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Acorn System 4 Maintenance Guide

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Acorn System 4 backplane
Acorn System 4 14-slot backplane (photo: Chris's Acorns)

The Acorn System 4 is a double-height rack with a 14-slot buffered backplane, power supplies built in, and two floppy drives above the cards.[1] The cards are the same types as in the Acorn System 3, so card and disc care follow Acorn System 3 Maintenance Guide and Acorn System 2 Maintenance Guide.

Safety

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The System 4 was sold assembled, with its power supplies inside the rack.[1] Unplug the mains lead before removing any cover, and treat the supply primaries as live until their reservoir capacitors have discharged. The cards run at 5 V and have no regulators of their own.[2]

Cards and backplane

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The backplane has 14 slots and bus buffers; the four-card System 2 runs without buffers.[1][2][3] In the System 2 design, the left-hand backplane slot is the one reserved for the CPU card when bus buffers are fitted; check that the CPU card is in the slot the backplane expects.[2]

  • Unplug the supply, then slide each card out.
  • Clean the 64-way connectors. If side B of the backplane has wire-wrapped links, check that none has broken.
  • Reseat each IC, checking for folded pins.
  • Refit each card fully home.

Chris Whytehead's photographs show the top cage, card guides and power switch removable for access to the backplane and supply.[3]

Power supply

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Measure the 5 V rail at the backplane with the system running, and the drive supply at each drive's power connector. Because no card has a regulator, a low 5 V rail affects every card.[2]

Disc drives

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The two drives in the upper frame connect to the floppy disc controller card (200,004). The controller checks are on Acorn System 3 Maintenance Guide. For the drives themselves, follow the drive maker's manual; general care is on Floppy Drive Repair.[4]

Capacitors

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See Acorn System 4 Capacitor Replacement Guide.

Preventive maintenance checklist

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  1. Check the mains lead, then measure the supply outputs under load.
  2. Clean the card connectors and backplane; reseat the cards and ICs.
  3. Check the drive cables and power connectors.
  4. Back up important discs before running unknown drives.
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References

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  1. ↑ 1.0 1.1 1.2 Acorn Computers, Acorn System 4 Computer, one-page sales sheet, undated. Hosted on this wiki as File:Acorn System 4 brochure.pdf.
  2. ↑ 2.0 2.1 2.2 2.3 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 Acorn System 2 Technical Manual (PDF).
  3. ↑ 3.0 3.1 Whytehead, Chris. "Acorn System 4", Chris's Acorns, archived by The Centre for Computing History (retrieved 2026-10-01). 14-slot backplane; photographs of the case with the top cage, card guides and power switch removed and of the power supply; Chris's cards: 6502 CPU with System keyboard connector, 8K static RAM, 40-column VDU, Econet, Universal Interface Board, Bus Extender.
  4. ↑ 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.


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