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Acorn System 3 Capacitor Replacement Guide

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Acorn System 3
Acorn System 3 (photo: Chris's Acorns)

The Acorn System 3 cards have no regulators of their own; they run from the frame's 5 V supply, and each has one electrolytic across that supply with ceramic decouplers for the rest.[1][2]

Electrolytics

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System 3 card electrolytics
Card Board Designator Value Source
6502 CPU 200,000 C9 (parts list) or C8 (assembly text) 15 µF System 2 manual[1]
Teletext VDU 200,002 C1 10 or 15 µF System 2 manual; circuit diagram[1][3]
Memory 200,003 C8 15 µF (4.7 µF on the issue 1 drawing) System 2 manual; circuit diagram[1][4]
Floppy disc controller 200,004 C1 15 µF Circuit diagram[2]

Acorn's documents give no voltage rating for these parts. They sit across the 5 V supply, so a 16 V or higher replacement is suitable; the System 1 manual specifies 15 µF 16 V for the same CPU card position.[5] On the CPU card, check the board's + marking to see whether the electrolytic is at C8 or C9; the System 2 manual, hosted as a 1998 text transcription, names both.[1] The memory card's issue 1 drawing gives C8 as 4.7 µF where the parts list for the issue 2 board gives 15 µF, so read the fitted part.[4][1]

Ceramic capacitors

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The floppy controller card has C2, C3, C4 and C7 (47 nF decouplers), C5 (150 pF, on the 4 MHz oscillator) and C6 (100 pF).[2] The CPU, VDU and memory card ceramics are listed on Acorn System 2 Capacitor Replacement Guide.

Acorn's 6809 processor card and 80 × 25 VDU card, found in some System 3s, are not covered by these documents.[6] Read the values from the parts on those boards.

Procedure

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  1. Switch off and unplug the supply; slide the card out of the frame.
  2. Note the + marking for the electrolytic.
  3. Desolder from the solder side, fit a part of the same value at 16 V or more with the same polarity, and refit the card.
  4. Check the system starts and, after work on the controller card, that a disc reads.
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Acorn Computers, System 2 Technical Manual, issue 1, January 1980. Parts lists pp. 2-1 to 2-4 (CPU, VDU and memory cards); CPU card set-up p. 3-6; power supply p. 5-1. Text transcription by Ben Harris, hosted on this wiki as Acorn System 2 Technical Manual (PDF).
  2. ↑ 2.0 2.1 2.2 Acorn Computers, Floppy Disc Controller PCB Circuit Diagram, 200,004/C, issue 1, 7 February 1980. Hosted on this wiki as File:Acorn floppy disc controller Eurocard circuit diagram (200,004).png.
  3. ↑ Acorn Computers, V.D.U. Interface P.C.B. (Teletext), drawing 200,002/C, issue 1, 29 August 1979, hosted as File:Acorn teletext VDU interface Eurocard circuit diagram (200,002).png: C1 (electrolytic) and C2 to C7 across +5 V.
  4. ↑ 4.0 4.1 Acorn Computers, 8K RAM + 8K PROM, drawing 200,003/C, issue 1, hosted as File:Acorn 8K RAM and 8K PROM Eurocard circuit diagram (200,003).png: C1 to C7 47 nF and C8 4.7 µF electrolytic across +5 V.
  5. ↑ Acorn Computers, Technical Manual, System 1 Micro-Computer, 1979, CPU card parts list (C10 15 µF 16 V) and "Extending the system" (electrolytic moved to C8 in a rack), hosted on this wiki.
  6. ↑ Whytehead, Chris. "Acorn System 3", Chris's Acorns, archived by The Centre for Computing History (retrieved 2026-10-01). Chris's System 3 has a 6809 CPU card and an 80 × 25 VDU card.


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