Editing Acorn A7000 Troubleshooting Guide

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[[File:Acorn A7000+ main board.jpg|thumb|right|300px|A7000+ main board 1102,200 (photo: Chris's Acorns)]]

The [[Acorn A7000]] and [[Acorn A7000+]] fail in the same ways, and the fixes below apply to both. There is no published A7000 circuit diagram. Repairers trace the board against the drawings in the [[Acorn Risc PC Technical Reference Manual]], following the circuit; the component numbers differ.<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: floppy and IDE failures from battery damage near the Super I/O chip, three tracks under the C13 legend; a high-resistance track at R6 that made the machine take 3 to 10 minutes to start; a machine that only booted when the CMOS chip was pressed, cured by a new chip and reflowing nearby vias; IanJeffray: no published schematic, refer to the Risc PC's.</ref><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, cured by a dried-out small capacitor in the supply; then no start-up chime, keyboard LEDs blinking, static from the speaker, no video. IanJeffray: "it's not a RiscPC" so do not rely on the Risc PC component numbers; check the CMOS RAM supply, charge path and I²C lines (ARM7500FE pins 181 and 182 to RTC pins 5 and 6); CMOS RAM chips "very regularly die"; RISC OS may appear to do nothing if the CMOS RAM is dead; hold Delete at power-on until RISC OS has started once; connect the floppy drive to see POST flashes; with the machine held in reset, the address lines should toggle at falling frequencies if the ARM is alive. sirbod: RISC OS can wait about 10 minutes for an unresponsive CMOS chip.</ref> Look after the battery and corrosion first: see [[Acorn A7000 Maintenance Guide]].

== First checks ==

# Disconnect everything not needed to start: SIMM, network card, expansion card, drives except the floppy.<ref name="sd25165" />
# Check that the supply gives +5 V, +12 V and −12 V at the main board.<ref name="sd25165" />
# Keep the floppy drive connected: the power-on self-test reports faults by flashing the floppy drive light, and a missing drive hides them.<ref name="sd25165" />
# Hold Delete while switching on, to reset the CMOS RAM to defaults. Repeat at every power-on until RISC OS has started once.<ref name="sd25165" />
# Do not run the board for long from a bench 5 V supply alone: Ian Jeffray's A7000+ showed an unstable, overscanned picture until it was back on its own supply with the negative rails.<ref name="sd21945">[https://stardot.org.uk/forums/viewtopic.php?t=21945 "Another rotten A7000+"], stardot.org.uk forum thread, March 2021. IanJeffray: no start traced to a cracked ROM data track; DRAM errors from cracked tracks at vias (D3, RA9); POST "SELFTEST R2.05" output read with a POST box and a "CMOS unreadable" code cured by a new CMOS RAM chip; unstable video from a bench 5 V supply; network card and Super I/O dead until the backplane connector was cleaned; EMC gasket corrosion at the PS/2 sockets.</ref>

== Self-test ==

The A7000 runs the same style of power-on self-test as the Risc PC, and repairers read its results with the same POST interface box. Ian Jeffray's A7000+ reported "SELFTEST R2.05" and a "CMOS unreadable" fault in this way.<ref name="sd21945" /> The meaning of each fault bit is given in Acorn's Risc PC documentation, summarised on [[Acorn Risc PC 600 Troubleshooting Guide#Power-on self test]]. The self-test does not check that the floppy or IDE drives work.<ref name="sd25165" />

== Symptoms ==

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''A7000 and A7000+ symptoms'''
! Symptom !! Probable cause !! Action
|-
| Power LED flickers, speaker clicks, nothing else || Power supply not starting || Check the supply's small electrolytics; one dried-out capacitor stopped an A7000+ supply<ref name="sd25165" />
|-
| No start-up chime, keyboard LEDs blink, static or pops from the speaker, no video || Processor not running, or dead or unreadable CMOS RAM || Check the CMOS RAM supply and the I²C lines to the ARM7500FE; replace the CMOS RAM chip; check for cracked tracks; with an oscilloscope, check that the ARM's address lines toggle while the machine is held in reset<ref name="sd25165" />
|-
| Takes several minutes to start, or hangs at the * prompt with the keyboard dead || RISC OS waiting for an unresponsive CMOS RAM || Check the tracks near the battery; sirbod found high resistance on the track from R6 and bridged it to the via; then replace the CMOS RAM chip and reflow the vias round it<ref name="sd12506" />
|-
| Forgets settings at power-off || Backup cell dead or removed; damaged charging or supply path || Measure the CMOS RAM supply (see below); replace the cell or fit a remote one<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. 0.35 V on the cell and 1.46 V at the CMOS chip with the machine on; about 5 V expected on both sides of the diode and 4 to 5 V at pins 8 and 4 of the CMOS chip; C4 (Risc PC C70, 4.7 µF) measured 52 ohms and was replaced; R2 corresponds to the Risc PC's R133; C16 (Risc PC C500) is the 18 pF crystal load capacitor; C14 a 100 nF decoupler; later the board lost floppy and hard disc after IC7 and nearby tracks were damaged.</ref>
|-
| POST reports CMOS unreadable || Failed CMOS RAM chip, common after battery leakage || Replace the chip; Ian Jeffray reports replacing dozens<ref name="sd21945" /><ref name="sd25165" />
|-
| Floppy drive not found and hard disc gives disc error 8 or freezes || Tracks to the Super I/O controller broken by battery leakage || Check the three tracks under the C13 legend and the IDE buffer IC2; bridge broken tracks<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. Jumpers over the three corroded tracks cured disc error 8 and the freezes; disc error 2 followed until further tracks were found; danielj: error 08 is drive not ready, check IDE pin 27 (IORDY); probe each data line from the IDE connector to the HCT245 buffers and on to the Super I/O chip; only the lower 8 data bits reach the Super I/O chip.</ref>
|-
| Hard disc still not found with all tracks good; disc error 23 with a CF card || Super I/O controller damaged || Ian Jeffray reflowed the controller's pins, replaced the 245 buffers and the 74ACT32, and finally replaced the controller<ref name="sd21690">[https://stardot.org.uk/forums/viewtopic.php?t=21690 "A7000+ failed IDE - fixed!"], stardot.org.uk forum thread, February 2021. Disc error 8 with the original drive and 23 with a CF card; all tracks buzzed out; the 245 buffers (Risc PC IC12 and IC16) and the ACT32 (Risc PC IC11) replaced without effect; the Super I/O controller replaced, two of its pads (DENSEL and /MTR0) lifting as it came off.</ref>
|-
| DRAM errors in POST; SIMM not seen || Cracked tracks between the ARM7500, ROMs and SIMM socket, typically where a track meets a via || Buzz the data and address lines from end to end; Ian Jeffray found D3 to the RAM and RA9 broken, one break under a SIMM socket support leg<ref name="sd21945" />
|-
| Network card and Super I/O functions dead, expansion card works || Corrosion in the backplane connector || Clean the connector: undiluted vinegar for 30 seconds, then isopropyl alcohol, then compressed air<ref name="sd21945" />
|-
| Keyboard or mouse faults; corrosion at the back edge of the board || EMC gasket at the PS/2 sockets reacting with the case || Sand off the corrosion and check the tracks along the rear edge<ref name="sd21945" />
|-
| Keyed IDE cable will not fit || The header is fitted backwards on the A7000 || Turn the header round and remove pin 20<ref name="sd12506" />
|}

== Settings and CMOS RAM ==

The CMOS RAM circuit follows the Risc PC's. Stardot members have matched the parts on an A7000 board to the Risc PC drawing:<ref name="sd19461" />

{| class="wikitable styled-table" style="width:70%; text-align:left;"
|+'''A7000 CMOS circuit parts and their Risc PC equivalents (forum identification)'''
! A7000 !! Risc PC !! Part
|-
| C4 || C70 || 4.7 µF decoupler on the CMOS RAM supply
|-
| R2 || R133 || Series resistor in the supply to the CMOS RAM
|-
| C16 || C500 || 18 pF crystal load capacitor
|-
| C14 || (none identified) || Probably a 100 nF decoupler
|}

With the machine switched on, there should be about 5 V on both sides of the charging diode, and between 4 and 5 V across pins 8 and 4 of the CMOS RAM chip; one owner measured 1.46 V there and found C4 reading 52 ohms.<ref name="sd19461" /> Acorn's Risc PC drawing shows the same arrangement (battery, D2, R130, R133, R212, C70 and the PCF8583); see [[Acorn Risc PC 600 Maintenance Guide]].

== Related pages ==

* [[Acorn A7000]]
* [[Acorn A7000+]]
* [[Acorn A7000 Maintenance Guide]]
* [[Acorn Risc PC 600 Troubleshooting Guide]]
* [[Acorn A7000 Documentation]]

== References ==

<references />

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