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Old 1st May 2023, 7:30 am   #1
TonyDuell
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Default Osborne 1A CP/M luggable computer

Last year I was kindly given an Osborne 1A portable CP/M computer. At the time I promised to document the repairs/restoration, so here goes.

Fortunately the Osborne 1A is well-documented. As well as the user manual (which contains more useful information than many modern 'technical' manuals) there's a technical reference manual and a service manual. This gave me circuit diagrams of everything apart from the Zenith monitor. I did find a Sams Computerfacts for that, but the diagram is clearly wrong (it couldn't work). And the power supply information in the Osborne service manual is typical for Astec, it contains no details of the transformer connections. But I do have the diagrams of all the logic boards, the ROM and CP/M CBIOS sources and so on.

I started by dismantling the machine into the basic units. Being a 1A (as opposed to a 1) it has the later case which splits in half and is a lot easier to work on. Apart from the case/chassis, I had the logic assembly (Main PCB with added daughterboards for 80 column video and double density disks), a pair of disk drives, the monitor, the power supply, the mains input panel and the keyboard.

One part of the case needed more work. There's a little plate that the carrying handle is fixed to. I drilled out the rivets, took it apart, and de-rusted the handle brackets. I then re-assembled it with 8-32 UNC nuts and bolts. Since all the other fixings in the machine were UNC I insisted on using such threads for my repairs.

I worked out the transformer connections on the Astec SMPSU board for interest. I noticed that the original metalised paper 'smokebomb' mains filter capacitors had already been replaced, but badly. spotted some reside of the smoke from the originals on the PSU cover in the chassis. The replacements had a different lead pitch and the leads had been bent to fit the original PCB holes, but the PCB had been laid out to take them directly. I desoldered them, straightened the leads and fitted them to the 'right' holes.

I then tested the power supply with a lamp limiter in the mains input. I find that provided the SMPSU is lightly loaded it is quite safe to do this (the chopper transistor will not fail from overcurrent due to the reduced input voltage) and the limiter will save PCB tracks, fuses, and my nerves if the chopper transistor does fail. In the event the power supply worked first time with no problems.

The mains input panel consists of a combined mains input plug/fuseholder/voltage selector, a mains filter module, the on/off switch, and a thermal cutout halfway along the wiring harness. I took it apart and cleaned the bits. Amazingly the mains filter tested fine on the insulation tester and on the mains with a lamp limiter (again). I had expected the internal capacitors to break down. Everything seemed good so it went back together again and I tested it with the PSU board. All fine.

The monitor came apart easily into the PCB, flyback transformer, CRT and chassis parts. I cleaned them (in particular the CRT screen, I've been caught by that before). I then produced a better circuit diagram of the monitor PCB. Alas the transistors all have Zenith part numbers only. The only IC, though, is the horizontal oscillator, an NE555. Those I understand. Before reassembling the monitor I decided to power the PCB from a 12V bench supply and check some waveforms. The horizontal side was fine.But on the vertical side, the oscillator (which is an NPN and PNP transistor with collector-base connections to form the equivalent of a 4-layer PUT type thing) was running fine but nothing in the output stage. I finally found there was 12V or so on a leadout of one of the output transistors and about 0.5V on the end of a resistor it should have been connected to. There was a spectacular dry joint on the transistor lead. Resoldered it, the waveforms were fine so the monitor went back together.

At this point I couldn't resist trying the logic assembly with the PSU and monitor. It didn't work, but I got a locked screen display and the fact that the beeper onlty sounded briefly impled the Z8- had initialised some of the I/O chips.So the PSU and monitor were fine, the video timing chain was working, the processor was running code from the ROM. I suspected RAM problems and made a mental note of that. There are 33 off 4116 DRAMs on the board (64K*8 main ram, including the video RAM and 16K*1 half-intensity video attribute bit), a chip know to be somewhat unreliable. But I did not want to replace the whole lot for obvious reasons.

And so to the disk drives. These have an Osborne-made PCB on a Siemens FDD100-5 mechanism. I have the circuit of the PCB in the Osborne technical manual, I also have the Siemens manual for the normal drive which has excellent exploded diagrams of the mechanical parts.

The Osborne drive has a single 34 way edge connector, no separate power connector. The signals on the even numbered pins are a subset of those on a standard drive, and on the same pins. The odd numbered pins are not all grounds, there are grounds, +12V and +5V power supply inputs there. I made up a simple adapter with a 34 pin header to connect normal drive testers/exercisers to, a 34 pin edge connector to the Osborne drive and a 4 pin power input connector. This let me test the drives on my exerciser, they ran but the spindle motor was clearly 'labouring' and the hard positioner was sluggish, in fact I had to work it by hand before it would move on the stepper motor. All down to hardened grease in the ball races.

Now I have a thing called a Telematic Microtest. This lets you use any classic PC (at least 256K RAM, any video card, and one serial port) to do drive tests/alignments. It's a box containing an 8035, ROM and ADC that links to the serial port of the PC and to testpoints on the drive. The drive is connected as the B:drive on the PCB, you run the Microtest software and put a 'real' analogue alignment disk in the drive under test. It will check the spindle speed, track 0 switch setting, radial alignment and so on.

Amazingly this system had the settings for the Osborne drive (but not for an unmodifed FDD100-5) but warned you need an special cable adapter. Turns out my home-made one was fine. I checked I could see the alignment pattern on this system. I could, so I felt confident about being able to re-align the drive if I took it apart and reassembled it.

This I did, essentially down to the last nut and bolt. The Siemens manual even covers how to dismantle the stepper motor and the clamp cone. I'd marked the positions of the track 0 microswitch and the stepper motor so I could get them back in about the same positions to simplify the alignment. Cleaned the chassis parts and the head.

There are a total of 5 ball races in the drive. 2 for the spindle, 1 in the clamp cone and 2 for the positioner leadscrew. I measured them up, they are, unsurprisingly, standard parts. I would like to have replaced them but I was quoted a lead time into January next year for one of them, and 'no idea when we'll have stock' for one of the others. In the event I removed the wire circlips and side covers from the old ball races and soaked them overnight in white spirit. This shifted the old grease, I then repacked them with high melting point grease and put the covers back on.

Then I reassembled that drive and repeated the process with the other one.

Time to do the alignment. The drives sounded a lot better when running. The spindle speed was low, but a tweak of the trimpot on the PCB cured that. The marks I made when taking the thing apart paid off. One drive was close but I adjusted it anyway. The other drive, by chance, was almost spot-on, so I left it alone. The drives seem fine.

I do a quick check on the keyboard. This is a membrane switch assembly and Osborne keyboards are well-known for having shorted 'switches' which are painful to repair. Fortunately mine tests good with no shorts.

And so to the logic assembly. I reconnected it to the PSU and monitor and noted it behaved the same as before. I removed the 2 upgrade daughterboards and refitted the Z80 processor and character generator EPROM to their sockets on the mainboard. Oh and fitted a couple of jumper links to connect the single-density-only data separator to the floppy controller chip. I now had a base model Osborne 1A. Powered up again and it did the same things, so there was certainly a fault on the mainboard.

Running that bit of the ROM firmware the machine only uses the top 16K of RAM both for the firmware (stack, etc) and video display. Checking the RAM data outputs showed one bit that was changing betwee 'floating' and 'high', it never went low like all the others. I suspected that chip but it's best not to jump to conclusions.

One disadvantage of the Osborne is that the firmware ROM is only I/O drivers and the disk bootstrap. There is no machine code monitor. The manual points out that this is not a problem, you can load DDT or similar from disk. Well, yes, provided the machine is working properly. A machine code monitor is very useful on a non-working machine to do various tests...

No matter. I have a Z80 in-circuit emulator (A Zax ICD278-Z80). Plug that in in place of the Z80 chip in the Osborne and power up. Now at power up, the Osborne processor can access the top 48K of RAM, the bottom 16K is mapped out and replaced by the firmware ROM and memory mapped I/O. But for the moment being able to access the top 48K is good enough.

I find I can read the firmware ROM (so the emulator is indeed accessing the Osborne hardware and I am using it correctly). THe RAM from 4000-7FFF and from 8000-BFFF seems fine. The top 16K has a stuck bit and it's the chip I suspected all along. I replace it.

As an aside at this point I'll mention that the Osborne PCBs are very good quality and that I had no problems desoldering and removing ICs with no damage to either the chip or the board.

OK, refit the Z80 and power up again. I get the startup screen at last. So refit the daughterboards ( need the double-density disk upgrade at least), power up and it still seems to work. Connect a disk drive and and try to boot CP/M

It tries and then fails with 'BDOS error on T: select'. I now suspect RAM problems in the first 16K of RAM where CP/M would store system variables. Reconnect the in-circuit emulator and work out from the Osborne circuit diagrams how to map all the RAM in. A write to Z80 port 1 will do it. So get the emulator to do that and check I can no longer read the firmware ROM. I can't, so I try to read and write the first 16K of RAM. I find 2 dead bits. I replace that pair of 4116s, refit the Z80 in place of the emulator and try to boot it again.

It boots. CP/M, the help system, STAT, etc all work. I now have a non-portable Osborne -- a load of bits filling most of the bench.

I clean up the case parts. Take the keyboard apart, remove the keycaps and clean those. Let everything dry then reassemble the keyboard and refit the drives, the monitor, the mains harness and the PSU to the chassis/upper case.

I decide to look at the I/O ports. There's a minimal RS232 port (it only does 300 and 1200 baud) and a rather nice parallel port. This appears on a 26 pin card edge on the front of the mainboard. It's wired so that a straight-through cable will connect it to an IEEE-488 device and there are software drivers for that. But it can also be used as a Centronics port and as an 8 bit strobed input port.

Since suitable edge connectors are getting hard to find, I decide to make a cable from the one such connector that I can find to a 26 pin header socket. This I do but when I check it out I find a couple of signals that seem to be shorted to 0V. I suspect my wiring, but have the sense to check the Osborne mainboard with no cable connected. The signals are shorted there. I desolder the ICs they go to and find a shorted 74LS32 and 7438. My guess from the signals used is that the machine was connected to a Centronics printer which had a fault and zapped those chips. No matter, I have replacements.

Fit those, fit the logic to the chassis and refit the lower half of the case. Try the machine again, it boots fine and will copy the 4 distribution disks (CP/M, BASIC, Wordstar, Supercalc)that came with it. I try the serial port, which works. I try the Centronics output which does the right things on a logic analyser.

So refit the front bezel to complete the job -- for the moment. I want to do some more work on the parallel port...
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Old 1st May 2023, 8:20 am   #2
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Default Re: Osborne 1A CP/M luggable computer

Well done. A very interesting account. It would be nice to see some pictures.
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Old 1st May 2023, 1:31 pm   #3
TonyDuell
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Default Re: Osborne 1A CP/M luggable computer

I did take many photos of it in bits and will put them on flickr when I get a chance. I'll put a few here as well just for completeness.
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Old 4th May 2023, 10:30 am   #4
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Default Re: Osborne 1A CP/M luggable computer

Here are photos of the 5 main PCBs. The main logic board, 80 column uprade, double density disk upgrade, power supply and video monitor.

Lots more photos of the machine here :

https://www.***********/photos/tony_d...77720301441294
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Old 4th May 2023, 10:33 am   #5
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Default Re: Osborne 1A CP/M luggable computer

And some photos of the disk drive. The Osborne PCB, the bits of the positioned mechanism (to give some idea as to how far I went), top and bottom of the assembled Siemens mechanism and the the assembled drive with its PCB
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Old 4th May 2023, 10:39 am   #6
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Default Re: Osborne 1A CP/M luggable computer

And now some pictures of the assembled monitor and logic assemblies, the home-made adapter to connect the drive to a standard system and of aligning the disk drive
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Old 4th May 2023, 10:41 am   #7
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Default Re: Osborne 1A CP/M luggable computer

Finally, finding the memory faults with a Z80 in-circuit emulator, the 'non portable Osborne' -- bits spread out on the bench, It running CP/M, and the assembled machine.
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Old 4th May 2023, 10:45 am   #8
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Default Re: Osborne 1A CP/M luggable computer

Thanks.

Rather you than me. I know my limits!
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Old 4th May 2023, 11:00 am   #9
TonyDuell
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I refuse to be beaten by a bit of machinery
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Old 4th May 2023, 11:01 am   #10
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Default Re: Osborne 1A CP/M luggable computer

Well done Tony, a very thorough job indeed.

Nice to see the old Amstrad portable again. We bought one at work when they came out as a portable way to check our DEC ports for connectivity. Even with alkaline ā€œDā€ cells the battery life was only a few minutes. Therefore it was hardly ever used.

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Old 4th May 2023, 11:08 am   #11
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I find it moderately amusing that the Amstrad 'laptop' is actually wider than a PDP11 front panel . I've never used it on batteries, it came without a power supply but I wired upt a transformer/bridge rectifier/smoothing capacitor for it. I really only use it now to run the Microtest system. I've removed the second disk drive and fitted a DC37 socket wired to the floppy drive cabling so I can plug in the drive under test.

My portable terminal of choice is the HP95LX that you can see sitting on top of the Z80 in circuit emulator in one of the photos.
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