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Acorn Risc PC 600 Capacitor Guide

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Acorn Risc PC 600 motherboard
Acorn Risc PC 600 main board, 0197,000 (photo: Chris's Acorns)

The Acorn Risc PC 600 main board (0197,000) has 24 surface-mount aluminium electrolytic capacitors, three surface-mount tantalums and eight radial electrolytics. All values below are from the parts lists in Acorn's Risc PC Technical Reference Manual.[1] Positions are on Acorn's main PCB assembly drawing, hosted with the circuit diagrams as File:Acorn Risc PC Technical Reference Manual drawings.pdf (pages 8 and 9).[2]

Why recap

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Owners on stardot report electrolyte leaking from the surface-mount electrolytics on Risc PC boards. On one board there was leakage under most of them that could not be seen until they were removed, and none passed an in-circuit ESR test.[3] On another, all the 10 µF, 4.7 µF and 1 µF parts had leaked and the 47 µF parts had not.[4] A leaking C122 (4.7 µF) gave one machine an unstable picture in high-resolution modes.[3] Battery leakage destroys pads too: on a 600 with heavy corrosion around the clock chip and the RAM connectors, a surface-mount electrolytic had come off with its pads.[5] Not every board leaks. danielj, who reported the C122 failure, said it was the first leaking capacitor he had seen and that his four other Risc PC boards showed no sign of leakage, and Ian Jeffray names only the early A5000 and the A4 as Acorn machines to recap as a matter of course.[3][6] Inspect the small surface-mount electrolytics and the board around them, and replace the ones that have leaked.

Main board electrolytics and tantalums

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Risc PC 0197,000 main board polarised capacitors (parts list pp. 3-1, 3-2)
Value Voltage Type Acorn part no. Designators Qty
10 µF 16 V Aluminium, surface-mount 0647,001 C29, C53, C59, C60, C86, C90, C91, C133, C145, C164, C176 11
47 µF 16 V Aluminium, surface-mount 0647,002 C61, C74, C79, C87, C93, C116, C123, C136, C139, C172 10
4.7 µF 25 V Aluminium, surface-mount 0647,000 C70, C122 2
1 µF 50 V Aluminium, surface-mount 0647,005 C134 1
220 µF 16 V Aluminium, radial 0635,230 C2, C65, C73, C152, C161, C162 6
100 µF 25 V Aluminium, radial 0642,103 C168, C175 2
10 µF 16 V Tantalum, surface-mount 0681,101 C12, C64 2
47 µF 10 V Tantalum, surface-mount 0681,100 C88 1

C68, C76, C89 and C137 are listed as not fitted. The other main board capacitors are ceramics: 33 nF and 100 nF decouplers and small-value multilayer parts.[1] C70 decouples the supply to the PCF8583 clock chip, next to the battery.[2]

Cards and backplanes

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Polarised capacitors on the Risc PC cards (parts lists pp. 3-6, 3-10, 3-11)
Board Designators Value Type
2-slot backplane, 0197,101 C5, C6, C7 47 µF 16 V Aluminium, axial (0622,470)
4-slot backplane, 0197,001 C1, C2, C3 47 µF 16 V Aluminium, axial (0622,470)
ARM610 processor card, 0197,208 C1 47 µF 10 V Aluminium, axial (0621,470)
VRAM card, 1 MB and 2 MB C9, C10, C11, C12 47 µF 16 V Aluminium, surface-mount (0647,002)

Both backplanes carry a 7905 regulator (Q1) that supplies −5 V to the expansion cards.[1]

Recap procedure

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  1. Unplug the mains lead. Remove the top and centre sections of the case (twist-lock pins), unplug the drives, and take out the processor card, SIMMs and VRAM card. The main board clips in and is held by one screw.[1]
  2. Photograph the board and note the value, voltage and polarity of every part before removing it.
  3. Remove each surface-mount electrolytic with hot air, two irons or heated tweezers. danielj on stardot advises against twisting the parts off, which can lift pads.[3] The parts between the SIMM sockets are the hardest to reach.[4]
  4. Clean the residue from the pads and the board around them, and check the tracks and vias. On one board the via under C70 had failed open.[4]
  5. Fit new parts of the same capacitance and the same or higher voltage, with the polarity matching the board marking.
  6. Where a pad has gone, use a leaded capacitor wired to the nearest undamaged joints on the same circuit.[5]

Checks after recapping

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  1. Measure the supply at the main board with the machine running: +5 V between 4.90 and 5.20 V, +12 V and −12 V within 7 %.[1]
  2. Watch the power-on self test. A pass goes straight to the RISC OS start-up screen; a red screen with a flashing floppy light is a fault code (see Acorn Risc PC 600 Troubleshooting Guide).
  3. Check that the clock and settings survive a power cycle.
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 Acorn Computers, Risc PC Technical Reference Manual, part no. 0497,057, issue 1, September 1994, hosted on this wiki. Main PCB assembly parts list pp. 3-1 and 3-2; 2-way and 4-way backplane parts lists p. 3-6; ARM610 PCB parts list p. 3-10; VRAM 1 MB and 2 MB PCB parts lists p. 3-11. Part number series: 0647 surface-mount aluminium electrolytic ("ALEC SMD"), 0635 and 0642 radial aluminium, 0621 and 0622 axial aluminium, 0681 surface-mount tantalum.
  2. ↑ 2.0 2.1 Acorn Computers, Risc PC engineering drawings (Technical Reference Manual Appendix C), 1994: main PCB circuit diagram 0197,000/C (PDF pages 1–7), main PCB assembly drawing (pages 8–9), ARM610 card (10–11), VRAM cards (12–14), backplanes (15–18). Hosted on this wiki as File:Acorn Risc PC Technical Reference Manual drawings.pdf.
  3. ↑ 3.0 3.1 3.2 3.3 "RISC PC motherboard refurbish", stardot.org.uk forum thread, July 2020 to March 2022. sP1d3r: visible leakage, no surface-mount electrolytic passing an in-circuit ESR test, leakage under most of them not visible until removal; danielj: use hot air, two irons or heated tweezers rather than twisting the parts off; C122 leaking on an issue 1 board, giving an unstable picture in high-resolution modes.
  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 of a 1208,000 board: 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 is the hardest part; the via under C70 had failed open.
  5. ↑ 5.0 5.1 "Dissolving Risc PC 600", stardot.org.uk forum thread, January 2016. Battery corrosion around the CMOS chip and the edge of the RAM connectors; a surface-mount electrolytic, which 1024MAK identified as 47 µF 16 V, had come off with its pads, and he suggests a leaded capacitor wired to nearby undamaged joints instead.
  6. ↑ Stardot forums, "cap list for acorn a3010", July 2024 (reply by IanJeffray, 9 July 2024).


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