Acorn System 1 Capacitor Replacement Guide

This guide lists the capacitors on the Acorn System 1 (Acorn Microcomputer) and gives the replacement procedure. The values and designators are taken from the Acorn System 1 Technical Manual parts lists for the CPU card (200,000) and the keyboard/cassette card (200,001).^[1]

Why (and why not) recap a System 1

The System 1 has no mains wiring and no CRT and runs on a low-voltage supply, so capacitor work is low-risk bench work. Each card carries only ceramic decouplers plus a single 15 µF/16 V electrolytic on the +5 V regulator. There is no large reservoir capacitor and no notorious failure pattern; replace the two electrolytics only when a card is already open or shows ripple-related instability.

Capacitor list

**CPU card (200,000)**
Designator	Value	Type
C1, C2, C5, C6, C7, C8, C9	100 nF	Ceramic decoupler
C3, C4	10 nF	Ceramic
C10	15 µF / 16 V	Electrolytic (regulator)

**Keyboard / cassette card (200,001)**
Designator	Value	Type
C1	100 pF	Ceramic
C2, C4	1 nF	Ceramic
C3	10 nF	Ceramic
C5	15 µF / 16 V	Electrolytic (regulator)
C6, C9, C10	100 nF	Ceramic decoupler
C8	22 nF	Ceramic
C7	—	Not fitted

The Technical Manual notes that the 0.1 µF (100 nF) decouplers may be substituted with 0.047 µF if needed, and that the 15 µF electrolytics are polarised (observe the + marking).^[1]

Replacement summary

The only recap targets are the two electrolytics:

Value	Voltage	Type	Where
15 µF	16 V (or higher)	Aluminium electrolytic, radial	C10 (CPU card), C5 (keyboard card)

Use a 105 °C low-ESR part of equal capacitance and equal-or-higher voltage. The ceramic decouplers are stable and are left in place.

Procedure

Power off and separate the two cards.
Mark the polarity of each 15 µF electrolytic (the silkscreen marks the + end).
Add fresh solder/flux, remove the old cap, clear the holes with solder wick.
Fit the new cap, matching polarity; solder and trim.
Verify +5 V at the regulator and confirm the monitor prompt and keypad on power-up.

Related pages

References

↑ ^1.0 ^1.1 Acorn System 1 Technical Manual (Acorn Microcomputer), Acorn Computers — hosted on this wiki. CPU-card and keyboard-card parts lists: capacitor designators and values, and the note that the ten 0.1 µF decouplers may be substituted with 0.047 µF, and that the 15 µF electrolytics are polarised.

[s1tech-1] 1.0 ^1.1 Acorn System 1 Technical Manual (Acorn Microcomputer), Acorn Computers — hosted on this wiki. CPU-card and keyboard-card parts lists: capacitor designators and values, and the note that the ten 0.1 µF decouplers may be substituted with 0.047 µF, and that the 15 µF electrolytics are polarised.

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