Acorn Archimedes A3000 Capacitor Guide

This guide lists the capacitors on the Acorn Archimedes A3000 main PCB and covers the recap. The values, designators and Acorn part numbers come directly from the Acorn A3000 Service Manual PCB assembly parts list.^[1]

⚠️ Safety

The A3000 contains a mains-powered 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 itself carries mains-side capacitors that can hold charge — treat the PSU module with the same caution as any mains switch-mode supply (see the PSU section below).

Is the A3000 a recap candidate?

The A3000 board uses leaded aluminium electrolytics and tantalums, not the leaking surface-mount electrolytics of the later Acorn Archimedes A5000, A4 laptop and Risc PC. The board therefore ages gently; the urgent items on an A3000 are the CMOS backup battery (which leaks and corrodes the bottom-left corner — see Acorn Archimedes A3000 Maintenance Guide) and the PSU module. A board recap is still reasonable on a 35-year-old machine that is already open, or to chase ripple-related instability.

Visual inspection

Check each electrolytic for a bulged or vented top, leaked electrolyte, a cracked sleeve, or a tired joint. The radial electrolytics cluster around the supply and audio sections; the two axial parts (C39, C83) are 47 µF/10 V.

Acorn A3000 capacitor list (electrolytic and tantalum)

These are the recap targets — the ceramic decouplers (33 nF/100 nF SMD) and plate ceramics are stable and are left in place.

**A3000 main PCB — aluminium electrolytic capacitors**
Designator	Value	Voltage	Mounting
C6, C7, C8, C9, C10	10 µF	16 V	Radial
C19	10 µF	16 V	Radial
C67	4.7 µF	16 V	Radial
C15, C60, C63	47 µF	16 V	Radial
C39, C83	47 µF	10 V	Axial
C65, C70	100 µF	25 V	Radial
C12, C14, C61, C66, C69	220 µF	16 V	Radial

**A3000 main PCB — tantalum capacitors**
Designator	Value	Voltage
C4, C13, C22, C34, C55, C57, C78, C89	10 µF	10 V

The remaining board capacitors are ceramic and not normally replaced: 33 nF and 100 nF surface-mount decouplers (the "DCPLR 1210" parts, e.g. C1, C5, C11, C17, C18, C20, C21, C35–C37, C40–C54, C56, C58, C59, C64, C71–C74, C76, C84, C87) and plate ceramics (33 pF C2, 12 pF C16, 100 pF C23/C32/C33, 1 nF C79/C85, 2.2 nF C24–C31/C75/C81/C82/C86/C90/C91), plus a few 47 nF ceramics (C3, C62, C68, C74) and 100 nF/22 nF polyester (C80/C88, C77).^[1]

Replacement summary

Use 105 °C low-ESR aluminium electrolytics of equal capacitance and equal-or-higher voltage; tantalums may be replaced with tantalum or a good electrolytic of equal value.

**Board recap shopping list**
Value	Voltage	Type	Qty
4.7 µF	16 V	Electrolytic (radial)	1
10 µF	16 V	Electrolytic (radial)	6
10 µF	10 V	Tantalum	8
47 µF	16 V	Electrolytic (radial)	3
47 µF	10 V	Electrolytic (axial)	2
100 µF	25 V	Electrolytic (radial)	2
220 µF	16 V	Electrolytic (radial)	5

Replacement procedure

Disconnect the mains lead; open the case and remove the main PCB.
Mark the polarity of each electrolytic/tantalum before removal (the silkscreen marks the positive pad).
Wet both leads with fresh solder/flux, lift each end in turn, and clear the holes with solder wick.
Fit the new part with correct polarity, solder and trim. Reverse-fitted tantalums fail short, so double-check C4/C13/C22/C34/C55/C57/C78/C89.
Inspect for clean joints and no bridges before reassembly.

PSU module

The A3000's switch-mode PSU is a separate cased module. It contains mains-side suppression capacitors and bulk smoothing electrolytics. The Service Manual treats the PSU as a non-serviceable assembly (output 5 V at up to 4.4 A, 22 W; ripple should be within 50 mV pk-pk).^[1] If the +5 V shows ripple or the machine resets randomly, either replace the PSU module or, if you are competent with mains switch-mode supplies, recap it: discharge the primary bulk capacitor first, then replace the electrolytics and any cracked mains-suppression (RIFA-style) capacitor with correctly-rated parts. Read each value off the fitted part.

Post-recap verification

Reassemble; reconnect the mains lead.
Power on and confirm RISC OS reaches the desktop.
Measure the +5 V rail (4.9–5.1 V) and check ripple is within 50 mV pk-pk.
Confirm sound (the A3000 has stereo output) and a floppy access.

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

Related pages

References

↑ ^1.0 ^1.1 ^1.2 Acorn A3000 Service Manual, Acorn Computers — hosted on this wiki. PCB assembly parts list: capacitor designators C1–C91, values, types ("ALEC" aluminium electrolytic, "TANT" tantalum, "DCPLR" SMD ceramic decoupler, "CPLT" plate ceramic) and Acorn part numbers; also the PSU output spec and mains-hazard warning.

[sm-1] 1.0 ^1.1 ^1.2 Acorn A3000 Service Manual, Acorn Computers — hosted on this wiki. PCB assembly parts list: capacitor designators C1–C91, values, types ("ALEC" aluminium electrolytic, "TANT" tantalum, "DCPLR" SMD ceramic decoupler, "CPLT" plate ceramic) and Acorn part numbers; also the PSU output spec and mains-hazard warning.

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