Acorn System 1 Maintenance Guide

Acorn System 1 6502 CPU card, 200,000 (photo: Chris's Acorns)

The Acorn System 1 (1979) is two Eurocards with every IC in a socket, a single 5 V regulator and a low-voltage DC supply. Care is mostly cleaning, reseating ICs and checking the supply and the ribbon between the cards.

Safety

The System 1 runs on 7 to 35 V DC and has no mains wiring. If it is powered from a mains adaptor, check the adaptor case and lead before use. Many of the ICs are MOS parts, and Acorn's manual asks for an earthed soldering iron and no static-generating clothing or carpets when handling them.

The cards

• The 6502 CPU card (200,000) carries the processor, an INS8154 RAM/IO chip, two 2114 RAMs, the two 74S571 monitor PROMs, address decoding, the 1 MHz crystal and the LM340-T5 regulator (IC13). Sockets for a 2516 EPROM (IC7) and a second 8154 (IC8) are usually empty.
• The keyboard card (200,001) carries the 25 keys, the NSA1198 display, a 7445 driver and the CMOS and LM358 cassette circuits. It has no regulator; it takes +5 V from the CPU card on pin 20 of the connecting strip.

On the CPU card, IC1 (the 6502) faces the opposite way to its neighbours. Check its orientation after any reseating.

Cleaning and reseating

• Brush dust off both cards. Keep fingers off the keyboard area of the keyboard card; Acorn says to wipe it clean before fitting the key cage.
• Reseat each IC, pressing it fully home and checking that no pin has folded under it.
• Check the 20-way strip between the cards for cracked joints at both ends. Acorn bolts the keyboard above the CPU card to stop the strip flexing.
• Check the 16-way strip between the display and the keyboard card.
• If the 64-way edge connector is fitted, clean its pins.

Power supply

The cards need 7 to 35 V DC at 500 mA, positive to edge connector pin 31 and 0 V to pin 32. With Acorn's mains adaptor the white wire is positive. The LM340-T5 needs a heatsink (Acorn suggests a 1 × 2 inch piece of aluminium) if the supply is above 9 V, or if Acorn's home-made supply circuit is built without its 4.7 Ω resistor.

Checks:

1. Measure the DC input at pins 31 and 32.
2. Measure +5 V at the IC13 output and on the keyboard card at pin 20 of the strip.
3. If the input is above 9 V, check that the regulator has a heatsink.

In a rack, the CPU card is fed from the rack's 5 V supply instead: IC13 and C10 are removed and a link fitted. A card that has been converted this way must not be connected to an unregulated supply.

Cassette levels

The tape interface records and plays back at 300 mV RMS. R3 on the keyboard card sets the output level and R12 the input gain. Acorn's setting for the input is a waveform at IC7 pin 7 of more than 2 V peak to peak without clipping. Acorn recommends a recorder with line-level input and output over one with only microphone and earphone sockets.

Capacitors

Each card has one electrolytic, a 15 µF 16 V part: C10 on the regulator input of the CPU card, and C5 on the cassette amplifier bias on the keyboard card. Everything else is ceramic. See Acorn System 1 Capacitor Replacement Guide.

Preventive maintenance checklist

1. Inspect the supply and lead; measure the DC input and +5 V.
2. Check the regulator temperature and heatsink.
3. Reseat the ICs; check IC1's orientation.
4. Check the 20-way and 16-way strips for cracked joints.
5. Clean the edge connector if fitted.

Related pages

• Acorn System 1
• Acorn System 1 Troubleshooting Guide
• Acorn System 1 Capacitor Replacement Guide
• Acorn System 1 Technical Manual
• Acorn System 1 Circuit Diagrams