Acorn Risc PC 600 Capacitor Guide

This guide lists the capacitors on the Acorn Risc PC 600 main PCB (board 0197,000) and covers the recap. The values, designators and Acorn part numbers are taken directly from the Acorn Risc PC Technical Reference Manual main-PCB assembly parts list.

Acorn Risc PC 600 motherboard (0197,000). The surface-mount aluminium electrolytics scattered across the board leak with age and corrode the pads and tracks.

⚠️ Safety

The Risc PC 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 that hold charge — treat it as any mains switch-mode supply.

Why the Risc PC must be recapped

The Risc PC main board uses surface-mount aluminium electrolytic capacitors. As on the Acorn Archimedes A5000 and the Acorn A4 laptop, the electrolyte leaks with age and corrodes the pads and tracks, producing progressive and intermittent faults — the Power On Self Test "RAM control line failure" code (00020000) is a classic result of leaked electrolyte attacking the RAM-control tracks (see Acorn Risc PC 600 Troubleshooting Guide). Replacing the surface-mount electrolytics and cleaning the board is the key long-term maintenance task.

Capacitor list (recap targets)

The recap targets are the aluminium electrolytics and tantalums below. The many ceramic decouplers (33 nF "DCPLR", and the 100 pF/1 nF/etc. "CML" parts) are stable and are left in place.

Risc PC main PCB — surface-mount aluminium electrolytics

Value	Voltage	Designators	Qty
10 µF	16 V	C29, C53, C59, C60, C86, C90, C91, C133, C145, C164, C176	11
47 µF	16 V	C61, C74, C79, C87, C93, C116, C123, C136, C139, C172	10
4.7 µF	25 V	C70, C122	2
1 µF	50 V	C134	1

Risc PC main PCB — surface-mount tantalums

Value	Voltage	Designators	Qty
10 µF	16 V	C12, C64	2
47 µF	10 V	C88	1

Risc PC main PCB — through-hole radial electrolytics

Value	Voltage	Designators	Qty
220 µF	16 V	C2, C65, C73, C152, C161, C162	6
100 µF	25 V	C168, C175	2

The surface-mount aluminium electrolytics are the leakage-prone parts and the priority for replacement; the tantalums and the radial electrolytics are replaced on a full recap. (C68, C76, C89 and C137 are listed as "not fitted" in the parts list.)

Recap procedure

1. Disconnect the mains lead; remove the CPU card(s) and the drive slice, and lift out the main PCB.
2. Photograph the board and record every electrolytic (designator, value, voltage, polarity). The polarity band on a surface-mount electrolytic must agree with the marked pad.
3. Remove each surface-mount electrolytic with hot air, or by adding fresh solder to both terminals and rocking the part off with the joints molten. Do not lever a cold leaking part — it tears pads.
4. Clean the corrosion: neutralise and wash the residue (isopropyl alcohol; a mild alkaline clean then IPA for heavy electrolyte), inspect each pad and track, and repair corroded or open tracks (particularly the RAM-control lines) with fine wire before fitting new parts.
5. Fit new 105 °C parts of equal capacitance and equal-or-higher voltage, observing polarity. A reversed electrolytic fails short — double-check every one.
6. Re-inspect under magnification for bridges and lifted pads.

On-board fuses and PSU

While the board is out, check the on-board surface-mount fuses FS1 (2 A) and FS2 (800 mA) for continuity. The mains PSU is a separate module; if it is unstable, service it only if competent with mains switch-mode supplies (discharge the primary bulk capacitor first), or exchange it. Replace the NiMH backup battery (BT1) at the same time if it has not been done (see Acorn Risc PC 600 Maintenance Guide).

Post-recap verification

1. Reassemble; reconnect the mains lead.
2. Power on and watch the POST: the screen should pass through purple/blue/purple and start RISC OS (black screen with the memory-size message), with no red screen or floppy-LED fault code.
3. Confirm sound is restored and run a memory check.

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

Note for the Risc PC 700

The Risc PC 700 uses the revised 1208,000 board (with on-board 16-bit sound). It shares the same surface-mount-electrolytic recap profile; the list above (from the original 0197,000 board) is the basis, but verify the sound-related positions against the actual 700 board. See Acorn Risc PC 700 Capacitor Guide.

Related pages

• Acorn Risc PC 600
• Acorn Risc PC 600 Maintenance Guide
• Acorn Risc PC 600 Troubleshooting Guide
• Acorn Risc PC 700 Capacitor Guide · Capacitor Failure Symptoms