Editing Acorn System 1 Maintenance Guide

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[[File:Acorn System 1 CPU board.jpg|thumb|360x360px|Acorn System 1 6502 CPU Eurocard (part 200,000): R6502P, the two INS8154 RAM/IO devices, 2114 RAM, 74S571 ROMs and the LM340-T5 regulator.]]

This guide covers preventive maintenance for the '''[[Acorn System 1]]''' (Acorn Microcomputer, 1979), a two-Eurocard 6502 machine. The System 1 has '''no mains wiring and no CRT''': it runs on a low-voltage external supply that is regulated to +5&nbsp;V on each card by an LM340-T5 (7805-type) regulator. Maintenance is cleaning, contact integrity, socket reseating and supply-voltage checks.<ref name="s1tech">[[Acorn System 1 Technical Manual]] (Acorn Microcomputer), Acorn Computers &mdash; hosted on this wiki. Source for the CPU-card and keyboard-card IC and capacitor parts lists, the LM340-T5 regulator and heatsink, the construction notes and the circuit description.</ref>

== Safety ==

The System 1 contains no mains voltage and no CRT. It is powered from an external low-voltage supply and regulates +5&nbsp;V on the board. Standard anti-static precautions apply (the 6502, the 8154 RAM/IO devices and the 2114 RAM are sensitive to ESD). The Technical Manual specifies a heatsink on the LM340-T5 regulator.<ref name="s1tech" />

== The two cards ==

The System 1 is the '''6502 CPU card (200,000)''' and the '''keyboard/cassette card (200,001)''', joined by a ribbon. Both cards are socketed throughout, so reseating ICs is the first response to an intermittent fault.

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{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''System 1 ICs'''
! Card !! Devices
|-
| CPU (200,000) || IC1 R6502; IC2/IC8 INS8154 RAM/IO; IC3/IC4 2114 RAM; IC5/IC6 74S571 ROM; IC9 74LS20; IC10 74LS139; IC11 74LS04; IC12 74LS00; IC13 LM340-T5 regulator
|-
| Keyboard/cassette (200,001) || IC2 4024; IC3 4011; IC4 4013; IC5 4001; IC6 4024; IC7 LM358 (cassette amp); IC8 7445 (keypad/display decoder)
|}

== Regular cleaning ==

* Clear dust with a soft brush and low-pressure air.
* Clean the EuroConnector edge fingers and the inter-card ribbon connector with a soft eraser or contact cleaner &mdash; edge-connector oxidation is a common cause of intermittent faults on Eurocard machines.
* Reseat every socketed IC, especially the 6502 (IC1), the ROMs (IC5/IC6) and the RAM (IC3/IC4).
* Clean the cassette DIN socket and the keypad contacts.

== Power and the regulators ==

Each card regulates +5&nbsp;V with an LM340-T5. Verify '''+5&nbsp;V''' at the regulator output and at the IC VCC pins before suspecting a logic fault. Confirm the regulator heatsink is fitted; a regulator running hot into thermal shutdown causes intermittent resets.<ref name="s1tech" />

== Display, keypad and cassette ==

* The 8-digit 7-segment LED display and the 25-key hex keypad are decoded by the 7445 (IC8) on the keyboard card; clean the keypad and reseat IC8 if digits or keys misbehave.
* The cassette interface is the CUTS 300-baud circuit around the LM358 (IC7). Keep the DIN socket clean; most loading problems are the tape or the external recorder.

== Capacitor health ==

Each card carries ceramic decouplers plus a single 15&nbsp;µF/16&nbsp;V electrolytic on the regulator. These are low-risk, leaded parts; replace the two electrolytics pre-emptively only if a board is already open or shows ripple-related instability. Full list and procedure: [[Acorn System 1 Capacitor Replacement Guide]].

== Recommended tools ==

* Posidrive/Philips screwdriver and anti-static strap.
* DC multimeter for the +5&nbsp;V checks.
* Contact cleaner and a soft eraser for the EuroConnector.
* Temperature-controlled soldering iron and solder wick for the two electrolytics.

See [[Recommended Tools]] for the general toolkit.

== Preventive maintenance checklist ==

# Verify +5&nbsp;V at both regulators; confirm the heatsinks are fitted.
# Reseat all socketed ICs (6502, ROMs, RAM, 8154s).
# Clean the EuroConnector fingers and the inter-card ribbon.
# Clean the keypad and cassette DIN socket.
# Consider replacing the two 15&nbsp;µF electrolytics while a card is open.

== Related pages ==

* [[Acorn System 1]]
* [[Acorn System 1 Troubleshooting Guide]]
* [[Acorn System 1 Capacitor Replacement Guide]]
* [[Acorn System 2]] &mdash; the rack-mounted successor that reuses these cards
* [[Acorn System 1 Technical Manual]] · [[Acorn System 1 Circuit Diagrams]]
* [[Capacitor Failure Symptoms]] · [[Recommended Tools]]

== References ==

<references />

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