Acorn System 2 Troubleshooting Guide

Acorn System 2 in a 19-inch Eurocard card frame.

This guide gives component-level diagnosis for the Acorn System 2 (1980). Because the System 2 is a set of Eurocards on an 8-slot backplane, diagnosis is by card: isolate the faulty card, then probe it against its circuit diagram. The CPU and cassette cards are the Acorn System 1 cards, so the Acorn System 1 Troubleshooting Guide applies to them directly.

Method: isolate the card

A racked system is best diagnosed by reducing it to a minimum and adding cards back:

1. Confirm the backplane rails (+5 V, and any ±12 V used by the VDU) at an empty slot.
2. Run with only the CPU card and the memory/BASIC card; confirm the machine starts.
3. Add the VDU card, then the cassette card, retesting after each.
4. A fault that appears only when one card is fitted isolates to that card (or a dirty slot).

No display

Symptom	Probable cause	Action
No video at all	VDU card, its rails, or the monitor	Confirm +5 V/±12 V at the VDU slot; reseat the MC6845 and SAA5050; try another monitor
Garbage / wrong characters	SAA5050 or video RAM on the VDU card	Reseat the SAA5050; check the VDU card's RAM
Dead machine, no activity	CPU card (clock/6502) or no rails	Apply the System 1 CPU-card checks; verify the backplane supply

Memory and BASIC

The memory card carries the RAM and the Acorn System BASIC ROM. Corrupt operation or crashes point to the RAM; failure to enter BASIC points to the BASIC ROM or its select. Reseat the card, then substitute RAM. Adding the optional second 4 KB RAM or the floating-point ROM changes the memory map, so verify links are set per the card documentation.

CPU and cassette cards

These are the System 1 cards. Use the Acorn System 1 Troubleshooting Guide: verify the 1 MHz clock and 6502, the monitor ROMs, the 2114 RAM, and the LM358 cassette read level. In the System 2 the keypad and LED are unused; the CPU card's keyboard interface drives the external keyboard instead.

Backplane and connectors

• A machine that works only when a card is pressed or wiggled has a dirty/loose edge connector — clean the fingers and the backplane socket.
• Check the backplane for cracked solder at the connectors, especially around the power-input pins.

Common fault catalogue

• Dead, no rails — power supply or backplane power wiring.
• Dead, rails present — CPU card (clock/6502/ROM); apply the System 1 checks.
• No display — VDU card, its rails, or the monitor; reseat MC6845/SAA5050.
• Crashes / corrupt — memory card RAM; reseat/substitute.
• Intermittent when jostled — dirty edge connector or backplane joint.

Related pages

• Acorn System 2
• Acorn System 2 Maintenance Guide
• Acorn System 2 Capacitor Replacement Guide
• Acorn System 1 Troubleshooting Guide · Acorn System Catalogue