Acorn Archimedes A3000 Maintenance Guide

An A3000 main board (0180,000 Issue 3) with corrosion from a leaking backup cell at the bottom left.

The Acorn Archimedes A3000 is Acorn's ARM2 keyboard-case machine of 1989, with the main board, power supply, floppy drive and keyboard in one case. Its backup cell, B1, is a NiCd soldered to the main board, and leakage from it corrodes the board. Acorn's Acorn A3000 Service Manual gives the disassembly steps, test points and parts list for the machine.

Safety

Warning: the power supply in the case runs from the mains. Acorn's service manual says dangerous voltages are exposed once the cover is off, and the computer must be unplugged before the cover is removed.

• Acorn specifies antistatic matting and a wrist strap for all servicing.
• The supply is set for 240 V or 120 V by a link wire to the pin marked 240 or 120 on the supply. If the setting is changed, Acorn asks for a label on the outside of the case and a mains plug suited to the new supply.
• After refitting or replacing an assembly, Acorn requires an earth continuity test on the assembled computer (Appendix B, p. 57).

Identify your board

The part number and issue are printed on the board. Chris Whytehead documents two: 0280,000 Issue A, with VL2333-QC ARM2, and 0180,000 Issue 3, with VL86C010-10QC ARM2. Acorn's service manual describes Issue 1 boards and lists what changed from Issue A on p. 23.

A3000 main board, principal parts (Service Manual parts list, pp. 52-53)

Designator	Part	Notes
IC37	ARM2	Surface-mounted
IC44	MEMC1a	Surface-mounted
IC41	VIDC1a	Surface-mounted
IC13	IOC	Surface-mounted
IC14-IC17	RISC OS ROMs 1-4	Socketed
IC20-IC27	256K × 4 DRAM, 1 MB in total
IC36	1772 floppy disc controller
IC2	Keyboard controller (8051)
IC6	8583 clock and CMOS RAM	Backed by B1
IC32, IC38	LM386 audio amplifiers
X3	24 MHz crystal	System clock
X2	32.768 kHz crystal	Clock chip
B1	NiCd 1.2 V 280 mAh, part 0817,013	Soldered to the board

Acorn says not to remove the four surface-mounted chips without surface-mount training and equipment.

Opening the case

Acorn's procedure for removing the cover:

1. Disconnect all peripherals and unplug the computer from the mains.
2. Unplug the mouse from the underside.
3. Remove the centre fixing screw on the underside.
4. Remove the two screws from the clips at the rear.
5. Unhook the two clips and lift the back of the cover slightly, pivoting on the front.
6. Find the three rectangular holes on the front underside. Insert a screwdriver at the rear edge of each in turn and push and twist until the catch comes free.
7. Lift the cover clear. Refitting is the reverse, skipping step 6.

The keyboard connector is easily damaged. Hold it by the white plastic strengthening strip only, and do not touch the track faces or press on the cable. The floppy drive is held by four recessed screws in the base. To take out the main board, remove the cover, keyboard, floppy drive and any RAM upgrade, unplug the red (+5 V) and black (0 V) power leads at the board and the two speaker leads, remove the right-hand speaker, and undo the three self-tapping screws; the board and rear panel then lift out.

Backup battery

B1 keeps the clock and CMOS RAM alive. It is a 1.2 V 280 mAh NiCd, Acorn part 0817,013, soldered to the board at the bottom left beside the ROMs. Leakage from it corrodes the board in that corner; Chris Whytehead's Issue A board has corrosion there and does not work.

1. Inspect the bottom-left corner of the board, around B1, the keyboard connectors and the ROM sockets, for green or white deposits and darkened tracks.
2. Check the cell with Acorn's figures. LK6 is the battery test point: pin 2 should read 1.2 V ± 0.2 V against pin 1. With the power off, IC6 pin 8 should read about 1.1 V; if it is absent, check R3 and R7, and if it is below 1 V, change B1 and check D2 and C15.
3. To remove B1, use a soldering iron and a desoldering gun. Acorn says sites without the equipment and experience should not attempt it because the board is easily damaged, and warns against shorting the cell's terminals even on a cell being thrown away.
4. Clean any leakage, then check the tracks in the affected area for continuity before fitting a new cell. NiCd electrolyte is alkaline; Battery Explosion, Capacitor or Corrosion Damage covers neutralising and cleaning it.

LK5 on the board is Acorn's connection point for an external battery, provided in case the on-board NiCd became a problem. The case base has a battery compartment with a screwed cover to the right of the mouse socket, unused on standard machines. Battery Refurbishment lists the cells Acorn fitted across the range.

Cleaning

• Brush and blow dust from the board, the supply's vents and the floppy drive.
• Clean the rear connectors and the internal expansion sockets (SK3, SK8, SK9, SK11) with contact cleaner if a card is unreliable.
• The keyboard comes out after the cover; handle its connector as described above.

Floppy drive

The drive is a one-inch-high 3.5-inch unit, 1 MB unformatted, running from +5 V on a 4-pin power connector (pin 1 +5 V, pins 2 and 3 ground). Chris Whytehead's two machines have Citizen OSDC-65C drives. Acorn's checks when the drive misbehaves: the configuration items STEP and FLOPPIES are set correctly, the drive ID switch is at zero, the cables are connected at both ends with +5 V and 0 V present on the power cable, and the drive ID plate has not fallen off inside the drive and jammed it. General drive care is on Floppy Drive Repair.

Power supply checks

The supply has one output, +5 V, delivered to the board on two faston tabs: PL3 is 0 V and PL4 is +5 V.

A3000 power supply specification (Service Manual p. 19)

Parameter	Value
Output voltage	4.9-5.1 V (5 V nominal)
Output current	0.5-4.4 A
Ripple and noise	50 mV peak to peak (0-50 MHz)
Over-voltage threshold	5.8-7.0 V
Power	22 W continuous, 29 W surge
Input	198-264 V at 47-53 Hz, or 99-130 V at 57-63 Hz

If +5 V is missing at PL4, Acorn's flowchart has you disconnect the floppy drive and check again. If the rail returns, the drive is at fault; if not, test the board for a short circuit, and replace the supply if the board is clear. Acorn's manual treats the supply as a unit and gives no component data for it.

Capacitors

The board's electrolytics are leaded radial and axial parts, with twelve leaded tantalums. Acorn's service manual notes that C23-C33 on the printer port occasionally go open circuit. The full list with part numbers is on the Acorn Archimedes A3000 Capacitor Guide.

Common failure points

A3000 failure points named by Acorn and Chris Whytehead

Item	Symptom	Check
B1 backup cell	Clock and settings lost; corrosion at the bottom left	LK6 1.2 V ± 0.2 V; IC6 pin 8 about 1.1 V with power off
Printer port capacitors C23-C33	Printer problems	Check for open-circuit capacitors
Configuration	Unstable or scrolling display	Type *CON. SYNC 1 and press Reset
ROM sockets IC14-IC17	Dead after a ROM change	Data and address activity on all four ROMs; a screwdriver used for ROM removal can damage tracks, sockets or legs
Floppy drive	Drive not recognised	Configuration, ID switch, cables, +5 V, jammed ID plate

Tools

Acorn's list for fault diagnosis is a 100 MHz oscilloscope, a DC voltmeter, an earth continuity tester, the serial port loopback plug (part 0280,087), 32 Ω headphones, IC extraction tools, antistatic matting and wrist straps, and standard hand tools. Use an IC extraction tool on the ROMs. Removing B1 needs a desoldering gun or station. See also Recommended Tools.

Maintenance checklist

1. Open the case and inspect B1 and the board around it. Remove a leaking cell and clean the board.
2. Measure +5 V at PL4 (4.9-5.1 V).
3. Inspect the electrolytics for swelling or leakage.
4. Clean the expansion sockets and rear connectors.
5. Check the floppy drive reads a known good disc.
6. Refit the cover and carry out Acorn's earth continuity test.

Related pages

• Acorn Archimedes A3000
• Acorn Archimedes A3000 Troubleshooting Guide
• Acorn Archimedes A3000 Capacitor Guide
• Acorn A3000 Service Manual and Acorn A3000 Technical Reference Manual
• Battery Explosion, Capacitor or Corrosion Damage
• Recommended Tools