RetroTechCollection

Acorn System 4 Capacitor Replacement Guide

This guide covers recapping the Acorn System 4 (1980). As a Eurocard rack it has per-card capacitors (a regulator electrolytic plus ceramic decouplers on each card), and as a cased machine it also has bulk smoothing electrolytics in its integral mains power supply.[1]

Acorn System 4 double-height Eurocard rack.

Per-card capacitors

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On each Eurocard the recap target is the +5 V regulator electrolytic; the ceramic decouplers are stable and left in place. Read each electrolytic's value from the card and its circuit diagram and fit a 105 °C low-ESR part of equal capacitance and equal-or-higher voltage, observing polarity.

⚠️ Mains power supply

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The System 4's integral PSU is mains-powered and contains the bulk smoothing electrolytics (and mains-side suppression parts). Disconnect the mains lead and discharge the bulk capacitor before any PSU work. Recap the PSU only if competent with mains supplies; otherwise replace the unit. Read each value from the fitted part.

Procedure (per card)

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  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.
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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 card capacitor values.


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