Acorn Archimedes A4000 Capacitor Guide

Acorn A4000 "Roadrunner" main PCB. The logic is surface-mount, but the electrolytics are conventional leaded parts; the A4000 is not affected by the SMD electrolyte leakage of the A5000/A4/Risc PC.

This guide covers the capacitors on the Acorn Archimedes A4000 (ARM250 "Roadrunner" board, 0194,600) and its separate cased PSU. Unlike the Acorn Archimedes A3000, Acorn documented the A4000 only at module level, so there is no machine-readable per-component parts list; the exact per-designator values are on the Acorn A3010, A3020 and A4000 Technical Reference Manual circuit diagrams, which are hosted on this wiki.

Safety: mains PSU — risk of fatal shock

The A4000's power supply runs directly from the mains. Its primary side, the reservoir (bulk) capacitor and the mains-suppression capacitors can hold a lethal charge even after the machine is unplugged, and a shock from the primary side can kill you. Always disconnect the mains lead before opening the case, and never work on the supply while it is plugged in.

Recapping the PSU is a legitimate part of restoring these machines. Acorn's original service manual told dealers to exchange the whole PSU module rather than repair it, but that advice assumed replacement modules were on the shelf; none have been made for decades, so for restoration the supply is recapped, not discarded. Only attempt PSU work if you are competent with mains switch-mode supplies. Before touching the primary side, discharge the reservoir capacitor through a suitable bleeder resistor and confirm 0 V with a meter, and treat the X/Y mains-suppression capacitors as charged until proven otherwise. After any internal work, re-apply the earth-continuity and DC-insulation (class 1) safety tests before returning the machine to service.

Is the A4000 a recap candidate?

The A4000 board is almost entirely surface-mount logic, but its electrolytics are conventional leaded parts plus surface-mount ceramic decouplers — it is not one of the Acorn machines that suffer from leaking surface-mount electrolytic capacitors. Those are the Acorn Archimedes A5000, the A4 laptop and the Risc PC; the through-hole-capacitor machines such as the A3020/A4000 are generally unaffected. On the A4000 the recap priorities are therefore the PSU module and any individually aged or leaking board electrolytic, not a blanket board recap.

Board capacitors

The A4000 main board ("Roadrunner", 0194,600) carries:

• Surface-mount ceramic decouplers of 33 nF and 100 nF across the supply pins of the ICs — these are stable and are not replaced.
• A small number of leaded aluminium electrolytics on the supply-filtering, audio and video sections. These are the recap targets.

Because Acorn published the A4000 only at module level, there is no official tabular parts list giving each electrolytic's designator and value. The exact values are legible on the Acorn A3010, A3020 and A4000 Technical Reference Manual circuit-diagram sheets (on the wiki), and can be read directly off each fitted capacitor. When recapping, record the value, voltage and polarity of each electrolytic before removing it and replace like-for-like with a 105 °C low-ESR part of equal capacitance and equal-or-higher voltage. This guide does not reproduce a per-designator table rather than risk publishing unverified values.

PSU module

The A4000 uses a separate cased switch-mode PSU supplying +5 V, +12 V and −5 V, with an internal fuse FS1 and thermal-shutdown protection. Acorn originally handled a faulty supply by exchanging the whole module, but with no replacement modules made for decades the practical fix for an unstable supply (random resets, ripple, thermal cut-outs) is to recap it. Observe the mains-shock warning at the top of this page: the supply is dangerous until discharged.

1. Unplug the mains lead.
2. Open the PSU case and discharge the reservoir (bulk) capacitor through a bleeder resistor; confirm 0 V with a meter before touching the primary side.
3. Replace the smoothing electrolytics, reading the value, voltage and polarity off each fitted part; use 105 °C low-ESR parts of equal capacitance and equal-or-higher voltage.
4. Replace any cracked or leaking mains-suppression (RIFA-style) capacitor with a safety-rated X/Y-class part of the same rating — never an ordinary capacitor.
5. Reassemble and re-apply the earth-continuity and DC-insulation (class 1) safety tests before returning the machine to service.

Replacement procedure (board electrolytics)

1. Disconnect the mains lead; remove the two cover screws and lift out the board.
2. Record each electrolytic's value, voltage and polarity (the silkscreen marks the positive pad).
3. Wet both leads with fresh solder/flux, lift each end in turn, and clear the holes with solder wick.
4. Fit the new part with correct polarity; solder and trim. The A4000 board is multi-layer, so use a temperature-controlled iron and short dwell times to avoid lifting pads.
5. Inspect for clean joints and no bridges.

Post-recap verification

1. Reassemble; reconnect the mains lead.
2. Power on and confirm RISC OS 3.11 reaches the desktop.
3. Check the +5 V, +12 V and −5 V rails.
4. Run the audio and video functional tests; confirm the hard disc and floppy.
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 electrolytic first.

Related pages

• Acorn Archimedes A4000
• Acorn Archimedes A4000 Maintenance Guide
• Acorn Archimedes A4000 Troubleshooting Guide
• Acorn Archimedes A3000 Capacitor Guide — the discrete-chip predecessor (full cap list)
• Capacitor Failure Symptoms