Editing Acorn System 1 Troubleshooting Guide

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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]].<ref name="s1tech">[[Acorn System 1 Technical Manual]] (Acorn Microcomputer), Acorn Computers &mdash; hosted on this wiki. Source for the IC complement (IC1 R6502, IC2/IC8 8154 RAM/IO, IC3/IC4 2114 RAM, IC5/IC6 74S571 ROM, IC9 74LS20, IC10 74LS139, IC11 74LS04, IC12 74LS00, IC13 LM340-T5; keyboard card IC7 LM358, IC8 7445), the address decoding (IC8 chip selects on A7/A8), the cassette read level (IC7 pin 7), and the +5&nbsp;V regulation.</ref>

[[File:Acorn System 1 circuit diagram (1 of 3).png|center|thumb|680px|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 ==

# '''Supply''': verify +5&nbsp;V at the LM340-T5 (IC13) output on the CPU card and at the IC VCC pins. Out of range &rarr; regulator or supply.
# '''Clock''': scope the 1&nbsp;MHz crystal and the 6502 (IC1) clock input. No clock &rarr; the master oscillator is dead and nothing runs.
# '''Reset''': check the 6502 reset line releases after power-on.
# '''Address activity''': with the clock present, the 6502 address lines should toggle. If static, reseat or substitute IC1 (6502).
# '''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 &mdash; reseat IC5/IC6.

== Display and keypad ==

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{| class="wikitable styled-table" style="width:100%; text-align:left;"
! 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&nbsp;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).<ref name="s1tech" />

== 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&nbsp;V; a low or absent level there points to the read amplifier or the input level.<ref name="s1tech" /> 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 [[Acorn System 2|System 2]]/3/4 frame. A System 1 that misbehaves only when fitted to a backplane usually has a dirty or bridged edge connector &mdash; clean the fingers.

== Common fault catalogue ==

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

== References ==

<references />

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