Editing Acorn A7000 Maintenance Guide

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[[File:Acorn A7000 main board.jpg|thumb|right|360px|A7000-series main board (photo: Deep Fried Brains, CC BY-SA 2.0)]]

The [[Acorn A7000]] and the [[Acorn A7000+]] share a case, power supply and board layout, and the same maintenance applies to both.<ref name="chrisplus">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/A7000+.html "Acorn A7000+"], Chris's Acorns (retrieved 2026-10-01). The A7000+ is an upgraded A7000; photographs of the case interior and the 1102,200 main board.</ref> Acorn published no service manual or circuit diagram for either board that has come to light, and stardot repairers work from the [[Acorn Risc PC Technical Reference Manual|Risc PC Technical Reference Manual]] and its drawings, which cover much of the same circuitry.<ref name="sd12506">[https://stardot.org.uk/forums/viewtopic.php?t=12506 "A7000 floppy / HD issues"], stardot.org.uk forum thread, February 2017 to December 2024. sirbod: battery damage near the Super I/O chip broke three tracks under the C13 legend (left to the via under the R6 legend, middle to pin 4 and right to pin 5 of IC2, south of the IDE connector); a high-resistance track at R6 caused a long delay at power-on; reflowing the vias round the CMOS chip; stevepick: the IDE header is fitted backwards, so a keyed cable needs the header turned round and pin 20 removed; IanJeffray, December 2024: the A7000 schematics were never published as far as is known, but the Risc PC schematics cover many parts.</ref> Faults are diagnosed on [[Acorn A7000 Troubleshooting Guide]].

== Safety ==

The power supply is an internal mains switch-mode unit.<ref name="ds002">Acorn Risc Technologies, datasheet "A7000", DS002/ART/RK, February 1996. Hosted on this wiki: [[:File:Acorn A7000 datasheet (ART DS002).pdf]]. Integral 35 W switch-mode supply, IEC inlet and outlet, auto-ranging 110 to 240 V; 240 bytes of battery-backed CMOS RAM.</ref> Unplug the machine before removing the cover, and treat the supply's primary capacitor as charged until it has been discharged and measured. See [[Capacitor Failure Symptoms]].

== Inside the case ==

[[File:Acorn A7000 with cover removed.jpg|thumb|right|260px|Cover removed: CD-ROM, floppy and hard disc, power supply at the left (photo: Chris's Acorns)]]

The power supply sits at one side of the case, the floppy drive at the front, and the space in the middle takes either a CD-ROM drive or a single expansion card on the optional backplane, but not both. The main board lies in the bottom of the case with the speaker; the network card plugs into its own socket in the middle of the board.<ref name="chris">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/A7000.html "Acorn A7000"], Chris's Acorns (retrieved 2026-10-01). Photographs with the cover off and of the main board: SIMM, RISC OS ROMs, backplane slot, IDE and floppy connectors with the CMOS battery above them, network slot, ARM7500, power connectors along the right-hand edge.</ref><ref name="chrisplus" /> The power supply produces +5 V, +12 V and −12 V.<ref name="sd25165">[https://stardot.org.uk/forums/viewtopic.php?t=25165 "A7000+ with new caps, but it's dead Jim"], stardot.org.uk forum thread, July to August 2022. cosworthchris: flickering power LED and clicking speaker until a dried-out small capacitor in the supply was replaced, then 12 V, −12 V and 5 V present; he replaced the main board's surface-mount electrolytics, some of which had begun to leak, and four radial capacitors, and fitted a CR2032 with a diode in place of the rechargeable cell. IanJeffray: the cell is not needed for the machine to run, only to keep settings; routine recapping of the main board is not needed ("Acorn machines aren't Amigas"); check the CMOS RAM supply and charge path and the I²C lines to the ARM7500FE (pins 181 and 182 to the RTC); CMOS RAM chips "very regularly die"; hold Delete at power-on until RISC OS has started once.</ref>

The A7000's IDE header is fitted the wrong way round. A modern keyed IDE cable will only fit if the header is turned through 180° and pin 20 removed.<ref name="sd12506" />

== CMOS backup battery ==

The rechargeable backup cell sits on the main board above the IDE and floppy connectors.<ref name="chris" /> The CMOS RAM (240 bytes) and the clock run from it when the machine is off.<ref name="ds002" /> On stardot, A7000 and A7000+ boards with a leaked cell are the most common repair: the electrolyte attacks the tracks to the Super I/O controller, which runs the floppy and IDE interfaces, and the area round the CMOS RAM chip.<ref name="sd12506" /><ref name="sd12617">[https://stardot.org.uk/forums/viewtopic.php?t=12617 "A7000+ revival, pointers on where to start"], stardot.org.uk forum thread, 2017 to 2019. Battery damage; vinegar then isopropyl alcohol with a toothbrush; jumper wires over the three corroded tracks cured disc error 8 and the freezing; later IDE tracing with the Risc PC schematic; danielj suggests a NiMH AA cell in a holder away from the board.</ref>

# Remove the cell from any machine that still has the original.
# Neutralise the residue with white vinegar, then wash it off with isopropyl alcohol and a soft brush.<ref name="sd12617" />
# Check every track near the cell for continuity, end to end, including the three thin tracks that run under the C13 legend beside the battery position (sirbod's map: left to the via under the R6 legend, middle to pin 4 of IC2, right to pin 5 of IC2). Bridge any that read open or high with fine wire.<ref name="sd12506" />
# Fit a replacement cell away from the board, in a holder on short leads. danielj suggests a NiMH AA cell.<ref name="sd12617" /> The original cell was rechargeable and the board charges it, so use a rechargeable cell unless the charging path is blocked; one owner fitted a CR2032 lithium cell with a series diode.<ref name="sd25165" />
# If the machine still forgets its settings, see [[Acorn A7000 Troubleshooting Guide#Settings and CMOS RAM]].

The machine runs without the cell; it only loses its settings and clock at power-off.<ref name="sd25165" />

== Corrosion elsewhere ==

* Keyboard and mouse sockets: Ian Jeffray reports that the metal EMC gasket round the PS/2 sockets reacts with the bare metal case. The corrosion has to be sanded off, and it starts copper corrosion along the back edge of the main board.<ref name="sd21945">[https://stardot.org.uk/forums/viewtopic.php?t=21945 "Another rotten A7000+"], stardot.org.uk forum thread, March 2021. IanJeffray: EMC gasket corrosion at the keyboard and mouse sockets; tracks cracked where they meet via annular rings (D0 to the ROM, D3 to the RAM, RA9, two breaks in /IOW, one under a SIMM socket support leg); "CMOS unreadable" cured by a new CMOS RAM chip; network card and Super I/O dead until green corrosion was cleaned out of the backplane connector with undiluted vinegar for 30 seconds followed by isopropyl alcohol and compressed air; video unstable on a bench 5 V supply but correct on the proper supply.</ref>
* Backplane connector: green corrosion in the expansion connector stopped the network card and the Super I/O on Ian Jeffray's A7000+ while expansion cards still worked. He flooded it with undiluted vinegar for 30 seconds, then with isopropyl alcohol, and blew it out with compressed air.<ref name="sd21945" />
* Tracks: on badly corroded boards, thin tracks crack where they meet a via. Ian Jeffray found breaks on a data line to the ROM, a data line to the SIMM, a RAM address line and the I/O write strobe, far from the battery. Check continuity from one end of each track to the other, across every via on the way.<ref name="sd21945" />

== Capacitors ==

There is no published list of the main board capacitors. The board has surface-mount electrolytics and four larger radial electrolytics; cosworthchris found some surface-mount parts starting to leak when he replaced them.<ref name="sd25165" /> Ian Jeffray advises against routine recapping of these boards, and recommends finding the actual fault first.<ref name="sd25165" /> In the CMOS circuit, the A7000's C4 corresponds to C70 on the Risc PC, a 4.7 µF decoupler on the clock chip's supply, and one owner found it nearly short circuit after battery damage.<ref name="sd19461">[https://stardot.org.uk/forums/viewtopic.php?t=19461 "A7000 doesn't retain CMOS settings"], stardot.org.uk forum thread, May to June 2020. markdryan, DutchAcorn and philpem map the A7000 CMOS circuit to the Risc PC's: C4 = C70 (4.7 µF, measured 52 ohms across it), R2 = R133, C16 = C500 (18 pF crystal load), C14 a 100 nF decoupler; about 5 V on both sides of the diode with the machine on; the supply at pins 4 and 8 of the CMOS chip should be between 4 and 5 V.</ref> Check the supply's electrolytics if the machine will not start: a dried-out small capacitor stopped one A7000+ supply.<ref name="sd25165" />

== Storage ==

* Remove the backup cell, or replace it with a remote one, before storing the machine.
* Keep the machine dry. Ian Jeffray describes the corrosion at the sockets and in the backplane connector as environmental damage, separate from the battery leakage.<ref name="sd21945" />

== Related pages ==

* [[Acorn A7000]]
* [[Acorn A7000+]]
* [[Acorn A7000 Troubleshooting Guide]]
* [[Acorn Risc PC 600 Maintenance Guide]]
* [[Battery Explosion, Capacitor or Corrosion Damage]]

== References ==

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[[Category:Acorn Computers]]
[[Category:Maintenance Guides]]