Acorn System 3 Capacitor Replacement Guide

This guide covers recapping the Acorn System 3 (1980). As a Eurocard rack, its capacitors are per-card: each card has ceramic decouplers plus an electrolytic on its +5 V regulator. The CPU and cassette cards use the verified Acorn System 1 values; the VDU, memory and floppy-controller cards each carry their own regulator electrolytic.

Acorn System 3, a disc-based Eurocard system.

Approach

Recap is low-risk per-card bench work (no mains or CRT on the cards). On each Eurocard the recap target is the regulator electrolytic; the ceramic decouplers are stable and left in place. Read each electrolytic's value from the card and its circuit diagram before replacing, and fit a 105 °C low-ESR part of equal capacitance and equal-or-higher voltage, observing polarity.

Cards

• CPU and cassette cards: the Acorn System 1 cards — a 15 µF/16 V electrolytic each (see Acorn System 1 Capacitor Replacement Guide).
• VDU card (40×25 or 80×25): regulator electrolytic plus ceramic decouplers.
• Memory card (8 KB RAM + 4 KB BASIC ROM): regulator electrolytic plus decouplers.
• Floppy-disc controller card (Intel 8271): regulator electrolytic plus decouplers.

Procedure

1. Power off; remove the card from the backplane.
2. Record each electrolytic's value, voltage and polarity.
3. Desolder, clean the holes, fit the new part with correct polarity, solder and trim.
4. Verify +5 V on the card and refit.

Mains PSU

A cased System 3 with an Acorn mains supply has bulk smoothing electrolytics in the PSU; recap that only if competent with mains supplies (discharge the bulk capacitor first).

Related pages

• Acorn System 3
• Acorn System 3 Maintenance Guide
• Acorn System 3 Troubleshooting Guide
• Acorn System 1 Capacitor Replacement Guide · Capacitor Failure Symptoms