m very nervous of anything that close to a CRT.  I took all the precautions I could (unplugged, insulated screwdriver, one hand behind my back) and adjusted it small amounts between testing and repeating.  The tube would have still had a charge in it, potentially lethal, so I was very carefully to keep all parts of me away from anything conductive.

After 3 or 4 adjustments I got something I was happy with:

(Notice the lines I drew on the screen with felt-tip pen to show where the bezel will go.  That was a tip from the service manual!)

I then cleaned off the lines, reassembled everything, and I now seem to have one fully working Osborne :-)

I’ve still got a few more things on my to-do list:

• Fix the floppy interface on the first Osborne
• Fix the broken key on the second Osborne
• Test the composite video out on the second Osborne
• Work out the best way to get more software onto the machines (I don’t really want to open it up every time to connect a USB drive). Perhaps a serial cable? Perhaps a floppy extension cable out through a ventilation slot?  I’m not sure yet.

But all-in-all a pretty good day :-)