It's taken me 27 years to get an original Osborne 1, largely because they always sell for £LOL. However, offer me a broken one for a good price and that's what I call a challenge. It's a single board Z80 machine choc-full of everyone's favourite triple-voltage RAM chip, the 4116 DRAM. To add more fun all 33 chips (yes, not 32) are made by ITT which these days have a high failure rate. No fault on ITT of course, they wouldn't have expected idiots like me to still be trying to make their stuff work 44 years after it was manufactured.

I went to the December 2025 Bring&Byte sale at the computer museum purely as a social visit because I knew a lot of the sellers and I had some things to give people. I was NOT buying things. I LEFT with this Osborne 1 and some Sideways ROM/RAM boards for the BBC Micro. Go me. Label on the top of the Osborne said 'RIFAs in PSU removed, 4 screws missing and no sign of life when powered up.' OK let's go.

First off, PSU. Osbornes use my nemesis, a supply beloved by system builders in the early 80s. It's the Astec AC8151 and you'll find them in Osbornes, STC Executels, Memotech FDX floppy drives, Tandy TRS80 models 2 and 3, the list goes on. The RIFAs had indeed been replaced, but their spooge still needed to be cleaned up. That was done and new caps fitted; to my utter surprise the supply fired right up without upsetting my current limiter and lit up my test brakelight bulb which has both 5V and 12V filaments.

Before hooking the PSU up to the board I checked the voltage rails for shorts and discovered 12V was indeed shorted. In machines of this age this is usually caused by tantalum capacitors being used as decouplers to quieten down noise on a power rail. Tantalums have a high failure rate and either fail open circuit or go short resulting in a little explosion. There's only 5 tants on this board and typically it was the 5th that was bad. Swapped it with a new one and I had resistance across the rails once again.

Attached the PSU and switched on. Definitely no sign of life, and it soon became apparent why - Osbornes have a front-facing connector that allows you to use a bigger external composite monitor. When this isn't in use you need to fit a loopback plug which was missing. Thankfully I have an Executive too, so I borrowed its loopback plug and got life on the screen. It wasn't 'good' life, it was garbage but it showed that at minimum there was a good clock and the character generator was working.

Armed with the schematic and my trusty oscilloscope I set about probing the address and data buses, control lines, /RESET circuit etc. Half the data bus (D4-7) looked wild on the scope while everything else looked fine. While all the repair blogs screamed 'RAM' at me I didn't fancy desoldering and testing 33 chips so eliminated everything else I could think of first, helped by dismantling the Exec too so I could part swap between the two. Of course, in the end it HAD to be RAM and I noticed one of the chips had been replaced in the past. That was the ideal place to start and it turned out that that chip was bad. Great.

Over one morning I desoldered 272 pins and found 5 bad chips, all 5 being in the top half of the data bus as I predicted. I'd been doing test powerups after each column was tested and was overjoyed to see the happy welcome screen after only 4 columns. So was my left arm which had been wielding the desoldering gun for 4 hours straight.

Next, keyboard. Osborne 1's are notorious for their early keyboard design known as a 'Full Travel Membrane" - yep, an early membrane keyboard. These fail over the years and are rendered useless by their materials breaking down. People do attempt repairs, and I may still in the future, but in recent years others have instead designed replacement PCBs with proper keyswitches. It's expensive but it's a road I'd have gone down if I didn't have the Executive to steal its keyboard from.

Naturally, I didn't have any boot disks or any knowledge on how the disk bus works. In the Osborne they designed their own and moved the usual Shugart signals around meaning you can't just replace a bad drive with another eg Shugart SA400. You'll put 12V up it in the wrong direction if you do, because the 34 pin Osborne cable carries the voltages needed by the drive. Fortunately adapters are available for Goteks and other floppy drive replacement devices.

Quizzed some folk on the Short Circuit Discord and was given the clue that helped me sort the Gotek out - each drive has a resistor network (RN3) which signals the bus number of the drive. If RN3 is present the drive is DS0, otherwise it's DS1+. Four drives are supported. This means both the Gotek and signal converter need to be strapped as drive 1 to work in place of the right hand drive. The machine booted!

I did try to troubleshoot the floppy drives and wrote an SSSD floppy disk with my Greaseweazle, but since they both had RN3 installed and were from different manufacturers I reasoned that the original drives had been used as spares for another machine because this machine had died through bad RAM. Two DS0's don't half upset the bus, so I attempted pulling RN3 from the right hand drive (it was socketed) and it promptly fell to bits. Splendid. Both drives are toast, perhaps unsurprisingly. Neither of them read which hints at either physical read head or read amplifier issues, and the left hand drive (now definitely DS0) doesn't even seek. It makes me wonder if they were both damaged by someone putting the drive cable on backwards resulting in 12V going where it shouldn't. Since the Gotek works I didn't pursue it any more.

So! The machine works as long as you give it another keyboard and a floppy drive replacement :oD I'll take that.