Acorn System 1 Troubleshooting Guide

This guide gives component-level diagnosis for the Acorn System 1 (Acorn Microcomputer). The System 1 has no video and no POST: faults are diagnosed by the monitor program's behaviour on the 8-digit LED display, by probing the 6502 bus against the circuit diagram, and by substituting socketed parts. The IC references below are from the Acorn System 1 Technical Manual and the Acorn System 1 Circuit Diagrams.

Acorn System 1 circuit diagram, sheet 1 of 3 (Technical Manual). All three sheets are on the wiki at Acorn System 1 Circuit Diagrams. (Source: The Centre for Computing History.)

Power-up behaviour

On power-up the monitor program in the 74S571 ROMs (IC5/IC6) drives the 8-digit LED display through the 7445 (IC8) and the 8154 (IC2) on the keyboard card, and waits for hex-keypad input. A healthy machine shows a prompt/address on the display and responds to the keypad. A dark or frozen display points to a supply, clock, CPU, ROM or RAM fault.

Dead machine

1. Supply: verify +5 V at the LM340-T5 (IC13) output on the CPU card and at the IC VCC pins. Out of range → regulator or supply.
2. Clock: scope the 1 MHz crystal and the 6502 (IC1) clock input. No clock → the master oscillator is dead and nothing runs.
3. Reset: check the 6502 reset line releases after power-on.
4. Address activity: with the clock present, the 6502 address lines should toggle. If static, reseat or substitute IC1 (6502).
5. ROM fetch: the monitor lives in IC5/IC6 (74S571). The address decoder IC8 (74LS139 on the CPU card) and the glue (IC9 74LS20, IC11 74LS04, IC12 74LS00) select the ROM; check for activity on the ROM pins. A blank display with clocks present is often a ROM-select or ROM fault — reseat IC5/IC6.

Display and keypad

Symptom	Probable cause	Action
Whole display dark, machine seems alive	7445 decoder (IC8) or 8154 (IC2) on the keyboard card	Reseat IC8 / the 8154; check the display drive lines
One digit or segment dead	LED display or its drive	Check the segment/digit lines from IC8
Keypad dead or wrong keys	8154 (IC2) keypad port, or keypad contacts	Clean the keypad; reseat the 8154; check the ribbon to the CPU card
Garbled values / crashes	2114 RAM (IC3/IC4)	Reseat; substitute a known-good 2114

RAM and ROM

• RAM: the 1 KB of RAM is two 2114 devices (IC3/IC4). A failed 2114 corrupts the monitor's scratch/zero-page area and crashes it. Reseat, then substitute.
• ROM: the 512-byte monitor is in two 74S571 bipolar ROMs (IC5 "Blue", IC6 "Yellow"). These are robust; a blank or wrong display with good clocks and RAM points to a ROM seating or select fault. Reseat IC5/IC6 and verify the decode (the Technical Manual notes the CS lines on address lines A7/A8 at IC8).

Cassette interface

The CUTS 300-baud cassette circuit is built around the LM358 (IC7) on the keyboard card. When reading a known-good tape, the signal at IC7 pin 7 should exceed about 2 V; a low or absent level there points to the read amplifier or the input level. If save/load fails with a known-good recorder, probe IC7 and check the DIN wiring before suspecting the 8154.

EuroConnector / expansion

The CPU card's second 8154 (IC8) and the EuroConnector let the card run in a System 2/3/4 frame. A System 1 that misbehaves only when fitted to a backplane usually has a dirty or bridged edge connector — clean the fingers.

Common fault catalogue

• Dead, +5 V present — 1 MHz clock/crystal or the 6502 (IC1); reseat IC1.
• Blank display, clocks present — ROM (IC5/IC6) seating/select, or the 7445 (IC8) display decoder.
• Garbled values / crashes — a 2114 (IC3/IC4); substitute.
• Keypad dead — 8154 (IC2) or keypad contacts.
• Won't load tapes — recorder/tape first, then the LM358 (IC7); confirm IC7 pin 7 read level.
• Intermittent when racked — dirty EuroConnector; clean.

Related pages

• Acorn System 1
• Acorn System 1 Maintenance Guide
• Acorn System 1 Capacitor Replacement Guide
• Acorn System 1 Circuit Diagrams · Acorn System 1 Technical Manual
• Capacitor Failure Symptoms