Acorn A5000 Maintenance Guide

A5000 main board (25 MHz, 0192,100): four RISC OS 3 ROMs and an empty fifth socket, the IDE and floppy connectors at lower right, and the battery near the socketed FPA10.

The Acorn Archimedes A5000 is Acorn's ARM3 desktop of 1991, with a separate keyboard, a four-slot backplane on hard disc models, an internal fan-cooled power supply and a surface-mount main board. The two jobs that matter most on a stored machine are the backup cell, BT1, which leaks onto the board, and the surface-mount electrolytics, which have leaked on some early boards. Acorn's Acorn A5000 Service Manual covers disassembly and fault finding, and the Acorn A5000 Circuit Diagrams give the component values.

Safety

Warning: the power supply runs from the mains. Acorn treats it as a sealed service replacement module and gives no component data for it. Disconnect the mains and all peripherals before removing the cover. After refitting or replacing any assembly, and in particular the supply, Acorn requires an earth continuity test at 25 A between the mains plug earth pin and the base metalwork, rear panels and top cover, and a DC insulation test (Appendices C and D).

Identify your machine

Chris Whytehead's 25 MHz A5000 has main board 0192,100 and his 33 MHz model of 1993 has board 0192,420, which has a socketed MEMC1a for 8 MB upgrades that use a second MEMC. Both have the four-slot backplane 0192,004. The first models (ALB10, ALB12, ALB14) shipped with RISC OS 3.00, which has a bug in the ADFSBuffers code that corrupts discs; on those machines set ADFSBuffers to 0 or fit RISC OS 3.10 or 3.11 ROMs.

A5000 main board, principal parts (circuit diagram 0192,000)

Part	Function
ARM3, MEMC1a, VIDC1a, IOC	Processor, memory controller, video and sound, I/O
RISC OS 3 ROMs 1-4 and a fifth socket	Operating system; ROM size and type set by LK10-LK12 and LK14
82C710	Floppy, IDE, serial and parallel controller
MAX241 (IC7)	Serial line driver
PCF8583 (IC58)	Clock and CMOS RAM
BT1	Backup cell, 1.2 V 280 mAh
LM324 (IC3), LM386 (IC1)	Audio filter and speaker amplifier

Opening the case

Acorn's sequence:

1. Shut the computer down (press F12 and type SHUTDOWN, or use Shutdown from the Task Manager) so the hard disc is parked. Switch off and disconnect the mains, the keyboard and all peripherals.
2. Remove the top cover.
3. On hard disc models, which are the ones fitted with the backplane, remove it: four screws, then pull it up out of SK9.
4. Disconnect the six power fastons PL2-PL7, the speaker (LK8), the LEDs (LK22), the floppy cable (PL10) and the hard disc cable (PL9).
5. Undo the screw holding the backplane and drive tray, and lift the tray out; it has two tabs at the front and four clips at the sides. Each drive is held to the tray by four screws.
6. To remove the main board, take out the board fixing screws near SK3 and SK8 (some models), the two screws on the rear panel and the two on the lower blanking panel, then lift the rear panel slightly and slide the board out to the rear along its guides. Take care with the EMC gasket fingers on the rear panel.

When refitting, connect the fastons in this order: PL2 -5 V (purple), PL3 +12 V (yellow), PL4 and PL5 0 V (black), PL6 and PL7 +5 V (red). Make sure the drive tray's front tabs are fully in their slots, or the drives can be damaged when the machine is moved.

Backup battery

BT1 is a single 1.2 V 280 mAh cell. It is charged from +5 V through D15 and R272 (180 Ω), with R301 (180 Ω) on its negative side, and supplies the PCF8583 clock chip (IC58) through R281 when the machine is off. It sits near the socketed FPA10 position. On the Stardot forum, danielj replied to one owner that battery damage on these boards is all too common and that the IDE and floppy interfaces, which are in the same area, may also have gone; that owner had found the clock chip's pins 5-8 lifted from the board.

1. Remove BT1 and inspect the board around it, including IC58 and the IDE and floppy connectors.
2. Neutralise alkaline residue with a mild acid such as white vinegar, then clean with isopropyl alcohol. Battery Explosion, Capacitor or Corrosion Damage has the full method.
3. Check the nearby tracks and vias for continuity and repair any that are broken.
4. Fit a replacement. Because the board charges the cell through D15, a non-rechargeable cell needs its own blocking diode. Battery Refurbishment covers the options.
5. Hold Delete while switching on to reset the CMOS RAM to its defaults; Acorn notes that the configuration may need resetting after repairs.

Surface-mount electrolytics

On Stardot, Ian Jeffray named the early A5000 as one of the two Acorn machines whose surface-mount electrolytics should be replaced as a matter of course, and an owner found all the 47 µF surface-mount parts on his board leaking; another member replied that there was not much evidence of all the surface-mount capacitors failing on these machines. Inspect each one for residue and darkened solder mask. The list of values and positions is on the Acorn A5000 Capacitor Guide.

Power supply checks

The supply has three outputs. Acorn lists what each feeds:

A5000 supply rails (Service Manual pp. 1-3 and 2-4)

Rail	Faston	Feeds
+5 V	PL6, PL7 (red)	Main board, floppy drive and hard disc
-5 V	PL2 (purple)	Backplane expansion connector pin A31
+12 V	PL3 (yellow)	Video connector SK1 pin 12 (SCART TV), backplane pins B31 and B32, hard disc
0 V	PL4, PL5 (black)

Acorn's checks:

• Complete failure (no fan noise, no green LED, no Caps Lock response): check the mains plug fuse, 5 A on UK machines. If a new fuse blows at switch-on, the supply is faulty.
• One rail missing: check that the fastons are secure on the board and the drive connectors are firmly attached.
• Runs, then shuts down: a controlled thermal shutdown, caused by a failed fan or a failed part in the supply. Listen for the fan and check that its aperture is not blocked.

Memory links

LK23 and LK24 set the DRAM size. With no upgrade card, 1 MB has both not connected, 2 MB has LK24 at 2-3 and 4 MB has LK24 at 1-2. Acorn's manual lists illegal combinations with a RAM upgrade card: 2 MB on the main board with a 1 MB or 3 MB card, 1 MB on the board with a 2 MB card, and any card with 4 MB on the board.

Cleaning

• Blow dust out of the supply's fan aperture and vents, the drives and the backplane.
• Clean the backplane slots and the rear connectors with contact cleaner. Acorn's backplane check is to inspect the plug pins and expansion sockets for damage or blockage.
• The keyboard is a Panasonic unit on most models and a Keytronic on some early ones. Its case halves are held by eight cross-head screws; the reset switch cap must be moved to any replacement keyboard.

Hard disc

The hard disc connects to PL9 by a 40-way cable and takes power on a 4-way connector with +12 V on pin 1 (yellow), 0 V on pins 2 and 3 (black) and +5 V on pin 4 (red). Acorn's checks for a drive that fails its test are the configuration (IDEdiscs 1), the cables and those voltages. Chris Whytehead's machines have a 99 MB Fujitsu M2616ET and an 85 MB Conner CP30084E. See Hard Drive Maintenance and Repair.

Common failure points

A5000 failure points

Item	Symptom	Check
BT1 backup cell	Settings and time lost; corrosion near the FPA socket and the drive connectors	Remove the cell, clean and repair tracks
Surface-mount electrolytics	Residue on the board; faults in the area affected	Inspect and replace leaking parts
Supply	Dead, or shuts down after running	Plug fuse (5 A), fan and aperture
RAM upgrade	Memory faults after an upgrade	Card seated, ZIP chips in their sockets, LK23 and LK24, legal combination
RISC OS 3.00	Disc corruption	ADFSBuffers 0, or fit RISC OS 3.10 or 3.11

Maintenance checklist

1. Shut down, unplug and open the machine.
2. Remove BT1 if it is the original, clean any leakage and repair the tracks.
3. Inspect the surface-mount electrolytics and replace any that have leaked.
4. Clean the fan aperture, backplane slots and connectors.
5. Check the +5 V, -5 V and +12 V rails at the fastons.
6. Reassemble with the fastons in Acorn's order, do a Delete power-on, and carry out the earth continuity and insulation tests.

Related pages

• Acorn Archimedes A5000
• Acorn A5000 Troubleshooting Guide
• Acorn A5000 Capacitor Guide
• Acorn A5000 Service Manual and Acorn A5000 Circuit Diagrams
• Battery Explosion, Capacitor or Corrosion Damage
• Recommended Tools