Acorn A4 Maintenance Guide

A4 with the keyboard removed: battery bay (lower left), floppy drive (lower right), hard disc and the vertical DC/DC converter board (photo: Chris's Acorns)

Three parts of the Acorn A4 age badly: the main battery pack, the nickel-cadmium backup cell soldered to the main board, and the surface-mount electrolytic capacitors, which leak onto the board. Capacitor replacement is on Acorn A4 Capacitor Guide and diagnosis on Acorn A4 Troubleshooting Guide.

Everything inside is plastic and fragile. Theo Markettos, who has dismantled many A4s, sums up the method as "be gentle".

Safety

• Remove the battery pack and unplug the adaptor before opening the case.
• The battery is nickel-cadmium. Acorn's guides warn that the cells contain toxic chemicals, must not be pierced, burnt or heated above 50 °C in use or 80 °C in storage, and must go back to a supplier or proper disposal, not household waste. Wear gloves when handling leaked cells and the residue from them.
• The power adaptor is a mains switch-mode unit. Unplug it and let it stand before opening it.
• See Battery Explosion, Capacitor or Corrosion Damage for neutralising and cleaning leakage.

Opening the A4

You need a small crosshead screwdriver (2 to 3 mm), a small flat screwdriver, a larger crosshead (about 5 mm), something thin for levering, and a small pair of pliers for the pillars on the D-type connectors.

1. Shut down, remove all cables and slide out the battery pack.
2. Flip out the Econet module or its blanking plate behind the keyboard, and unplug the Econet module if fitted.
3. Release the two white display cables from the main board. Push up both ends of the top half of each connector so that it slides up about 1 mm, then the cable comes free. If it does not, do not pull: work on the connector again.
4. Remove the three rear screws and the three base screws, then lift away the lid assembly.
5. Release the keyboard cables in the same way. These are part of the keyboard membrane and cannot be replaced, so take particular care. Lift the keyboard out, pushing it up from the battery bay if needed.
6. Remove the metal screen over the floppy drive. The DC/DC converter, floppy drive and hard disc are now accessible.
7. The floppy drive is held by four screws from underneath. Its cable can usually stay connected.
8. Lift out the small vertical DC/DC converter board in the middle of the main board, and the screen behind it.
9. Unplug the hard disc cable, then remove the hard disc cradle (one screw at the rear, and on some machines one under the floppy drive).
10. To remove the main board, unscrew the pillars on the monitor, parallel and serial connectors (keep the nuts behind them), unplug all the cables noting their orientation, then remove the three base screws at the rear and the single screw at the bottom rear left. The board has components on both sides and only comes out at one angle.

When reassembling, check that the DC/DC converter board sits on its pins correctly: Theo Markettos warns that being one pin out in any direction may damage the machine. Check the hard disc cable is not off by one, that every flexible cable is square in its connector, and that no screws are loose inside.

The A4 is held together with self-tapping screws into plastic. Turn each screw anticlockwise in its hole until it clicks into the existing thread before tightening it, and avoid power screwdrivers.

Removing the screen and LCD panel from the lid is a separate procedure, described by Theo Markettos. When refitting the lid front, check that no part of the LCD glass sits on the L-shaped plastic guides, or the panel breaks as the cover goes on.

Backup cell B1

Main board seen through the expansion bay, with the display cables at the left (photo: Chris's Acorns)

B1 is a 1.2 V 11 mAh NiCd cell soldered to the main board (Acorn part 0817,015). On the circuit diagram it is charged from +5 V through D2 and from the battery management supply (Vbmu) through D7 and R190 (22 kohm), so it is kept charged from the main battery pack even when the A4 is switched off, and it feeds the PCF8583 clock and CMOS RAM through R171 (4.7 kohm).

These cells leak, and owners advise removing the original and fitting a rechargeable 1.2 V AA or AAA cell in a holder away from the board. Because the circuit charges the cell, use a rechargeable cell. Insulate and fix the new cell and its leads: Ian Jeffray found a replacement cell on a long wire whose tape had come loose, leaving its contacts free to touch live parts of the board.

If B1 has leaked, neutralise and clean the area, then check the tracks around it, the PCF8583 and its crystal X2. On the circuit diagram the clock chip is IC5; the parts list calls it IC6.

Battery pack

Battery pack, 14.4 V 1800 mAh NiCd (photo: Chris's Acorns)

The pack is labelled "Nickel Cadmium Battery 14,4 V 1800 mAh". Inside are twelve sub-C NiCd cells in series, with a thermal fuse in the middle of the string and a pair of thermistors on a separate lead for temperature sensing. Acorn's manual gives the thermistors as a series pair of PTC devices, nominally 2 kohm at 25 °C, read by the BMU; the BMU charges at 60 to 70 mA (trickle) or 320 to 380 mA (quick). Of the three contacts that look like positive terminals, two are the temperature sensor.

To rebuild a pack:

1. Open the case by pushing in the tabs around three sides and peeling the lid off the silicone on the cells.
2. Remove the cells, wearing gloves. Neutralise and clean the case; Adam Wilson used white vinegar.
3. Keep the thermal fuse and the thermistors. Test the thermistors (about 1 kohm each at room temperature, rising with temperature) and refit them against the cells.
4. Fit twelve new sub-C cells in series. Cells with solder tags are much easier to join: Ian Jeffray found soldering untagged cells difficult and the joints not very stable, and KenLowe found tagged cells easy to solder.
5. Insulate the joints and the string, close the case and charge the pack fully in the A4 (eight to ten hours) before relying on it.

Original-type NiCd cells match the BMU's charging. Forum members disagree about NiMH: some have used NiMH cells successfully, and replacement packs have been sold with them, but the charge control was designed for NiCd and owners report that a NiMH pack is not charged to its full capacity.

A charged pack loses its charge in about two months when left in the A4 and about three months when out of it. Take the pack out of a machine that is going into storage. Ian Jeffray describes an A4 stored upside down whose leaking pack ran into the LCD.

Power adaptor

Power adaptor with its 9-way plug (photo: Chris's Acorns)

The adaptor connects through a 9-way D-type plug and supplies both the machine and the charging current. Acorn's pinout, as given in the Portable Guide:

Power adaptor connector

Pin	Signal	Function
1	UMAIN	Charger system supply
2	0V
3	OE	Charger output enable
4	0V
5	CRSEL1	Charge current select
6	UBAT	Charger battery supply
7	CRSEL2	Charge current select
8	IBAT	Current feedback
9	0V

UMAIN is about 21 V. Theo Markettos suggests that a substitute supply for running the machine without charging could provide UMAIN alone, at about 21 V and around 720 mA, and that a lower voltage risks the internal power diodes.

Philip Pemberton found adaptors failing through a 1 µF 50 V electrolytic near the mains inlet, and replaced it with a 105 °C low-ESR part; he suggests replacing all the adaptor's electrolytics at the same time. The adaptor case plastic may be brittle: turn the screws without pressing down, and reinforce cracked screw posts with epoxy. The adaptor's own circuit is not in Acorn's manual.

Warning: the adaptor contains mains-voltage circuitry and a charged primary capacitor. Unplug it and discharge the capacitor before working inside. See Capacitor Failure Symptoms.

Hard disc

The A4 takes one 2.5-inch IDE drive as master, assigned ADFS drive 4. The original Conner 60 MB drives are known for sticking heads. Owners replace them with a CompactFlash card on an IDE adapter. Adam Wilson found that several SD and CF adapters gave disc errors 20 and 23 under the RISC OS 3.1 ADFS, and got a Transcend CF card working after fitting a Wizzo expansion ROM with the IDEFS filing system in place of the original extension ROM; RISC OS 3.1 limits a partition to 512 MB. The extension ROM also holds the battery manager, so a replacement image must include it.

See Hard Drive Maintenance and Repair.

Cleaning

Acorn's Portable Guide gives these instructions:

• Case: switch off and unplug, then wipe with a soft, slightly damp cloth.
• Mouse: remove the ball by turning the retainer 90° and clean the rollers with tape head cleaner on a cotton swab.
• LCD: wipe lightly with a soft cloth moistened with isopropyl or ethyl alcohol. Do not use water, ketones or aromatic solvents, or a dry or coarse cloth. If liquid leaks from a damaged panel, do not swallow it and wash it off skin with soap and water.

Clean the connectors and controls with contact cleaner when the machine is apart. The brightness and contrast knobs are often missing; Ian Jeffray has published a 3D-printable replacement under CC BY 4.0.

Checklist

1. Remove the battery pack from storage machines.
2. Remove the board-mounted cell B1 if it is still fitted; clean any leakage.
3. Recap the main board (Acorn A4 Capacitor Guide).
4. Check the power adaptor output and, if it is original, consider replacing its electrolytics.
5. Rebuild or replace the battery pack.
6. Check every flexible cable for cracks and seat it squarely.
7. Replace the hard disc with solid-state storage if the Conner drive sticks.

Related pages

• Acorn A4
• Acorn A4 Troubleshooting Guide
• Acorn A4 Capacitor Guide
• Acorn A4 Technical Reference Manual
• Battery Refurbishment