Acorn System 4 Capacitor Replacement Guide
The Acorn System 4 uses the same Acorn Eurocards as the Acorn System 3, so the card capacitors are the same: one electrolytic per card across the 5 V supply and ceramic decouplers for the rest. The cards have no regulators.[1] The power supplies are built into the rack.[2]
Card electrolytics
| Card | Board | Designator | Value |
|---|---|---|---|
| 6502 CPU | 200,000 | C9 (parts list) or C8 (assembly text) | 15 µF[1] |
| Teletext VDU | 200,002 | C1 | 10 or 15 µF[1] |
| 8K RAM and 8K PROM | 200,003 | C8 | 15 µF[1] |
| Floppy disc controller | 200,004 | C1 | 15 µF[3] |
The ceramic values for these cards are on Acorn System 2 Capacitor Replacement Guide and Acorn System 3 Capacitor Replacement Guide. Acorn's documents give no voltage rating for the electrolytics; they are on the 5 V rail, so 16 V or higher replacements are suitable.[1]
Other cards found in System 4s, such as the static RAM, Econet, Universal Interface Board and bus extender cards in Chris Whytehead's machine, are not covered by these documents.[4] Read the values from the parts on those boards.
Power supplies
Acorn's sales sheet gives no details of the power supplies. Before working on one, unplug the rack and let the primary reservoir capacitor discharge, then measure it. Note the value and voltage of each electrolytic from the part itself, and replace like for like with parts of the same or higher voltage rating.
Procedure for a card
- Unplug the rack and slide the card out.
- Note the + marking for the electrolytic.
- Desolder it from the solder side, fit a replacement with the same polarity and refit the card.
- Check the system starts and, after work on the controller card, that both drives read.
Related pages
- Acorn System 4
- Acorn System 4 Maintenance Guide
- Acorn System 4 Troubleshooting Guide
- Acorn System 3 Capacitor Replacement Guide
References
- ↑ 1.0 1.1 1.2 1.3 1.4 Acorn Computers, System 2 Technical Manual, issue 1, January 1980, parts lists pp. 2-1 to 2-4 and power supply p. 5-1. Text transcription by Ben Harris, hosted on this wiki as File:Acorn System 2 Technical Manual (text transcription).pdf.
- ↑ Acorn Computers, Acorn System 4 Computer, one-page sales sheet, undated. Hosted on this wiki as File:Acorn System 4 brochure.pdf.
- ↑ 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.
- ↑ Whytehead, Chris. "Acorn System 4", Chris's Acorns, archived by The Centre for Computing History (retrieved 2026-10-01). Cards in Chris's System 4.
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