Acorn System 2 Maintenance Guide
The Acorn System 2 (1980) is four Acorn Eurocards on an eight-slot backplane, run from one 5 V supply. Maintenance is cleaning and reseating the cards and ICs, checking the backplane and supply, and setting the VDU and cassette levels.[1]
Safety
[edit | edit source]The cards run at 5 V. If a mains power supply is fitted in or beside the frame, unplug it before working on the system; its primary side carries mains voltage. Acorn's manual asks for the card frame to be earthed through the backplane's earth wire if the supply's 0 V is not earthed.[1] Many of the ICs are MOS parts; handle them with earthed tools.[1]
Cards and positions
[edit | edit source]From the left of the frame, Acorn's layout for the four-card System 2 is: power supply space, an empty slot (the left-hand backplane slot, whose address bus is isolated for bus buffers), then the CPU, cassette interface, VDU interface and memory cards, with the components facing right.[1]
| Card | Board | Notes for maintenance |
|---|---|---|
| 6502 CPU | 200,000 | IC1 (6502) faces the opposite way to its neighbours. The COS ROM is in IC7; IC5, IC6 and the regulator IC13 are not fitted.[1] |
| Cassette interface | 200,001 | 64-way plug with pins A2–A31, B1 and B21–B31 cut off; a wire link from IC6 pin 3 to B14.[1] |
| VDU interface | 200,002 | MC6845, SAA5050, two 2114-3 RAMs; preset R3 sets character width.[1] |
| Memory | 200,003 | 2114 RAM in IC3–IC10, BASIC ROM in IC17, link at RAM 1 and ROM 6.[1] |
The CPU card connects to the cassette card through four wired links on side B of the backplane: A29 to B11 (φ2 clock), B19 to B12 (CASIN), B20 to B13 (CASOUT) and B21 to B14 (NHITON). On issue 3 CPU cards, three of these signals reach the CPU card's connector through fine wires from IC2 pins 16, 17 and 18.[1]
Cleaning and reseating
[edit | edit source]- Unplug the supply, then slide each card out of its guides.
- Clean the 64-way connectors and inspect the backplane solder joints. Side B pins carry hand-wired links; check that none has broken off.[1]
- Reseat each IC, making sure no pin folds under it.[1]
- Refit the cards with components facing right. If a connector does not mate freely, Acorn suggests loosening the backplane connector screws and moving the backplane slightly.[1]
Power supply
[edit | edit source]Measure 5 V at the backplane with the system running. Acorn specifies at least 1.5 A, or 2 A with 8 KB of RAM fitted.[1] The cards have no regulators of their own, so a low or noisy 5 V affects every card.
Display and cassette
[edit | edit source]- The composite output is 1 V into 75 Ω on pin 15 of the VDU card's front connector, with 0 V on pin 14. If a BNC socket is fitted to a front panel, its body must be insulated from the panel.[1]
- With the COS running, adjust R3 on the VDU card for stable characters of the correct width.[1]
- Cassette levels are 300 mV RMS in and out. R3 on the cassette card sets the output, R12 the input; IC7 pin 7 should show more than 2 V peak to peak without clipping on playback.[1]
Capacitors
[edit | edit source]Each card has one electrolytic; the rest are ceramic. See Acorn System 2 Capacitor Replacement Guide.
Preventive maintenance checklist
[edit | edit source]- Check the supply and earth connection; measure 5 V at the backplane under load.
- Clean the card connectors and check the side B wire links.
- Reseat the ICs.
- Set R3 on the VDU card, and the cassette levels if tapes are unreliable.
Related pages
[edit | edit source]- Acorn System 2
- Acorn System 2 Troubleshooting Guide
- Acorn System 2 Capacitor Replacement Guide
- Acorn System 1 Maintenance Guide
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 Acorn Computers, System 2 Technical Manual, issue 1, January 1980. Text transcription by Ben Harris (1998), hosted on this wiki as Acorn System 2 Technical Manual (PDF). Parts lists pp. 2-1 to 2-4; electrical assembly chapter 3 (capacitor polarity p. 3-5, CPU card p. 3-6, cassette card p. 3-8, memory card p. 3-10, backplane and earthing pp. 3-11 and 3-12); card frame and card positions pp. 4-2 to 4-4; power supply, keyboard, cassette and display pp. 5-1 to 5-4; Cassette Operating System chapter 6.