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StrongARM RISC PC Capacitor Guide

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Acorn StrongARM CPU cards 1111,150 and 1111,160 (LinuxJedi)
StrongARM cards 1111,160 (top, Intel SA-110, recapped) and 1111,150 issue 3 (bottom, Digital SA-110). The four tantalums C1, C7, C16 and C17 and the regulators Q1 and Q2 are at the right of each card; the small aluminium electrolytics are C2, C9, C14 and C19 (photo: LinuxJedi)

The StrongARM RISC PC has electrolytic capacitors on four assemblies: the 1208,000 main board, the StrongARM processor card, the backplane and the power supply. Acorn published a parts list for the earlier 0197,000 Risc PC main board and for the backplanes in the Risc PC Technical Reference Manual, but none for the 1208,000 board, the StrongARM card or the supply.[1] The card values below were read from photographs of two cards; the main board values come from owners' repair reports and from Acorn's list for the older board.

What fails

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  • On the main board, the stardot user philpem found that every 10 µF, 4.7 µF and 1 µF surface-mount electrolytic on his 1208,000 board had leaked, while the 47 µF parts had not; the via under C70, the 4.7 µF capacitor by the clock chip, had also failed open.[2] Another owner found leakage under most of the surface-mount electrolytics on a 1208,000 board that could not be seen until the parts were off.[3]
  • LinuxJedi bought an Intel StrongARM card with deposits over its metal parts. After cleaning it he replaced its electrolytics because they had been "physically abused", and the card then worked.[4]
  • On a StrongARM machine restored by LinuxJedi, a wire added by a previous owner had snapped the leg off one of the backplane's axial electrolytics, and another looked as if it had leaked.[5]
  • Electrolyte from the backup cell damages the board around it; see StrongARM RISC PC Maintenance Guide.

StrongARM processor card

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The card is a surface-mount board with two three-terminal regulators, Q1 and Q2, at one end and four tantalum capacitors beside them. Four small aluminium electrolytics sit elsewhere on the card. The layout is the same on the Digital-built 1111,150 issue 3 card and the Intel-built 1111,160, but the parts differ.[6]

StrongARM card polarised capacitors (read from photographs)
Ref Type 1111,150 issue 3 (Digital) 1111,160 (Intel)
C1 Tantalum, surface-mount 68 µF 16 V 68 µF 10 V (marked 686, 10)
C7 Tantalum, surface-mount 68 µF 16 V 68 µF 10 V
C16 Tantalum, surface-mount 68 µF 16 V 68 µF 10 V
C17 Tantalum, surface-mount 68 µF 16 V 68 µF 10 V
C2 Aluminium, surface-mount 10 µF 16 V 10 µF 16 V
C9 Aluminium, surface-mount 10 µF 16 V 10 µF 16 V
C14 Aluminium, surface-mount 10 µF 16 V 10 µF 16 V
C19 Aluminium, surface-mount 10 µF 16 V 10 µF 16 V

Source: LinuxJedi's photographs.[6] The regulators are marked LT1117 on the Digital card and AS1117 on the Intel card. The other capacitors on the card are small unpolarised chip parts. Acorn's 1994 Technical Reference Manual predates the card, and Acorn published no values for it, so check each value and the polarity mark on your own card before ordering. The four aluminium parts are the ones LinuxJedi replaced.[4]

Main board 1208,000

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Acorn's parts list covers only the 0197,000 board of the Risc PC 600. On that board the surface-mount aluminium electrolytics are eleven 10 µF 16 V, ten 47 µF 16 V, two 4.7 µF 25 V (C70 and C122) and one 1 µF 50 V; there are also six 220 µF 16 V and two 100 µF 25 V radial parts and three surface-mount tantalums.[1] The full designator list is on Acorn Risc PC 700 Capacitor Guide. The 1208,000 board in the StrongARM Risc PC adds 16-bit sound,[7] and philpem found a leaking capacitor beside R133 that is not on Acorn's diagram,[2] so read every part on your own board and record its value, voltage and polarity before removing it.

Backplane

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Both backplanes carry three axial electrolytics of 47 µF 16 V, Acorn part 0622,470: C5, C6 and C7 on the two-slot backplane, and C1, C2 and C3 on the four-slot backplane.[1] They are through-hole parts and easy to replace.

Power supply

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The StrongARM Risc PC uses the 70 W or 103 W Risc PC supply; Chris Whytehead's single-slice SRP26 has the 103 W unit.[1][7] Acorn gave only the ratings, and owners on the RISC OS Open forum in 2024 knew of no circuit diagram for either supply. In that thread a 103 W supply measured +5.06 V, +11.45 V and −12.1 V off load in a machine that rebooted after about 20 seconds, and one reply suggested checking the 400 V electrolytic at the mains input.[8] No capacitor list for these supplies has been published, so read the parts in your own supply. Warning: the input electrolytic holds mains-derived voltage after the supply is unplugged; discharge it and measure it before touching the primary side.

Procedure

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  1. Unplug the mains lead and take off the top cover. Remove any expansion cards, then the PC card if fitted, then the StrongARM card.[9]
  2. Wear an earthed wrist strap while handling the card and main board.[9]
  3. Photograph each assembly and write down the value, voltage and polarity of every electrolytic before removing it.
  4. Remove the surface-mount parts with hot air, two irons or heated tweezers, without twisting them off the pads.[3]
  5. Clean off any electrolyte and check the pads, tracks and vias under each part; on the main board check the via under C70.[2]
  6. Fit parts of the same capacitance and the same or higher voltage, with the polarity matching the board.
  7. Refit the StrongARM card in PROC I/F slot 0, power up holding Delete, and check that the machine starts.[9]
  8. Soak test the machine for several hours, as Acorn's Application Note 297 recommends after any work on a StrongARM system.[10]

Checks after recapping

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Supply limits at the main board (Technical Reference Manual p. E-1)
Rail Nominal Tolerance Ripple and noise (pk-pk)
+5 V +5.1 V +2 %, −4 % 50 mV
+12 V +12.0 V ±7 % 100 mV
−12 V −12.0 V ±7 % 50 mV

Measure with the machine running; the supply needs at least 0.5 A on +5 V and 0.1 A on +12 V to regulate.[1] Then check that the clock and CMOS settings survive a power cycle. If the machine shows data aborts or crashes after the work, follow StrongARM RISC PC Troubleshooting Guide.

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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 0197,000 parts list, pp. 3-1 and 3-2; 2-way and 4-way backplane parts lists, p. 3-6; supply ratings, tolerances and ripple, p. E-1. No parts list for the 1208,000 board, any processor card or the supply.
  2. ↑ 2.0 2.1 2.2 "(Repair) RISC PC Series 3 (1208,000) motherboard", stardot.org.uk forum thread, July 2020 to November 2022. philpem's repair: the 10 µF, 4.7 µF and 1 µF surface-mount electrolytics all leaked, the 47 µF parts had not; the via under C70 open circuit; a leaking capacitor beside R133 that is not on Acorn's diagram.
  3. ↑ 3.0 3.1 "RISC PC motherboard refurbish", stardot.org.uk forum thread, July 2020 to March 2022. Leakage under most of the surface-mount electrolytics on a 1208,000 board; removal with hot air, two irons or heated tweezers; danielj found C122 leaking on one issue 1 board and none on four others.
  4. ↑ 4.0 4.1 LinuxJedi, "Intel Inside (but not how you would expect)", LinuxJedi's /dev/null, 30 March 2023 (retrieved 2026-10-19). Intel StrongARM card with deposits on its metal parts; ultrasonic cleaning, deionised water rinse and oven drying; electrolytic capacitors replaced "because they have been physically abused"; card working at 236 MHz afterwards. Photographs of this card before and after, and of his Digital card.
  5. ↑ LinuxJedi, "Acorn RiscPC #2: Restoration Part 1", LinuxJedi's /dev/null, 3 May 2023 (retrieved 2026-10-19). A double-slice Risc PC sold as a StrongARM machine (revision 3 board, StrongARM 233 card); a grounding cable screwed to the supply and case top had snapped the leg off an axial capacitor on the podule backplane, with possible leakage from another; leaking battery.
  6. ↑ 6.0 6.1 Values read from LinuxJedi's photographs of a 1111,150 issue 3 card (Digital SA-110K) and a 1111,160 card (Intel SA-110, before and after recapping), in "Intel Inside (but not how you would expect)", 30 March 2023, hosted here as File:Acorn StrongARM CPU cards 1111,150 and 1111,160 (LinuxJedi).jpg. The aluminium parts on the Intel card are marked 10 µF 16 V both before and after his recap. Chris Whytehead's photographs of 200 MHz and 233 MHz cards show the same layout: File:Acorn ART10 233MHz StrongARM CPU card.jpg.
  7. ↑ 7.0 7.1 Whytehead, Chris. "Acorn StrongARM Risc PC", Chris's Acorns (retrieved 2026-10-19). Both SRP26 machines have the issue 3 main board, part 1208,000, with 16-bit sound; one with a corroded CMOS battery.
  8. ↑ "Trouble with power supply of my RiscPC", RISC OS Open forum, January 2024 (retrieved 2026-10-19). 103 W supply reading +5.06 V, +11.45 V and −12.1 V unconnected, machine rebooting after about 20 seconds; no circuit diagram known to the posters; a stardot parts pack of diodes and capacitors for the supply mentioned; Jean-Michel Brück suggesting the 400 V input electrolytic and measuring under load on the main board.
  9. ↑ 9.0 9.1 9.2 Acorn Computers, Acorn StrongARM card and RISC OS 3.7 Upgrade Instructions, part no. 1403,222, issue 1, August 1996, hosted as File:Acorn StrongARM Card and RISC OS 3.7 Upgrade Instructions.pdf. Static precautions with a wrist strap on the serial port shell; avoid touching the pins of the chips and components on the processor card, p. 3; expansion cards out before the processor card is changed, pp. 7–8.
  10. ↑ Acorn Risc Technologies, Engineering Support Application Note 297, Hardware Modification and Testing Procedures to enhance StrongARM / PC Card compatibility, issue 1.00, 14 January 1997, hosted as File:Acorn Application Note 297 StrongARM PC Card Compatibility.pdf. Soak testing "for several hours, eg overnight", with the continuous soak test in the Risc PC Dealer Test suite, p. 4.


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