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 mains transformer with an 18 Vac output, but these are difficult to find – however, due to the design of the internal power supply board, it is possible to power it from a DC supply instead if necessary.

The Electron power supply board has AC rectification (via a full-wave bridge rectifier), smoothing capacitors, and DC regulation (via a 7805 linear regulator) – as such, if you feed DC into the power supply board, only half of the bridge rectifier will be used. This side will run marginally hotter, but that shouldn’t matter too much in practice. It won’t even matter what polarity the DC supply is (centre-negative or centre-positive) – this will just determine which side of the bridge rectifier gets used.

An 18 Vdc PSU with a 2.1mm DC jack would be optimal (as this matches the voltage of the original AC PSU), but if you’re not using any expansions it’s possible to use a 9 Vdc PSU instead, either a modern regulated one or an older unregulated one (i.e. a ZX Spectrum PSU). The 7805 regulator can handle a DC input ranging from 7 Vdc to 25 Vdc, but it will run hotter at higher voltages.

I’m not sure whether powering an Electron via DC would be compatible with all expansions, as the AC input is fed out through the expansion connector, so it would depend on how the AC voltage gets used by a specific expansion.

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

My name is Adam Wilson - I'm an electronics engineer based in the North East of England, UK, and I like tinkering with old junk. In my spare time, I collect, repair, refurbish, and (sometimes) sell vintage computer systems and peripherals, typically from the 1980s (the likes of Commodore, Sinclair, Acorn, Apple, Amstrad, and Atari).

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