Acorn A7000 Troubleshooting Guide
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.[1][2] Look after the battery and corrosion first: see Acorn A7000 Maintenance Guide.
First checks
[edit | edit source]- Disconnect everything not needed to start: SIMM, network card, expansion card, drives except the floppy.[2]
- Check that the supply gives +5 V, +12 V and −12 V at the main board.[2]
- Keep the floppy drive connected: the power-on self-test reports faults by flashing the floppy drive light, and a missing drive hides them.[2]
- Hold Delete while switching on, to reset the CMOS RAM to defaults. Repeat at every power-on until RISC OS has started once.[2]
- 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.[3]
Self-test
[edit | edit source]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.[3] 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.[2]
Symptoms
[edit | edit source]| 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[2] |
| 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[2] |
| 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[1] |
| 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[4] |
| POST reports CMOS unreadable | Failed CMOS RAM chip, common after battery leakage | Replace the chip; Ian Jeffray reports replacing dozens[3][2] |
| 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[1][5] |
| 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[6] |
| 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[3] |
| 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[3] |
| 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[3] |
| Keyed IDE cable will not fit | The header is fitted backwards on the A7000 | Turn the header round and remove pin 20[1] |
Settings and CMOS RAM
[edit | edit source]The CMOS RAM circuit follows the Risc PC's. Stardot members have matched the parts on an A7000 board to the Risc PC drawing:[4]
| 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.[4] 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
[edit | edit source]- Acorn A7000
- Acorn A7000+
- Acorn A7000 Maintenance Guide
- Acorn Risc PC 600 Troubleshooting Guide
- Acorn A7000 Documentation
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 "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.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 "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.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 "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.
- ↑ 4.0 4.1 4.2 "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.
- ↑ "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.
- ↑ "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.