Acorn Electron Repair & Restoration

A little while back, I was given a 1984 Acorn Electron by a friend – it had just been pulled out of his parents’ loft, so as you could imagine, it was in pretty rough shape. I’ve owned a few Electrons now, and they’re fun to work on, so I decided to fix it up.

As you can see, the Electron was in pretty rough shape – however, it appeared to be complete and didn’t have any visible damage, so was a good candidate for restoration.

It was so mucky with loft crud that I decided to give it a quick clean before I’d dare to even touch it. Then, I decided to pull it apart and check the voltages on the internal power supply before trying to power it up, to avoid the possibility of damage.

The Electron appeared to be a bog-standard Issue 4, nothing special here. From this side, the board doesn’t appear to have had any previous rework.

I immediately found a problem that would prevent the computer from working – the power socket was very loose, and wiggled around significantly.

All three solder joints on the power socket were so dry and cracked that the socket was barely held onto the PCB, so there was no way that power would reliably transfer from the external PSU to the internal PSU – this was probably due to a combination of poor quality solder from the factory, and heavy use.

I pulled out the original DC jack, and replaced it with a new one – I usually do this when restoring a computer, as the originals tend to get tarnished and loose. I made sure to put plenty of solder on the new jack, and I resoldered all the joints on the board.

I also pulled the mainboard, so I could check for any potential problems before attempting power-on. It turned out that the board had, in fact, seen rework in the past – there was a lot of flux residue underneath the 6502 CPU and the ULA, which would not have been there from the factory. All the work looked OK apart from the flux remnants, so I cleaned off the underside of the board with IPA, and cleaned the top of the board with compressed air and an anti-static brush, to remove any dirt and dust.

I also resoldered the joints on the power header, cleaned both sides of the edge connector using a white eraser, and cleaned all of the ports using contact cleaner.

Acorn Electron mainboard and internal power supply reinstalled.

I reassembled the computer in preparation for testing with a suitable power supply.

The original Electron PSU is a simple transformer with an 18Vac output, but these are difficult to find – however, because the Electron has internal rectification, it’s possible to power it from a DC supply instead, and it won’t even matter what polarity the supply is. An 18Vdc PSU with a 2.1mm DC jack would be optimal, but if you’re not using any expansions it’s possible to use a 9Vdc PSU instead (i.e. a ZX Spectrum PSU).

I powered the Electron up for the first time, hooked up to my TV using an RGB SCART video cable – the power LED came on and the computer gave a normal start-up beep, but nothing was displayed on-screen (no signal). That’s weird.

I switched over to the composite video output and tried again – this time, the Electron seemed to boot up correctly, and displayed video as expected.

Acorn Electron booting correctly over the composite video output.

If the composite video output worked, that meant that the RGB signals were being correctly generated by the ULA. I checked the continuity of all signals to the RGB video connector against the schematic, and they all seemed OK; I checked that LK3 was set to GND to make sure that CSYNC was being generated correctly, and it seemed OK; I checked the CSYNC output signal from the ULA, and it seemed OK.

Then, I noticed that the RGB SYNC signal is generated by pin 11 of U14 (74LS86). I checked this pin using my logic probe, and the signal was constantly low – this meant that the attached screen would never display video, and that U14 was likely faulty. I removed U14 using my desoldering station and tested it using my MiniPro TL866II – sure enough, gates 2 and 4 of the IC had failed. I installed a socket and replaced U14, then tested the machine again, and the RGB video output now worked perfectly.

Acorn Electron mainboard with U14 socketed and replaced.

I noticed that a couple of the keyswitches were a bit unreliable – I pulled all the keys from the keyboard (a simple job using my keycap puller, as there aren’t any springs underneath the keys like there are on many computers), then sprayed some contact cleaner into the top of the offending switches, then worked them up and down several times, which seemed to clear up the problem. I also removed the keyboard from the upper case, and gave it and all of the keycaps a thorough clean.

Then, it was time for some testing.

Finally, I gave the whole case a thorough clean using Cillit Bang – I use a microfibre cloth for non-patterned plastic, and a toothbrush to get into all the nooks and crannies on patterned plastic. After rinsing the case off and reassembling the computer, it came up really well – it was unrecognisable in comparison to how it was before.

I also installed a new set of rubber feet, as one was missing and the others were sticky.

And with that, the restoration was complete!

Published by themightymadman

A conscientious, intelligent and committed graduate engineer, with excellent interpersonal skills, an eye for detail and a keen interest in hardware design, mathematics, and software development.

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