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Acorn Risc PC 700 Maintenance Guide

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Acorn Risc PC 700 motherboard
Acorn Risc PC 700 main board, 1208,000, with 2 MB VRAM and SIMMs fitted (photo: Chris's Acorns)

The Acorn Risc PC 700 (1995) uses the same case, power supply, backplanes and chip set as the Acorn Risc PC 600, on a revised main board, 1208,000, with 16-bit sound and a CD audio input.[1][2] Most maintenance is the same as on the 600 and is described in full on Acorn Risc PC 600 Maintenance Guide: opening the case, the power supply limits, cleaning and reseating.

Acorn's Risc PC Technical Reference Manual and its drawings document the 600's 0197,000 board.[3] The battery-circuit designators owners report on the 1208,000 board (D2, R133, R212, C70, IC20) match the 0197,000 diagram; check any other part on the board itself before relying on the 600 list.[4]

Safety

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The power supply is the same mains unit as on the 600. Unplug the mains lead before opening the case. Warning: the supply's primary reservoir capacitor holds a dangerous charge after unplugging; discharge and measure it before working on the primary side.

Backup battery

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The 1208,000 board keeps the 600's backup arrangement: a rechargeable 1.2 V cell charged from +5 V through D2, a PCF8583 clock and CMOS RAM chip (IC20), R212 in the negative lead and C70 (4.7 µF) on the clock chip's supply.[4] On the 0197,000 board this cell is BT1, a 1.2 V 280 mAh nickel-metal hydride part, trickle charged while the machine is on.[3]

The stardot user philpem traced a "CMOS unreadable" fault on a 1208,000 board through this circuit. The battery read 1.3 V but only 0.13 V reached the clock chip. The faults he found were diode D2 failed, a via under C70 gone open circuit, and a small decoupling capacitor next to R133, not shown on Acorn's diagram, that had leaked.[4] Check each of these on a board that loses its settings with a good battery.

Battery leakage on a 700 board can run a long way: one owner found it had spread down to the reset switch. Remove the cell, clean with white vinegar and a toothbrush, rinse, and dry the board for a day or two before powering it.[5] See also Battery Refurbishment.

Surface-mount capacitors

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On philpem's 1208,000 board all the 10 µF, 4.7 µF and 1 µF surface-mount electrolytics had leaked, and the 47 µF parts had not.[4] Another owner found leakage under most of the surface-mount electrolytics that could not be seen until they were removed.[2] Not every board is affected: one owner who found C122 leaking on a Risc PC board saw no leakage on his four other boards.[2] The capacitor list and procedure are on Acorn Risc PC 700 Capacitor Guide.

Hard disc

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The 700 shipped with RISC OS 3.60, which supports ADFS hard discs larger than 512 MB, up to 128 GB.[6] The drive connects to the 40-way IDE connector, as on the 600.[3]

Preventive maintenance checklist

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  1. Inspect the backup cell and remove it at once if it has leaked.
  2. Inspect the small surface-mount electrolytics (10 µF, 4.7 µF, 1 µF) for leakage.
  3. Measure the supply rails with the machine running, against the limits on Acorn Risc PC 600 Maintenance Guide.
  4. Reseat the processor card, SIMMs and VRAM; clean the backplane connector.
  5. Back up the hard disc.
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References

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  1. ↑ Whytehead, Chris. "Acorn Risc PC 700", Chris's Acorns, archived by The Centre for Computing History (retrieved 2026-10-01). 40 MHz ARM710, RISC OS 3.60, VRAM and 16-bit sound standard; main board 1208,000 in all three of Chris's machines.
  2. ↑ 2.0 2.1 2.2 "RISC PC motherboard refurbish", stardot.org.uk forum thread, July 2020 to March 2022. A 1208,000 "Series 3" board with CD audio input and 16-bit sound; leakage under most of the surface-mount electrolytics, not visible until they were removed; none passed an in-circuit ESR test. danielj (March 2022): C122 leaking on his issue 1 board, with no leakage on his four other boards.
  3. ↑ 3.0 3.1 3.2 Acorn Computers, Risc PC Technical Reference Manual, part no. 0497,057, issue 1, September 1994, hosted on this wiki. Battery p. 1-10; power requirements p. 1-21; parts lists Part 3 (0197,000 board); power supply and case pp. E-1 and E-2.
  4. ↑ 4.0 4.1 4.2 4.3 "(Repair) RISC PC Series 3 (1208,000) motherboard", stardot.org.uk forum thread, July 2020 to November 2022. philpem's repair of a 1208,000 board: "CMOS unreadable" with 1.3 V across the battery but 0.13 V at the clock chip; D2 failed short in both directions; the via under C70 had failed open; a leaking decoupling capacitor next to R133 that is not on Acorn's diagram; the 10 µF, 4.7 µF and 1 µF surface-mount electrolytics had leaked and the 47 µF parts had not.
  5. ↑ "Where to start resurrecting a RiscPC 700", stardot.org.uk forum thread, April–May 2018. A corroded battery on a Risc PC 700 whose leakage had spread down the board to the reset switch; cleaning with white vinegar and a toothbrush, rinsing, using distilled water in hard-water areas, and drying for 24–48 hours before power-up.
  6. ↑ "The Acorn Risc PC and A7000", 4corn Computers (retrieved 2026-10-01). Risc PC 700 shipped with RISC OS 3.60; RISC OS 3.60 supports ADFS hard discs larger than 512 MB, up to 128 GB.


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