Acorn Archimedes A3020 Capacitor Guide

A3020 main PCB assembly drawing (Technical Reference Manual). The component-placement sheet gives the reference designator for each fitted capacitor; the matching values are on the circuit-diagram sheets.

This guide covers the capacitors on the Acorn Archimedes A3020 (ARM250 "Heron E" board, part 0194,500), which carries its mains power supply on the main PCB. The A3020 is not a routine recap machine: the usual source of corrosion is the backup battery, not the capacitors. Recap the board only where a capacitor is visibly aged, and treat the on-board PSU section as the one part worth planned attention.

⚠️ Safety

The A3020's PSU is built onto the main board, so the board carries mains voltages. Disconnect the mains lead before opening the case and discharge the PSU primary bulk capacitor before working near the power (left-hand) end of the board. Re-apply the earth-continuity and DC-insulation (class 1) safety tests after any internal work.

Is the A3020 a recap candidate?

The board's logic is surface-mount, but its electrolytics are mostly conventional leaded parts, plus a small number of surface-mount aluminium electrolytics and many surface-mount ceramic decouplers. It is not one of the Acorn machines that suffer routine surface-mount electrolyte leakage — those are the A5000, the A4 laptop and the Risc PC. The recap priorities are therefore the on-board PSU electrolytics and any individually aged or leaking board part — not a blanket board recap.

The real corrosion problem on the A3020 is the NiCd backup battery (see the Acorn Archimedes A3020 Maintenance Guide), which leaks and destroys tracks whether or not the capacitors are healthy.

Board capacitor list

Acorn documented the A3020 at module level, so there is no per-designator parts list in the Service Manual. The values below are the electrolytic and tantalum families listed on Acorn's own parts list for the shared A3010/A3020/A4000 board (the Training Reference Manual bill of materials). The list is transcribed from a scanned bill of materials, so confirm each value and designator against the fitted part or the Acorn A3010, A3020 and A4000 Technical Reference Manual circuit diagram before ordering.

A3020 main-board electrolytic and tantalum capacitors (from Acorn's parts list)

Value	Voltage	Type	Notes
10 µF	16 V	Radial aluminium electrolytic (leaded)	General supply filtering
47 µF	16 V	Radial aluminium electrolytic (leaded)	Several fitted
220 µF	16 V	Radial aluminium electrolytic (leaded)	Several fitted; local rail smoothing
100 µF	25 V	Radial aluminium electrolytic (leaded)	×2
4.7 µF	16 V	Radial aluminium electrolytic (leaded)	—
47 µF	16 V	Surface-mount aluminium electrolytic	A few fitted; check for base leakage
10 µF	16 V	Surface-mount aluminium electrolytic	A few fitted; check for base leakage
10 µF	10 V	Tantalum bead (leaded, 5 mm pitch)	Decoupling
10 µF	16 V	Tantalum (surface-mount)	Decoupling
47 µF	—	Tantalum (surface-mount)	Decoupling

The many surface-mount ceramic decouplers (33 nF and smaller values such as 47 nF, 1 nF and various picofarad parts) are stable and are not replaced. Do not copy the Risc PC capacitor list onto the A3020 — they are different boards.

On-board PSU

The PSU section is part of the main PCB and includes the mains transformer, the primary bulk electrolytic, the low-voltage smoothing electrolytics for the +5 V and +12 V rails, and mains RFI-suppression capacitors on the input. If the machine will not start reliably from cold, cuts out when warm, or shows ripple on its rails, the PSU electrolytics are the place to look.

Working on the PSU section:

• Unplug the mains lead and discharge the primary bulk capacitor before touching the primary side.
• Replace any smoothing electrolytic that is bulged, leaking or out of tolerance, reading the value, voltage and polarity off the fitted part.
• Replace any cracked mains-suppression capacitor with a correctly-rated safety (X/Y-class) part. Acorn's documentation does not name the suppression-cap brand fitted, so inspect the actual parts and replace on condition.
• Re-apply the earth-continuity and insulation safety tests afterwards.

Replacement procedure (board electrolytics)

1. Disconnect the mains lead; discharge the PSU primary capacitor; lift out the board.
2. Record each electrolytic's value, voltage and polarity (the silkscreen marks the positive pad); the PCB assembly drawing gives the designators.
3. Wet both leads with fresh flux and solder, lift each end in turn, and clear the holes with solder wick.
4. Fit the new part with correct polarity — a 105 °C low-ESR aluminium electrolytic of equal capacitance and equal-or-higher voltage.
5. Use a temperature-controlled iron and short dwell times; the board is multi-layer, so avoid lifting pads.
6. Inspect for clean joints and no bridges.

Post-recap verification

1. Reassemble and reconnect the mains lead.
2. Power on and confirm RISC OS 3.11 reaches the desktop.
3. Check the +5 V and +12 V rails are in tolerance and free of excessive ripple.
4. Run the audio, video, floppy and hard-disc functional tests.
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 A3020
• Acorn Archimedes A3020 Maintenance Guide
• Acorn Archimedes A3020 Troubleshooting Guide
• Acorn Archimedes A4000 Capacitor Guide — three-box sibling (separate PSU module)
• Acorn Archimedes A3000 Capacitor Guide — discrete-chip predecessor (full parts list)
• Capacitor Failure Symptoms