Acorn A5000 Maintenance Guide

Acorn A5000 motherboard (25 MHz, board 0192,100). Four RISC OS 3 ROMs (with an empty fifth socket), the ARM3, MEMC1a, VIDC1a and IOC, the IDE/floppy connectors at lower right, the podule-backplane connector, and the battery pack.

This guide covers preventive maintenance for the Acorn Archimedes A5000 (launched September 1991). The A5000 is an ARM3 machine with a surface-mount main board, an internal mains switch-mode PSU, and an internal IDE hard disc. Three things dominate A5000 maintenance: the mains PSU, the rechargeable battery pack (which leaks), and the surface-mount electrolytic capacitors (which also leak and corrode the board).

⚠️ Safety: mains PSU

The A5000 contains a fan-cooled mains switch-mode PSU. It runs directly from the mains, and its primary side, reservoir capacitor and mains-suppression capacitors can hold a lethal charge even after the machine is unplugged — a shock from the primary side can kill you. Disconnect the mains lead before opening the case, and never work on the supply while it is plugged in.

Acorn's service manual told dealers to exchange the whole PSU module rather than repair it, but that assumed replacement modules were available; none have been made for decades, so on a surviving machine the supply is recapped, not discarded. This is a legitimate restoration job, but only for someone competent with mains switch-mode supplies: discharge the reservoir capacitor and confirm 0 V before touching the primary side (procedure in the Acorn A5000 Capacitor Guide). Re-apply the earth-continuity and DC-insulation (class 1) safety tests after any internal work, because the A5000 is mains-earthed equipment.

The machine

The A5000 was Acorn's mid-range ARM3 machine, an offshoot of the Acorn A4 portable project. It introduced several changes over the earlier Archimedes and A3000: a 15-pin VGA monitor connector (replacing the 9-pin RGB), a PC-standard serial port (replacing RS423), a bidirectional parallel port, a high-density floppy (the Acorn 1.6 MB F format, plus MS-DOS 1.44 MB), and an on-board IDE controller (replacing the ST506/MFM interface). It was the first Acorn computer to ship with RISC OS 3.

Acorn A5000 key facts

Item	Detail
Processor	ARM3 @ 25 MHz (4 KB cache); 33 MHz on the 1993 "Alpha"
Support chips	MEMC1a, VIDC1a (VL2303A), IOC
RAM	2 MB or 4 MB (one MEMC1a addresses 4 MB; 8 MB needs a second MEMC1a, Alpha only)
ROM	RISC OS 3 (2 MB; four ROMs + an unused fifth socket)
Storage	HD floppy (1.6 MB) + on-board IDE hard disc
I/O	PC-style Super I/O (IDE, floppy, serial, parallel); 15-pin VGA; bidirectional parallel
Expansion	4-podule backplane (0192,004)
Option	FPA10 floating-point accelerator (socketed, near the battery)
PSU	internal fan-cooled mains switch-mode module

⚠️ The battery pack: check it first

The A5000 keeps its configuration and clock in CMOS RAM, backed by a rechargeable battery pack on the board, near the FPA socket. These packs leak with age, and the electrolyte corrodes the surrounding tracks. Inspect the pack on any unrestored machine; if it has leaked, remove it, neutralise and clean the residue with isopropyl alcohol, check the nearby tracks for continuity, and repair any breaks before refitting power. Replace the pack (period packs can be substituted with a modern cell and holder, or a CR2032 + diode/holder modification). Lost configuration, a wrong clock, or boot problems point to a flat or removed pack.

⚠️ Surface-mount capacitor leakage

The A5000 main board is surface-mount, and its small surface-mount aluminium electrolytic capacitors are the machine's defining long-term fault. As they age the electrolyte escapes and corrodes the pads and tracks beneath and around each capacitor, which causes intermittent and progressive faults (corrupted RAM control lines, loss of sound, instability and outright failure to start). On any A5000 that has not been recapped, plan to replace the surface-mount electrolytics and clean the board; full procedure and the verified data available: Acorn A5000 Capacitor Guide.

Opening and cleaning

1. Unplug the mains lead and peripherals.
2. Remove the cover. The PSU is at one rear corner, with the floppy and hard disc on a removable drive tray.
3. Lift the drive tray (it hooks into slots on the case sides and is held by a single screw) to reach the board; disconnect the IDE, floppy and power cables first.
4. Brush and air-blow dust from the board, the PSU fan and vents, and the drives. Keep the PSU fan aperture clear — the Service Manual notes a blocked aperture as a cause of PSU shutdown.
5. Clean the four podule backplane connectors and the monitor, serial and parallel sockets with contact cleaner.

Hard disc

The internal IDE drives (Conner, Fujitsu and others, from 40 MB up) are now decades old and the usual faults are mechanical (failure to spin up, bearing noise). When fitting a replacement, 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. Back up the original drive where possible.

Power supply checks

The PSU is a self-contained fan-cooled module. Start with the Service Manual's "action if PSU fails" routine:

1. Switch off and unplug the mains lead. Check the 5 A fuse in the mains plug. If a new fuse blows on switch-on, the PSU is faulty.
2. For an individual rail failure, check the rails at the faston power connectors: +5 V (main PCB, floppy, hard disc), −5 V (backplane), +12 V (video connector SK1 pin 12, and the disc motors).
3. If the PSU runs then shuts down, check the fan and the fan aperture; a failed fan or blocked aperture causes thermal shutdown. Repeated shutdowns mean a faulty PSU.

A faulty or ripple-prone PSU is recapped; Acorn's original exchange-only advice assumed spare modules that no longer exist. ⚠️ The PSU primary side and its reservoir and mains-suppression capacitors can hold a lethal charge after the machine is unplugged — a shock can be fatal. Only open the PSU if you are competent with mains switch-mode supplies, and discharge the reservoir capacitor and confirm 0 V with a meter before touching the primary side (procedure in the Acorn A5000 Capacitor Guide).

RISC OS version

The first A5000 models (ALB10/12/14) shipped with RISC OS 3.00, which has a serious ADFSBuffers bug that can corrupt discs; on a 3.00 machine, set ADFSBuffers to 0, or upgrade to RISC OS 3.10/3.11. Check the version with *FX 0 and upgrade ageing 3.00 ROMs to the stable 3.11 set.

Common failure points

Acorn A5000 — failure-prone items

Item	Symptom	Action
Surface-mount electrolytics	Sound loss, RAM-control faults, instability, corrosion	Recap; clean residue; repair corroded tracks
Battery pack	Lost config/clock; corrosion near FPA socket	Clean, repair tracks, replace pack
Mains PSU module	Dead, random resets, fan/thermal shutdown	Check plug fuse; clear fan aperture; exchange PSU
IDE hard disc	Won't spin up, noisy, boot errors	Replace drive (mind the ~512 MB RISC OS 3.1 limit)
RISC OS 3.00 ROMs	Disc corruption	Set ADFSBuffers 0 or upgrade to 3.11
Podule backplane / contacts	Expansion unreliable	Clean contacts; reseat backplane and podules

Recommended tools

• Posidrive/Philips screwdrivers and an anti-static strap.
• DC multimeter (rail checks); oscilloscope for bus-level work.
• Hot-air or fine-tip soldering equipment for the surface-mount recap, plus solder wick, flux and isopropyl alcohol.
• Replacement battery/holder; replacement IDE drive or solid-state adapter.
• Earth-continuity and insulation testers if returning the machine to service.

See Recommended Tools for the general toolkit.

Preventive maintenance checklist

1. Inspect/replace the battery pack; clean any corrosion.
2. Recap the surface-mount electrolytics and clean the board (Acorn A5000 Capacitor Guide).
3. Check the plug fuse; verify +5 V, −5 V and +12 V; confirm the PSU fan runs.
4. Confirm the RISC OS version; upgrade 3.00 or set ADFSBuffers 0.
5. Clean the backplane contacts; back up and assess the IDE drive.
6. Re-apply the safety tests after internal work.

Related pages

• Acorn Archimedes A5000
• Acorn A5000 Troubleshooting Guide
• Acorn A5000 Capacitor Guide
• Acorn Archimedes A4000 — the ARM250 sibling
• Acorn A5000 Service Manual · Acorn A5000 Circuit Diagrams
• Capacitor Failure Symptoms · Recommended Tools