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Acorn System 2 Capacitor Replacement Guide

From RetroTechCollection
Acorn System 2 in a 19-inch Eurocard card frame.

This guide covers recapping the Acorn System 2 (1980). The System 2 is a set of Eurocards, so the capacitors are per-card: each card carries ceramic decouplers plus a single electrolytic on its +5 V regulator. The exact per-card values are in the card circuit diagrams and the Acorn System Catalogue; the CPU and cassette cards use the verified Acorn System 1 values.[1]

Approach

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The System 2 has no mains and no CRT on its cards (only the optional Acorn PSU is mains-powered). Capacitor work is low-risk per-card bench work. On each Eurocard:

  • The recap target is the regulator electrolytic (the CPU and cassette cards use 15 µF/16 V; the VDU and memory cards each have their own regulator electrolytic).
  • The ceramic decouplers are stable and are left in place.

CPU and cassette cards

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These are the Acorn System 1 cards. Their full capacitor lists are in the Acorn System 1 Capacitor Replacement Guide; the only electrolytic on each is a 15 µF/16 V part (C10 on the CPU card, C5 on the cassette card).

VDU and memory cards

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The 40×25 VDU card and the memory (RAM + BASIC ROM) card each regulate their own +5 V and carry a regulator electrolytic plus ceramic decouplers. Read each value from the card itself and the card circuit diagram before replacing. Replace any electrolytic with a 105 °C low-ESR part of equal capacitance and equal-or-higher voltage, observing polarity.

Procedure

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

Optional mains PSU

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If an Acorn mains power supply is fitted, it has its own bulk smoothing electrolytics (and mains-side parts). Recap it only if competent with mains supplies; discharge the bulk capacitor first.

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References

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  1. ↑ Acorn System Catalogue and Acorn System 1 Technical Manual, Acorn Computers — hosted on this wiki. Source for the Eurocard set and the System 1 CPU/cassette-card capacitor values (a 15 µF/16 V electrolytic plus ceramic decouplers per card).


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