Acorn A5000 Capacitor Guide

Acorn A5000 motherboard. The surface-mount aluminium electrolytics scattered across this board are the machine's defining failure point: they leak corrosive electrolyte onto the pads and tracks.

This guide covers recapping the Acorn Archimedes A5000. The A5000 is the Acorn 32-bit machine most in need of a recap: its main board is surface-mount, and the small surface-mount aluminium electrolytic capacitors leak with age and corrode the board. The exact per-designator values are on the on-wiki Acorn A5000 Circuit Diagrams (from the A5000 Technical Reference Manual); this guide explains the leakage, the procedure, and how to read the values.

⚠️ Safety

The A5000 has a fan-cooled mains switch-mode PSU. Disconnect the mains lead before opening the case. Board-level capacitor work is low voltage once the supply is disconnected, but the PSU module carries mains-side capacitors; Acorn ships it sealed and prohibits opening it. Re-apply the earth-continuity and DC-insulation (class 1) safety tests after any internal work.

Why the A5000 must be recapped

The A5000 main board uses surface-mount aluminium electrolytic capacitors. As they age the electrolyte escapes, and because it is corrosive it attacks the pads and tracks beneath and around each capacitor. The result is the A5000's characteristic set of ageing faults: corrupted RAM control lines, loss of sound, instability, and eventual failure to start. The damage is progressive, so a machine that still works is not safe to leave — the longer leaked electrolyte sits on the board, the more track repair it needs later. Recapping (and cleaning the residue) is the single most valuable maintenance task on an A5000.

This is the same class of fault seen on the Acorn A4 laptop and the Acorn Risc PC; the earlier through-hole-capacitor machines (Acorn Archimedes A3000, A3020, Acorn Archimedes A4000) are not affected in the same way.

Visual inspection

Inspect the board under magnification:

• Look for crusty or discoloured residue around the base of each surface-mount electrolytic, and for darkened or green-tinged pads and tracks.
• Check the laminate near each capacitor — leaked electrolyte wicks under solder mask and corrodes tracks out of sight.
• Pay attention to the areas around the audio circuitry and the RAM control lines, where leakage most often shows up as a functional fault.

Capacitor values

Because the A5000 board is surface-mount, Acorn documented the machine at module level and there is no separate tabular component parts list in the Service Manual. The per-designator capacitor values are given on the A5000 Technical Reference Manual circuit diagrams, which are hosted on this wiki at Acorn A5000 Circuit Diagrams. For example, the TRM specifies C169 as a 10 µF 16 V tantalum.

The board's recap targets are the small surface-mount aluminium electrolytics (values in the roughly 1–47 µF range at 16–35 V) and the tantalums. Rather than reproduce a per-designator table that cannot be verified from a machine-readable source, the reliable method is to work from the circuit diagram and read each fitted part: record the designator, value, voltage and polarity of every electrolytic before removing it, and replace like-for-like. This avoids publishing unverified values and matches the fact that board revisions (25 MHz 0192,100 and 33 MHz "Alpha" 0192,420) can differ.

Recap procedure

1. Disconnect the mains lead; remove the drive tray and lift out the main PCB.
2. Photograph the board and record every surface-mount electrolytic (designator, value, voltage, polarity). The polarity band on a surface-mount electrolytic and the marked pad on the silkscreen must agree.
3. Remove each electrolytic. Twisting a leaking surface-mount electrolytic off can tear pads, so prefer hot air, or add fresh solder to both terminals and rock the part off with the joints molten. Do not lever a cold part.
4. Clean the corrosion: neutralise and wash the residue (isopropyl alcohol; a mild alkaline clean followed by IPA for heavy electrolyte), and inspect each pad and track. Repair corroded or open tracks with fine wire before fitting the new parts.
5. Fit new 105 °C parts of equal capacitance and equal-or-higher voltage, observing polarity. Surface-mount aluminium electrolytic or tantalum replacements are both used; a common, costly mistake is fitting a replacement reversed, which fails short — double-check every one.
6. Re-inspect under magnification for bridges and for any pad lifted during removal.

PSU module

The A5000's mains switch-mode PSU is a sealed, fan-cooled module supplying +5 V, −5 V and +12 V, with a 5 A mains fuse. Acorn ships it as a non-serviceable assembly. If it is unstable, the supported fix is exchange. If you are competent with mains switch-mode supplies and choose to recap it, unplug from the mains, discharge the primary bulk capacitor before touching the primary side, replace the electrolytics and any cracked mains-suppression (RIFA-style) capacitor with correctly-rated parts read off each fitted part, and re-apply the safety tests afterwards.

Post-recap verification

1. Reassemble; reconnect the mains lead.
2. Power on and confirm RISC OS reaches the desktop with the normal start-up.
3. Check the +5 V, −5 V and +12 V rails.
4. Run the audio and memory functional tests; confirm sound is restored and RAM is clean (Acorn A5000 Troubleshooting Guide).
5. Re-apply the earth-continuity and DC-insulation safety tests before returning the machine to service.

If a previously-working function fails after the recap, re-check the polarity of every replaced capacitor first.

Related pages

• Acorn Archimedes A5000
• Acorn A5000 Maintenance Guide
• Acorn A5000 Troubleshooting Guide
• Acorn A5000 Circuit Diagrams — per-designator capacitor values
• Acorn Archimedes A4000 Capacitor Guide · Capacitor Failure Symptoms