Acorn System 5 Troubleshooting Guide

Acorn System 5 Eurocard rack with two floppy drives.

This guide gives diagnosis for the Acorn System 5 (late 1982), a 10-slot Eurocard rack with two floppy drives and an integral mains power supply. Diagnose by card as for the System 4; the Acorn System 5 Handbook gives the per-board detail.

⚠️ Mains warning

The System 5 has a mains PSU. Disconnect the mains lead before opening; the PSU primary side is at mains potential.

First: power and backplane

1. With the mains off, then on (lid open, take care), confirm the backplane rails (+5 V and any ±12 V) at an empty slot. A dead rail points to the PSU or the backplane wiring.
2. Reduce to a minimum (CPU + DRAM + VDU) and add cards back to isolate a faulty or shorting card.

Card-level diagnosis

• CPU card (1 MHz 6502 or 2 MHz 6502A) — verify the clock and the 6502; on a 6502A board confirm the 2 MHz timing. Apply the Acorn System 1 Troubleshooting Guide approach.
• VDU card (80×25 or 40×25) — check the video output and reseat the CRTC/character generator.
• 32 KB DRAM cards — DRAM faults give corruption or crashes; reseat the card and check its regulator (DRAM cards draw more current and stress the regulator). Confirm the memory-size links match the fitted DRAM.
• Floppy subsystem — Intel 8271 controller, ribbon and drives (see Acorn System 3 Troubleshooting Guide).
• Econet — check the Econet card and the network if the machine is a server.

DRAM vs static RAM

Unlike the earlier static-RAM systems, the System 5 uses 32 KB DRAM cards, which need correct refresh and timing. A DRAM card that fails the memory test or causes intermittent crashes should be reseated and its links checked before substitution.

Backplane

A shorted card or dirty contact can take down the rack; pull all cards and add them back, cleaning edge fingers and backplane sockets, and inspect the backplane power-input joints.

Common fault catalogue

• Dead, no rails — mains PSU or backplane wiring.
• Dead, rails present — CPU card; apply the System 1 checks (confirm 1 vs 2 MHz board).
• Corruption / crashes — a 32 KB DRAM card; reseat, check links, substitute.
• No disc — FDC (8271), ribbon or drive.
• Not visible on Econet — Econet card or cabling/termination.
• Intermittent when jostled — dirty edge connector or backplane joint.

Related pages

• Acorn System 5
• Acorn System 5 Maintenance Guide
• Acorn System 5 Capacitor Replacement Guide
• Acorn System 5 Handbook · Acorn System 4 Troubleshooting Guide · Acorn System 1 Troubleshooting Guide