Acorn System 4 Troubleshooting Guide

Acorn System 4 double-height Eurocard rack.

This guide gives diagnosis for the Acorn System 4 (1980), a 14-slot double-height Eurocard rack with two floppy drives. Diagnose by card as for the System 3, with attention to the larger backplane and the integral mains power supply.

⚠️ Mains warning

The cased System 4 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 power wiring.
2. On a 14-slot rack, reduce to a minimum (CPU + RAM + VDU) and add cards back one at a time to isolate a faulty card or a shorted card pulling a rail down.

Card-level diagnosis

Use the System 1/2/3 guides for the individual cards:

• CPU card — Acorn System 1 Troubleshooting Guide (1 MHz clock, 6502, ROM, RAM).
• VDU / memory cards — Acorn System 2 Troubleshooting Guide.
• Floppy subsystem — Acorn System 3 Troubleshooting Guide (Intel 8271, ribbon, drives, alignment).

Econet file-server faults

A System 4 used as an Econet server depends on the Econet card and the network: check the Econet card seating, the clock/termination on the network, and the file-server ROM. A server that boots locally but is invisible on the network points to the Econet card or the cabling rather than the CPU.

Backplane

With many slots, a single shorted card or a dirty contact can take down the whole rack. Pull all cards and add them back; clean edge fingers and backplane sockets; inspect the backplane power-input joints for cracks.

Common fault catalogue

• Dead, no rails — mains PSU or backplane power wiring.
• Dead, rails present — CPU card; apply the System 1 checks.
• A rail collapses with one card fitted — shorted card; isolate by substitution.
• No disc — FDC (8271), ribbon or drive.
• Not visible on Econet — Econet card or network cabling/termination.
• Intermittent when jostled — dirty edge connector or backplane joint.

Related pages

• Acorn System 4
• Acorn System 4 Maintenance Guide
• Acorn System 4 Capacitor Replacement Guide
• Acorn System 3 Troubleshooting Guide · Acorn System 1 Troubleshooting Guide