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Acorn Risc PC 700 Capacitor Guide

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Acorn Risc PC 700 motherboard
Acorn Risc PC 700 main board, 1208,000 (photo: Chris's Acorns)

The Acorn Risc PC 700 uses main board 1208,000, a later design than the 600's 0197,000, with 16-bit sound and a CD audio input on the board.[1][2] Acorn's Technical Reference Manual gives a full parts list for the 0197,000 board only.[3] Read the value and voltage off each part on a 1208,000 board before ordering, and use the 0197,000 list below as a guide to what to expect.

What leaks

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philpem recapped a 1208,000 board on stardot and found every 10 µF, 4.7 µF and 1 µF surface-mount electrolytic had leaked, while the 47 µF parts had not. The hardest parts to reach were those between the SIMM sockets.[4] On the same board the via under C70, the 4.7 µF capacitor on the clock chip's supply, had failed open, and a small capacitor beside R133 that is not on Acorn's diagram had leaked.[4] Not every board leaks: danielj found a leaking C122 on one of his Risc PC boards and none on his four others, and Ian Jeffray names only the early A5000 and the A4 as Acorn machines to recap as a matter of course.[2][5]

Reference list from the 0197,000 board

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Polarised capacitors on the 0197,000 board (Technical Reference Manual pp. 3-1, 3-2)
Value Voltage Type Designators on 0197,000
10 µF 16 V Aluminium, surface-mount C29, C53, C59, C60, C86, C90, C91, C133, C145, C164, C176
47 µF 16 V Aluminium, surface-mount C61, C74, C79, C87, C93, C116, C123, C136, C139, C172
4.7 µF 25 V Aluminium, surface-mount C70, C122
1 µF 50 V Aluminium, surface-mount C134
220 µF 16 V Aluminium, radial C2, C65, C73, C152, C161, C162
100 µF 25 V Aluminium, radial C168, C175
10 µF 16 V Tantalum, surface-mount C12, C64
47 µF 10 V Tantalum, surface-mount C88

The 1208,000 board has the 16-bit sound circuit that the 0197,000 lacks, so expect extra or moved parts in that area.[1] Acorn part numbers and the full list are on Acorn Risc PC 600 Capacitor Guide.

The backplanes and VRAM cards are the same as the 600's: the 2-slot backplane has C5–C7 and the 4-slot C1–C3, all 47 µF 16 V axial, and each VRAM card has C9–C12, 47 µF 16 V surface-mount.[3] The manual has no parts list for the ARM710 processor card.

Procedure

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  1. Unplug the mains lead, open the case and take out the processor card, SIMMs and VRAM card before lifting the main board.
  2. Record the value, voltage and polarity of every electrolytic before removing it.
  3. Remove the surface-mount parts with hot air, two irons or heated tweezers.[2]
  4. Clean the pads and check the tracks and vias, especially around C70 and the clock chip.[4]
  5. Fit new parts of the same capacitance and the same or higher voltage, with the polarity matching the board.
  6. Power up and confirm a normal start, then check that the clock and settings survive a power cycle. If the self test fails, read the floppy-light code on Acorn Risc PC 700 Troubleshooting Guide.
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References

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  1. ↑ 1.0 1.1 Whytehead, Chris. "Acorn Risc PC 700", Chris's Acorns, archived by The Centre for Computing History (retrieved 2026-10-01). Main board 1208,000; 16-bit sound standard.
  2. ↑ 2.0 2.1 2.2 "RISC PC motherboard refurbish", stardot.org.uk forum thread, July 2020 to March 2022. Recap of 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, and none passing an in-circuit ESR test; danielj on using hot air, two irons or heated tweezers rather than twisting the parts off; danielj in March 2022: C122 leaking on his issue 1 board, the first leaking capacitor he had seen, with no sign of leakage on his four other boards.
  3. ↑ 3.0 3.1 Acorn Computers, Risc PC Technical Reference Manual, part no. 0497,057, issue 1, September 1994, hosted on this wiki. Main PCB (0197,000) parts list pp. 3-1 and 3-2; backplanes p. 3-6; VRAM cards p. 3-11.
  4. ↑ 4.0 4.1 4.2 "(Repair) RISC PC Series 3 (1208,000) motherboard", stardot.org.uk forum thread, July 2020 to November 2022. philpem's recap: the 10 µF, 4.7 µF and 1 µF surface-mount electrolytics had all leaked and the 47 µF ones had not; the tight gap between the SIMM sockets and the capacitors; the via under C70 (4.7 µF, beside the clock chip) had failed open; a leaking decoupling capacitor beside R133 not shown on Acorn's diagram; date codes up to early 1997.
  5. ↑ Stardot forums, "cap list for acorn a3010", July 2024 (reply by IanJeffray, 9 July 2024).


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