Acorn Electron Maintenance Guide

Acorn Electron opened: the issue 4 main PCB in the case base, with the ULA under its metal clip in the centre and the power supply board at the top right

Routine care of the Acorn Electron (1983, main PCB 205,000) is cleaning, checking the low-voltage supply and keeping the ULA connection sound. The mains transformer is inside the plug-in adaptor; the system unit runs from the adaptor's 19 V AC, and the internal power supply board turns that into +5 V and −5 V.

Safety

The adaptor is a mains transformer with a built-in UK plug, rated 14 W. Mains voltage is present only inside it; check its case and lead for damage before use. Inside the Electron the voltages are low, but the ULA, 6502A, DRAMs and ROM are MOS parts, so work on an earthed mat or wear a wrist strap. Acorn asks that ventilation under and behind the computer is not blocked.

Identifying the board

The main PCB is part 205,000 with an issue number printed on it. UK boards are found as issues 2, 4 and 6. On issue 2 and 4 boards the ULA (IC1) sits in a 68-way socket held down by a square metal clip. On issue 6 boards the ULA is under a black blob on a small carrier board soldered to the main board, and there is no socket to maintain.

Opening the case

1. Unplug the adaptor and all leads.
2. Turn the machine over and undo the four fixing screws: two recessed in the front corners and two in the cut-away on the back edge.
3. Lift the top half. The keyboard is fixed to it, and a short ribbon cable joins it to the main PCB. Unplug the ribbon at the main PCB before working on the board.
4. The keyboard comes out of the top case after removing five screws.
5. In the base, the power supply board connects to the main PCB through a three-wire socket (red +5 V, blue −5 V, black 0 V) and two faston tabs marked 18V AC, which feed only the expansion connector. The loudspeaker also plugs into the main PCB. The main PCB is held by four screws and the power supply board by three.

The Electron will not run without the keyboard connected, so plug the ribbon back in before any power-on test. Check that the connector covers all the pins and is not offset by one row.

Cleaning

• Brush and blow dust from both boards and the keyboard.
• Clean the rear expansion edge connector and the sockets with contact cleaner. Adam Wilson cleaned the edge connector with a white eraser.
• The keycaps pull off with a keycap puller, and there are no springs under them. Adam Wilson cleared unreliable switches by spraying contact cleaner into them and working them up and down.

ULA socket

On socketed boards the ULA connection is the part most worth looking after. The Classic Acorn repair notes describe crashes or a white screen once the machine is warm, failure to start, and a garbled display, all from poor contact in the socket.

To remove the ULA, Acorn's method is to slide a small screwdriver between the front left corner of the clip and the plastic holder and twist until that corner unclips, then do the same at the front right corner. The clip springs off, and the ULA drops out when the Electron is turned over. Keep the screwdriver away from the tracks, which Acorn describes as very easily damaged.

Clean the socket fingers and the contacts on the ULA with an alcohol-based cleaner. When refitting, put the notched corners of the ULA in the right positions in the carrier, check that the small metal spring in the remaining corner is not bent under the chip, and press the clip back on by hand. If the fault persists, check each socket contact for a broken pin.

Power supply checks

Acorn Electron supply figures (Service Manual pp. 2, 4 and 10)

Point	Acorn's figure
Adaptor output	19 V AC, 50 Hz, 14 W; at least 17 V RMS measured on a working Electron
Supply board input range	17.1 to 20.9 V RMS
+5 V (red)	4.75 to 5.25 V, 50 mV noise maximum, with a 10 Ω load; 1.5 A maximum
−5 V (blue)	Tested with a 47 Ω load; 100 mA maximum
Main board current	700–900 mA from +5 V; about 25 mA from −5 V

1. Measure the AC across the two grey leads where they join the power supply board, with the adaptor plugged into the Electron.
2. Measure +5 V and −5 V. The full loaded test is on Acorn Electron Troubleshooting Guide.
3. Check the joints of the power input socket. Adam Wilson found all three cracked on one machine, and the Classic Acorn notes list a worn joint where the power lead enters the supply board as a cause of the same symptoms as a bad ULA socket.

The power supply board is a switching regulator, and Acorn describes it as easy to work on and repairable. Its capacitors are listed on Acorn Electron Capacitor Replacement Guide.

Keyboard

The keyboard assembly has 56 switches, fourteen 1N4148 diodes (D1–D14), the CAPS LOCK LED (LD1, TIL212-2), four 15 kΩ resistors (R2–R5), a 470 Ω resistor (R1) and one 0.47 µF ceramic (C1). Acorn's check is with an ohmmeter and the keyboard circuit diagram: look for shorts, dry joints and lifted tracks, check that the diodes and resistors are fitted correctly, and replace faulty keys. Check the ribbon cable for damage and good contact at both ends.

Video links

Two links on the main PCB matter for maintenance. LK3 sets the polarity of the composite sync on the RGB socket; it is set to 0 V as standard, and moving it to +5 V inverts the sync for monitors that need it. LK4 is normally open; making it adds the colour signal to the composite output for a colour composite monitor.

Preventive maintenance checklist

1. Inspect the adaptor case and lead.
2. Check the power socket joints on the supply board.
3. Measure the adaptor AC and the +5 V and −5 V rails.
4. On socketed boards, remove, clean and refit the ULA if the machine is unreliable.
5. Clean the edge connector, the sockets and any unreliable keys.
6. Reconnect the keyboard ribbon squarely before testing.

Related pages

• Acorn Electron
• Acorn Electron Troubleshooting Guide
• Acorn Electron Capacitor Replacement Guide
• Acorn Electron Service Manual
• Recommended Tools