Acorn Atom General Maintenance
The Acorn Atom (1980–1983) has no disk drive, no CRT and no mains wiring inside the case. Routine care is mostly cleaning, reseating socketed ICs and checking the supply, across the double-sided main PCB (part 202,000), the keyboard, the tape interface, the two +5 V regulators and the external 8 V DC adaptor.
Safety
[edit | edit source]The system unit runs on 8 V DC from an external adaptor and has no mains voltage inside.[1] Mains voltage is present only inside the adaptor. Check an old adaptor's case and cable for damage before use; any supply that meets the figures below can replace it. The MOS ICs can be damaged by static, so work earthed.[1]
Identifying your board
[edit | edit source]The PCB is marked 202,000 with an issue number; Chris's Acorns photographs a basic Issue 4 board.[2] The two sides are marked SIDE 1 and SIDE 2. All IC sockets, connectors PL/SK1–7 and passive components are on side 2 and soldered on side 1; PL8 and the keyboard are on side 1 and soldered on side 2.[1]
The kit came with 52 IC sockets, one for every position, but only 24 socketed ICs plus the two regulators. These include two pairs of 2114 RAM (IC42/IC43 and IC51/IC52, 2 KB) and the 8 KB system ROM, an MM52164 at IC20. The empty sockets take the expansion RAM, the floating-point and utility ROMs, the 6522 VIA and the bus buffers.[1]
Opening the case
[edit | edit source]- Unplug the power lead from SK3 and disconnect the TV and tape leads.
- Turn the machine over and remove the two long screws in the base. Lift the base off. The component side (side 2) is now exposed, with the board still fixed in the top half of the case.[1]
- To reach the solder side, remove the four screws at the corners of the board and lift the board and keyboard out of the top moulding together.[1]
- The keyboard is held to the PCB by six self-tapping screws with insulating washers that stop the screws shorting tracks. Keep the washers if the keyboard is ever removed.[1]
To reassemble, fit the board loosely in the top with the four screws, move it until every key clears the cut-out, tighten the screws, then refit the base so the connectors sit centrally in their openings.[1]
Regular cleaning
[edit | edit source]- Brush dust from the board and keyboard and blow it out with low-pressure air.
- Clean the tape socket SK2 and the power socket SK3 with contact cleaner on a swab.
- Reseat each socketed IC: lift it slightly and press it home, checking that no pin folds under the IC. Acorn's assembly notes warn about bent pins when fitting ICs to their sockets.[1]
- If PL6 (the 64-way bus connector) or PL5 (the 26-way printer header) is fitted, check its pins for corrosion and bridges before connecting anything.
Keyboard
[edit | edit source]The 8255 (IC25) scans the key matrix through the 7445 decoder (IC26), which drives the ten rows in turn while the 8255 reads the columns.[1] A dead row or column therefore points at the track between the matrix and IC25 or IC26, or at a cracked joint on a key lead.
The key leads are soldered to the sides of large plated-through holes. When reflowing them, do not fill the holes with solder: Acorn warns that the solder can contract as it cools and break the plated-through connection.[1] Because each key is soldered to the board, replacing a switch means desoldering it.
Tape interface
[edit | edit source]The tape interface uses CUTS tones of 2.4 kHz and 1.2 kHz at 300 bits per second, generated from the 4 MHz crystal. The input passes through an amplifier (the LM358, IC46) and a Schmitt trigger to the 8255.[1][3]
- Use a 3-pin DIN lead in SK2: pin 1 out to the recorder, pin 2 common, pin 3 in from the recorder. Pins 4–7 of SK2 carry 8255 outputs, and Acorn warns they may damage a recorder with a 5- or 7-pin connector.[1]
- Input and output are both set to 300 mV RMS. To change the recording level, change R29 so that the recorder's meter reads 70 % (−3 dB). To raise the input gain for a hi-fi deck, change R34 from 47 kΩ to 220 kΩ.[1]
- Acorn recommends recorders with line input and output over those with only microphone and earpiece sockets.[1]
Power supply and voltage checks
[edit | edit source]The Atom takes 8 V DC on SK3. Two LM340T-5 regulators, IC53 and IC54, each supply one half of the board, with a heatsink between them and the PCB. C12 (22 µF) decouples the 8 V input.[1][3]
| Configuration | Current |
|---|---|
| Minimal system, 2 KB RAM | 750 mA |
| 10 KB expansion memory at 75 mA per KB | 750 mA |
| Bus buffers and 6522 VIA | 100 mA |
| Fully expanded, low-power RAM | 1600 mA |
Acorn's high-efficiency adaptor delivers up to 1600 mA, enough for a fully expanded Atom with low-power 2114-L RAM. Atoms supplied before October 1980 used normal-power RAM at 100 mA per KB, and a fully expanded one of those draws more than the adaptor can supply.[1] Chris Whytehead's Acorn adaptor is labelled 8 V 1.8 A.[2]
Checks:
- Measure the input at SK3 with the machine running. An LM340-5 needs at least 7.5 V in to hold regulation, so an adaptor that sags under load gives a low or noisy +5 V.[4]
- Measure +5 V at the output of each regulator; each feeds a separate half of the board. The datasheet limit for the part is 4.75–5.25 V.[4]
- Check that the heatsink is fitted and that the regulator screws do not short to nearby tracks.[1] The regulators run hot in normal use.[5]
Running from a regulated 5 V supply
[edit | edit source]Fit link LK6 to join the two halves of the supply and LK7 to bypass the regulators. SK3 then takes regulated 5 V, and the 8 V adaptor must never be plugged in; Acorn asks for a label saying so beside the socket.[1] The manual gives a circuit for a 5 V 3 A supply (9 V 50 VA transformer, bridge rectifier, 1500 µF 16 V capacitor, LM323K on a heatsink) able to run a fully expanded Atom with an Acorn Eurocard.[1] Some owners made this conversion to get rid of the regulator heat.[5]
Connectors
[edit | edit source]| Connector | Use | Note |
|---|---|---|
| SK1 | UHF modulator output | Channel 36 |
| SK2 | 7-pin DIN, tape and port C outputs | Use a 3-pin lead |
| SK3 | Power in, 8 V DC (5 V with LK6 and LK7) | Two lugs soldered, third connection a wire link |
| PL4 | 10-pin header: composite video (pin 9, 0 V on pin 10) and colour signals | 1 V into 75 Ω |
| PL5 | 26-way printer header (3M 3429-1302) | Needs the 6522 at IC1 and a 74LS244 at IC50 |
| PL6 | 64-way Acorn bus and VIA ports (Vero 17-3704L type) | Needs bus buffers IC2–IC5 |
| PL7 | Acorn bus for one internal Eurocard | In parallel with PL6 |
| PL8 | 20-way extension plug on side 1 | +5 V on pin 10, 0 V on pin 11 |
Sources: Technical Manual pp. 11, 13, 22–23 and the circuit diagram.[1][3]
Capacitors
[edit | edit source]The main board has eight small electrolytics (C2–C5, C8 and C11–C13), all 10 or 22 µF, and ceramic capacitors elsewhere. Acorn allows any value from 10 to 47 µF at 10 V or more in the electrolytic positions.[1] The list and procedure are on Acorn Atom Capacitor Replacement Guide.
RAM and ROM
[edit | edit source]Each 1 KB of RAM is a pair of 2114s at a fixed address, so a fault that appears at one address range points at one pair:[1]
| ICs | Addresses (hex) | Use |
|---|---|---|
| IC51, IC52 | 0000–03FF | Block zero (fitted in every Atom) |
| IC10, IC11 to IC18, IC19 | 2800–3BFF, 1 KB per pair | Lower text space |
| IC42, IC43 | 8000–83FF | Video and upper text (fitted in every Atom) |
| IC40, IC41 to IC32, IC33 | 8400–97FF, 1 KB per pair | Video graphics extension |
The lower text space pairs are added in order from IC10/IC11, and the Atom uses lower text space automatically if RAM is present there at power-up or reset.[1] The system ROM is IC20; the floating-point ROM goes at IC21 and the utility ROM at IC24.[1]
Clocks
[edit | edit source]X1 (3.58 MHz) runs the MC6847 video generator through an oscillator on IC9 (74LS04). X2 (4.00 MHz) runs an oscillator on IC45 (74LS04) that feeds the 74LS393 divider IC44, from which the 1 MHz processor clock and the tape tones are derived.[1][3] If +5 V is good and the machine is dead, check both oscillators before replacing LSI parts.
Recommended tools
[edit | edit source]- Cross-head screwdriver and an earthed wrist strap.
- Multimeter for the 8 V and 5 V checks.
- Contact cleaner and swabs.
- Temperature-controlled soldering iron, solder wick and fine flux-cored solder.
- A spare 2114 for substitution.
See Recommended Tools for the general toolkit.
Preventive maintenance checklist
[edit | edit source]- Inspect the adaptor case and cable, and check the SK3 joints.
- Measure the 8 V input under load and +5 V at both regulator outputs.
- Check the heatsink and regulator mounting.
- Reseat the socketed ICs.
- Clean SK2 and SK3, and the PL5 and PL6 pins if fitted.
- Use only a 3-pin tape lead.
Related pages
[edit | edit source]- Acorn Atom
- Acorn Atom Troubleshooting Guide
- Acorn Atom Capacitor Replacement Guide
- Acorn Atom Technical Manual
- Capacitor Failure Symptoms
- Recommended Tools
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 Acorn Computers, Atom Technical Manual, issue 2, October 1980, hosted on this wiki. Parts list pp. 6–7; construction notes pp. 7–12; TV, power supply and links p. 13; tape interface pp. 14–15; memory map p. 20; opening the case and RAM extensions pp. 21–22; connectors p. 23.
- ↑ 2.0 2.1 Whytehead, Chris. "Acorn Atom", Chris's Acorns, archived by The Centre for Computing History. Basic Atom with motherboard 202,000 Issue 4; Acorn mains unit rated 8 V 1.8 A DC; IC location diagram from The Acorn Atom Review.
- ↑ 3.0 3.1 3.2 3.3 Acorn Computers, Circuit Diagram for Atom Microcomputer, drawn 28 February 1980, revised to issue 3 on 14 January 1981. Hosted on this wiki as File:Acorn Atom circuit diagram.png.
- ↑ 4.0 4.1 Texas Instruments, LM340, LM340A and LM7805 Family Wide VIN 1.5-A Fixed Voltage Regulators, SNOSBT0L, revised September 2016, section 6.6 (LM340/LM7805, VO = 5 V): output 4.8–5.2 V at 25 °C; 4.75–5.25 V for 7.5 V ≤ VIN ≤ 20 V and 5 mA–1 A; input required to maintain line regulation 7.5 V; dropout 2 V typical at 1 A.
- ↑ 5.0 5.1 "Acorn Atom", Wikipedia (retrieved 2026-10-01): the two internal regulators "got uncomfortably hot", and some owners bypassed them and ran the Atom from an external 5 V supply.
