Acorn Atom General Maintenance

Acorn Atom main PCB (part no. 202,000 Issue 4) in base configuration. The empty sockets are for the RAM, ROM and 6522 VIA expansions.

This guide documents preventive maintenance for the Acorn Atom (1980–1983), covering the single double-sided main PCB (part 202,000), the integral keyboard, the cassette (CUTS) interface, the on-board +5 V regulators and the external 8 V DC mains adaptor. The Atom is a 6502-based 8-bit machine derived from the Acorn System 3; it has no internal disk drive, no CRT and no internal mains wiring, so most routine maintenance is cleaning, contact integrity, socket reseating and supply-voltage verification.

Safety

The Atom system unit contains no mains voltage and no CRT. It is powered from a low-voltage external adaptor that delivers 8 V DC (unregulated) to the 2.1 mm power jack (SK3); the only mains wiring is inside the sealed adaptor, which is not user-serviceable. The board can be opened, cleaned and serviced at the bench with the adaptor unplugged. Standard anti-static precautions apply because the machine uses NMOS LSI parts (6502, MC6847, INS8255) and 2114 static RAM that are sensitive to ESD.

Do not open or attempt to repair the moulded mains adaptor. If the adaptor is suspect, substitute a known-good 8 V DC (or 5 V regulated) supply of adequate current rather than dismantling the original.

Identifying your board

The Atom main PCB carries the part number 202,000 on the silkscreen, followed by an issue number. The construction (Technical) Manual documents Issue 2 (October 1980); production boards are commonly seen up to Issue 4. The board is double-sided, with the two sides marked SIDE 1 and SIDE 2 on the silkscreen: all IC sockets, the connectors PL/SK 1–7 and the passive components are mounted on side 2 (and therefore soldered on side 1), while PL8 and the keyboard are mounted on side 1 and soldered on side 2.

Acorn Atom component-location diagram, viewed from side 2, showing the IC positions used throughout this guide. (Source: The Acorn Atom Review, via Chris's Acorns.)

Almost every Atom shipped with empty sockets for later expansion. The base machine fits a minimum of four 2114 static RAMs (IC42, IC43, IC51, IC52 — 2 KB) and one 8 KB mask ROM (IC20, an MM52164 holding the operating system and Atom BASIC); the remaining RAM, the floating-point ROM, the utility ROM and the optional 6522 VIA printer interface go into the empty sockets.

Opening the case

Tools: posidrive/Philips screwdriver, anti-static strap.

1. Unplug the 8 V adaptor from SK3 and disconnect the UHF lead and any tape lead.
2. Turn the machine over onto a soft cloth and remove the four short and two long self-tapping case screws.
3. Lift the upper (keyboard) moulding away from the base. The keyboard is soldered directly to side 1 of the PCB, so the board and keyboard come away as one assembly.
4. Handle the board by its edges. The ICs sit in sockets; avoid flexing the board, which can crack solder joints on the through-plated holes that carry the keyboard leads.

Regular cleaning

• Clear dust from the board, the keyboard and the sockets with a soft brush and low-pressure compressed air.
• Clean the gold-plated fingers of the rear edge connector (PL8, the 64-way Acorn bus) and the printer/VIA connector with a soft eraser or a deoxidising contact cleaner on a foam swab. The Atom's edge fingers are exposed at the rear of the case and oxidise faster than the rest of the board.
• Clean the 7-pin DIN tape socket (SK2) and the UHF socket (SK1) with contact cleaner on a foam swab, sparingly.
• Reseat every socketed IC. Lift each chip a fraction and press it firmly back to wipe the contact. Intermittent faults on a 40-plus-year-old socketed machine are far more often a dirty socket than a dead chip.

Keyboard maintenance

The keyboard is a full-travel mechanical assembly soldered directly to the PCB. It is read as a matrix by the INS8255 PPI (IC25), with the 7445 BCD-to-decimal decoder/driver (IC26) driving the row-select lines and the 74LS138 decoders (IC23/IC30) handling address decode. The CTRL, SHIFT and REPT (repeat) keys and the BREAK key are part of this matrix.

• Single dead key: clean the switch contacts. Because the keys are soldered to the board, replacement means desoldering the individual switch from a donor.
• Whole row or column dead: suspect a broken track between the key matrix and IC25/IC26, or a cold joint on one of the key leads. Reflow the through-plated keyboard joints — do not flood them with solder, because the manual warns that solder contracting in the plated-through hole can break the connection.
• The Atom firmware provides software debouncing in the MOS ROM; a key that double-types or chatters after cleaning points to a worn switch rather than a software fault.

Cassette (CUTS) interface

The Atom stores data to an external cassette recorder over a 7-pin DIN lead (SK2) using the CUTS variant of the Kansas City standard at 300 baud only. There is no internal cassette deck and no motor relay — the interface is purely the audio read/write circuit built around the LM358 op-amp (IC46) and transistors Q1/Q2 (BC107). Maintenance is limited to keeping SK2 clean and verifying the read/write levels if loading is unreliable; the mechanical condition of the tape and the external recorder's own head alignment account for most loading problems.

Commercial BBC Micro cassettes cannot be loaded on an Atom: they run at 1200 baud and the Atom interface is 300 baud.

Power supply and voltage checks

The Atom takes 8 V DC on the 2.1 mm centre jack (SK3) and regulates it on the board to +5 V using two LM340T-5 (7805-type) regulators, IC53 and IC54, each feeding one half of the digital logic, with a heatsink fitted between them. Both regulators and the heatsink are fitted to every Atom.

Acorn Atom supply current budget (per Technical Manual)

Configuration	Current
Minimal 2 KB system	750 mA
10 KB expansion RAM (low-power), at 75 mA/KB	750 mA
Bus buffers + 6522 VIA	100 mA
Fully expanded total	~1.6 A (low-power RAM)

The genuine Acorn adaptor is rated 8 V at roughly 1.5–1.8 A, which is marginal for a fully expanded machine; a fully populated Atom (especially one fitted with the pre-October-1980 normal-power RAM at 100 mA/KB) can draw up to 3 A and exceed the adaptor. Random crashes or failure to start on an expanded machine are frequently an under-rated supply rather than a board fault.

Checks:

• Measure +5 V at the regulator outputs (and at the VCC pins of the ICs). It should read 5 V ±5%.
• Both 7805-type regulators run hot by design. Verify the heatsink is fitted and seated; a regulator running into thermal shutdown causes intermittent resets as it cools and recovers.

Running from a 5 V regulated supply

The Atom can be powered from an external 5 V regulated supply instead of 8 V DC. This requires fitting two links: LK6 joins the two on-board power sections, and LK7 bypasses the regulators. SK3 then expects regulated 5 V and the 8 V adaptor must not be connected — the manual instructs that this be clearly labelled at the rear of the machine. This conversion is the usual way to power a fully expanded Atom reliably, because a modern 5 V switch-mode supply of several amps removes the regulator heat and the adaptor current limit at once.

Connector and socket care

• SK3 (2.1 mm DC jack): the power input. Mechanically reflow its joints; the pin is soldered with one connection made by a wire link, and the joint fatigues with plug insertions.
• SK1 (UHF modulator) and the optional composite output: keep clean; the modulator is channel 36.
• SK2 (7-pin DIN): tape I/O.
• PL8 / rear 64-way edge connector: the Acorn bus for Eurocard expansion. Clean the fingers and verify there are no bridged contacts before inserting an expansion card.
• Printer / 6522 VIA header: fitted only on expanded machines.

Capacitor health

The Atom main PCB carries only small electrolytics (eight, in the 10–47 µF range) plus ceramic decouplers; there is no large mains-side reservoir and no CRT supply, so a recap is low-risk bench work. The eight electrolytics are 40-plus years old and are reasonable pre-emptive replacements when a board is already open. Full designator list and procedure: Acorn Atom Capacitor Replacement Guide.

ROM and RAM health

• ROM (IC20, MM52164 mask ROM): mask ROM does not degrade like UV-EPROM. A boot that hangs or shows a corrupt character pattern is more often a dirty ROM socket — reseat IC20 first.
• RAM (2114 static RAM, IC42/43/51/52 in the base machine, more in the expansion sockets): the 2114 is a common failure part. A single bad 2114 produces corrupted characters or crashes at specific addresses. Reseat first, then substitute a known-good 2114.

Common failure points

Acorn Atom — failure-prone components

Component	Designator	Symptom	Quick check
LM340T-5 (7805) regulators	IC53, IC54	Intermittent resets when warm; dead +5 V rail	Measure +5 V; confirm heatsink fitted
2114 static RAM	IC42, IC43, IC51, IC52 (+ expansion)	Corrupt text/graphics, crashes	Reseat, then substitute known-good 2114
MC6847 VDG	IC31	No or intermittent display	Reseat; check +5 V at VDG; verify 3.58 MHz at X1
INS8255 PPI	IC25	Keyboard and/or cassette dead, display OK	Reseat; check matrix lines to IC26 (7445)
Rear edge connector (Acorn bus)	PL8	Expansion unreliable; machine fails with card fitted	Clean fingers; check for bridged contacts
DC power jack	SK3	Dead or intermittent power	Reflow joints; verify 8 V present

Clocks

The Atom uses two crystals: X2, 4.00 MHz, divided down (by the 74LS163 / 74LS393 counters, IC47/IC44) to the 1 MHz 6502 clock, and X1, 3.58 MHz, for the MC6847 video timing. If the machine is completely dead with +5 V present, verify both crystals are oscillating before condemning any LSI part.

Recommended tools

• Posidrive/Philips screwdriver and anti-static strap.
• Digital multimeter (for the +5 V checks).
• Deoxidising contact cleaner and a soft eraser for the edge connector.
• Temperature-controlled soldering iron with a fine tip and solder wick, for reflowing keyboard and connector joints.
• A known-good 8 V DC adaptor (≥1.6 A), or a 5 V regulated supply of several amps for the LK6/LK7 conversion.
• Known-good 2114 RAM for substitution testing.

See Recommended Tools for the general vintage-repair toolkit.

Preventive maintenance checklist

1. Inspect and reflow the SK3 power-jack joints.
2. Verify +5 V at the regulator outputs; confirm the heatsink is fitted.
3. Reseat all socketed ICs (ROM, RAM, MC6847, 8255).
4. Clean the rear edge-connector fingers and the DIN/UHF sockets.
5. If the machine is heavily expanded, convert to a 5 V regulated supply (LK6/LK7) or use an adequately rated 8 V adaptor.
6. Consider the small-electrolytic recap while the board is open (Acorn Atom Capacitor Replacement Guide).

Related pages

• Acorn Atom
• Acorn Atom Troubleshooting Guide
• Acorn Atom Capacitor Replacement Guide
• Acorn Electron — Acorn's later low-cost 8-bit machine
• BBC Micro Model A/B — the Atom's successor
• Capacitor Failure Symptoms
• Recommended Tools