My 1982 Acorn Atom

I recently obtained my first Acorn Atom, another 1980s 8-bit computer to add to my collection. The machine was in very good, tested condition and came with several tasteful (and reversible) modifications to improve reliability and usability.

The Acorn Atom was produced by Acorn Computers Ltd from 1980 to 1982, when it was replaced by the BBC Micro. The Atom was a progression of the MOS Technology 6502-based machines that the company had been making from 1979, a cut-down Acorn System 3 without a disk drive but with an integral keyboard and cassette tape interface, sold in either kit or complete form.

The Atom wasn’t sold for very long or in large quantities – there are usually a couple up on eBay at any one time, but they often command high sale prices.

The machine was in very good, tested condition and came with several tasteful (and reversible) modifications to improve reliability and usability, as well as its original parts.

Firstly, it had been modified to use a regulated 5Vdc supply input, which requires bypassing the two internal 7805 linear regulators by fitting LK6 and LK7 on the mainboard. This is a common modification, as the internal regulators produce a lot of heat and struggle to power a fully-populated board, and can even burn the PCB.

As such, I use a regulated 5Vdc 4A PSU which I’ve modified to use the correct jack for the Atom (5.0/2.5mm, unlike the more standard 5.5/2.5mm), wired centre-positive.

Secondly, it had a ROM/RAM board installed (Atom Internal 32 RAM/ROMbox with Noisekiller, Mk 2.1), which replaces and upgrades the original system RAM to 32KB, replaces the original ROM, and allows the system to run at either 1MHz or 1.7MHz.

Thirdly, it had a video board installed (Atom Colour, Noisekiller and VidRAM, Mk 3.5LS), which replaces the original video RAM, and adds an RGB video output (conforming to the Acorn standard 6-pin DIN) and a monochrome composite video output.

Both of these are fitted into existing sockets on the board; the video board also requires a connection to the video header PL4 via a small ribbon cable.

Finally, it had an SD card interface installed (AtomMMC v2.0), which allows software to be easily loaded onto the Atom (a software archive is available here). This requires a connection to the system header PL8 via a ribbon cable.

I didn’t test the Atom immediately, as there were some items that I wanted to address first.

For one, the Atom Colour board uses a ribbon cable for its additional video outputs, which fits neatly through a gap in the back of the case – the previous owner had modified this cable to add a monochrome composite video output via an RCA jack, but it was not a neat looking job, so I reworked it using a bare plug. This cable can easily be removed from the header on the Atom Colour board to return the Atom to a “stock” look.

I also didn’t like the look of having the SD card daughterboard on the back of the case, so I moved it to the lower inside (secured with mounting tape).

I also inspected the mainboard before powering it on, and I noticed that the three jumper wires required by the Atom Colour board were not insulated – I replaced these in a more professional manner using wrapping wire.

After this, I tried powering on the machine, and it started up to a garbage screen.

On pressing the “BREAK” key, a BASIC prompt appeared with an “ACORN ATOM” header, but there was no cursor and the keyboard was unresponsive.

Apparently, the garbage screen on startup is normal for some Atoms – however, with an AtomMMC installed the startup banner should show “ACORN ATOM + ATOMMC2”.

The garbage screen on startup is apparently due to the system not resetting properly. The Atom reset circuit uses R38 (1kOhm) and C11 (10uF) to generate a reset pulse on power-on or when the BREAK key is pressed – on this board, C11 wasn’t even fitted, so I installed one (a 22uF 16V axial electrolytic capacitor) and the Atom now seemed to reset properly.

It’s also possible to improve the reliability of the original design by replacing the RC circuit (R38 and C11) with a solid-state device, such as the Dallas DS1813-5.

The Atom had been tested by the previous owner prior to shipping, so something had obviously happened since to cause it not to boot – I double-checked the work I’d done on it since it arrived, and everything was as it should be. I tried again without the AtomMMC installed in case this was causing problems, but there was no change in symptoms.

I noticed that the factory sockets on the Atom are of an incredibly poor quality, and I thought that these could be causing an intermittent connection, most likely on the ROM/RAM board. As such, I desoldered the two sockets that the ROM/RAM board uses (IC22 and IC44) using my desoldering station (a Duratool D00672), and installed replacements.

I always use high-quality double-sided sockets, which are more reliable than cheap single-sided sockets as they contact the IC legs on both sides – a lot of people push the use of turned-pin sockets, but I don’t like using them as they make swapping ICs difficult, they are difficult to desolder, and they are visibly obviously non-standard.

I also soldered the link wire between the two add-on boards, which ensures that only one noise-killer circuit is active at any one time.

After doing so, the Atom now seemed to boot up correctly, and all the keys worked.

However, just because a computer boots, doesn’t mean that it’s working properly. Thorough testing is necessary to verify correct operation, so I did as much testing as I could.

  • Machine powers on to a normal startup screen with cursor showing.
  • Monochrome composite video output works OK.
  • RGB video output works OK.
  • All keys register correctly.
  • Internal speaker works OK.
  • Computer reads and writes correctly from/to an external tape unit.
  • Computer loads correctly from internal SD card interface.

At this point, I did some finishing up: I thoroughly cleaned the mainboard with compressed air and an ESD-safe brush; I thoroughly cleaned the case inside and out using Cillit Bang general-purpose degreaser, a microfibre cloth for large areas, and a toothbrush for small areas; I also disassembled and cleaned the keyboard.

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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