Acorn Archimedes A4000 Maintenance Guide

Acorn A4000 "Roadrunner" main PCB (part 0194,600). The ARM250 is the large square package; the two RISC OS 3 ROMs sit beside it, with the RAM and expansion-RAM cards at lower right and the IDE/floppy connectors on the right edge.

This guide covers preventive maintenance for the Acorn Archimedes A4000 (launched September 1992). The A4000 is a three-box ARM250 machine sharing its "Roadrunner" main board with the A3010 and A3020. It has an internal IDE hard disc, a separate cased mains switch-mode PSU, and a highly integrated board with few serviceable parts. The maintenance priorities are the mains PSU, the CMOS backup battery, and the ageing IDE hard disc.

⚠️ Safety: mains PSU

The A4000 is powered by a separate cased switch-mode PSU inside the unit. The Service Manual is explicit that the PSU must not be opened or repaired: do not attempt to repair or modify the PSU — exchange it as a unit, because opening it invalidates the safety tests applied at manufacture. After any work, Acorn requires the full earth-continuity and DC-insulation (class 1) safety tests to be re-applied.

• Disconnect the mains lead before opening. The A4000 is unusually easy to open — two screws release the cover.
• Observe anti-static precautions for the ARM250 and the RAM.

The machine and its board

The ARM250 integrates the four chips that were separate on the Acorn Archimedes A3000 — the ARM2 core, MEMC1a, VIDC1a and IOC — into one package running at 12 MHz. Because of this, there are no socketed processor-family chips: a failure inside the ARM250 means replacing the whole device. The board is part 0194,600 ("Roadrunner"), the same board used by the A3020.

Acorn A4000 key facts

Item	Detail
Processor	ARM250 @ 12 MHz (integrates ARM2 + MEMC1a + VIDC1a + IOC)
RAM	2 MB standard, expandable to 4 MB
ROM	RISC OS 3.11 (2 ROMs)
Storage	3.5" floppy (Citizen OSDA-75G-U) + internal IDE hard disc
Hard disc (by model)	AGC20 80 MB, AGC40 105 MB (Conner CFS105A), AGC50 210 MB (Conner CFS210A)
Expansion	one mini-podule slot; optional Econet
PSU	separate cased switch-mode module

⚠️ CMOS backup battery

The A4000 keeps its configuration and clock in battery-backed CMOS RAM, with the cell at the top-right of the board. As on every Acorn machine of this era, an ageing backup battery can leak and corrode the surrounding tracks. Inspect it on any unrestored machine, clean any residue with isopropyl alcohol, check the nearby tracks for continuity, and replace the cell. Lost configuration, a wrong clock or boot problems are the usual symptoms of a failed cell.

Opening and cleaning

1. Unplug the mains lead and peripherals.
2. Remove the two cover screws and slide the cover off. Disconnect the floppy and hard-disc data and power cables and the PSU connector to lift the board.
3. Brush and air-blow dust from the board, the PSU vents, the hard disc and the floppy mechanism. Good airflow matters because the hard disc and PSU share the case.
4. Clean the mini-podule edge connector with a soft eraser; clean the keyboard, parallel, serial and video sockets with contact cleaner on a foam swab.

Hard disc

The internal IDE drives are Conner units (CFS105A / CFS210A). After decades, the common faults are mechanical: a drive that will not spin up (stiction) or that has become noisy. The IDE interface and the floppy share the on-board I/O. If replacing the drive, note that RISC OS 3.1's IDE filing system limits usable capacity to roughly 512 MB; larger drives, CompactFlash or SD adapters need third-party software or a podule. Keep a backup of the original drive's contents where possible.

Power supply checks

The PSU delivers +5 V (logic), +12 V (the video connector SK1 pin 12 and the disc motors) and −5 V. The Service Manual's "action if PSU fails" routine is the safe way to diagnose supply problems:

1. Switch off and unplug the mains lead.
2. Check the 5 A fuse in the mains plug. If a replacement blows on switch-on, the PSU is faulty — exchange it.
3. Check the internal fuse FS1.
4. If the PSU runs then shuts down, suspect thermal shutdown; if it keeps shutting down, the PSU is faulty.

Do not open the PSU module. A repeatedly-failing PSU is exchanged, not repaired.

Capacitor health

The A4000 board uses conventional leaded electrolytics plus surface-mount ceramic decouplers, and is not one of the Acorn machines that suffer leaking surface-mount electrolytics (those are the Acorn Archimedes A5000, the A4 laptop and the Risc PC). The recap priorities are therefore the PSU module and any visibly aged board electrolytic. Full detail: Acorn Archimedes A4000 Capacitor Guide.

Functional and safety testing

Acorn's service procedure uses the ARM250 Dealer Test Disc, which runs audio, battery-backed RAM, disc-interface, external-port, hard-disc, IDE, joystick, keyboard/mouse, memory, real-time-clock, UHF-modulator and video tests, and produces a test report. After any repair the earth-continuity and DC-insulation (class 1) safety tests must be re-applied because the A4000 is mains-earthed equipment.

Common failure points

Acorn A4000 — failure-prone items

Item	Symptom	Action
Mains PSU module	Dead, random resets, thermal shutdown	Check plug fuse and FS1; exchange the PSU (do not open it)
CMOS battery (top-right)	Lost config/clock; corrosion	Clean, repair tracks, replace cell
IDE hard disc (Conner)	Won't spin up, noisy, boot errors	Mechanical failure; replace drive (mind the ~512 MB RISC OS 3.1 limit)
Mini-podule edge connector	Expansion unreliable	Clean fingers; reseat
ARM250	No combination of CPU/MEMC/VIDC/IOC works	Whole-device replacement (no socketed sub-chips)

Recommended tools

• Posidrive/Philips screwdrivers and an anti-static strap.
• DC multimeter (rail checks); oscilloscope for bus-level diagnosis.
• Isopropyl alcohol, foam swabs, soft eraser, contact cleaner.
• Replacement CMOS battery; replacement IDE drive or solid-state adapter.
• Earth-continuity and insulation testers if returning the machine to service (per Acorn's safety tests).

See Recommended Tools for the general toolkit.

Preventive maintenance checklist

1. Inspect/replace the CMOS battery; clean any corrosion.
2. Check the plug fuse and FS1; verify the +5 V, +12 V and −5 V rails.
3. Clean the mini-podule edge connector and the floppy head.
4. Back up and assess the IDE hard disc.
5. Re-apply earth-continuity / insulation safety tests after any internal work.

Related pages

• Acorn Archimedes A4000
• Acorn Archimedes A4000 Troubleshooting Guide
• Acorn Archimedes A4000 Capacitor Guide
• Acorn Archimedes A3000 — the earlier discrete-chip machine
• Acorn A3010, A3020 and A4000 Service Manual · Acorn A3010, A3020 and A4000 Technical Reference Manual · Acorn A3010, A3020 and A4000 Training Reference Manual
• Capacitor Failure Symptoms · Recommended Tools