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Acorn Risc PC 700 Troubleshooting Guide

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Acorn Risc PC 700 front
Risc PC 700 front panel (photo: Chris Whytehead, Chris's Acorns)

Fault-finding on the Acorn Risc PC 700 (ARM710, main board 1208,000) follows the Risc PC 600 procedure: the same power-on self test, floppy-light codes and supply limits. The differences that matter are the 40 MHz ARM710 processor card, RISC OS 3.60 and the 1208,000 board, for which Acorn's Technical Reference Manual has no parts list or drawings.[1][2]

The power supply is a mains unit in the base of the case. 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 inside the supply.

Self-test codes

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A failed test leaves a red screen and flashes an eight-digit hexadecimal code on the floppy light: eight groups of four flashes, long for 1 and short for 0.[1]

POST fault bits (Technical Reference Manual p. A-1)
Bit Meaning
00000200 ROM failed checksum test
00001000 IOC register test failed
00004000 VIDC video (Virq interrupt) timing failed
00008000 VIDC sound (Sirq interrupt) timing failed
00010000 CMOS unreadable
00020000 RAM control line failure
00040000 Long RAM test failure

Status bit 00000080 (VRAM present) is normal on a 700, because every model has VRAM.[1][2] The meaning of each screen colour and what to check for each bit are on Acorn Risc PC 600 Troubleshooting Guide.

Power

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Measure the supply with the machine running: +5 V between 4.90 and 5.20 V, +12 V and −12 V within 7 %.[1] Check the fan. On the 0197,000 board the main board fuses are FS1 (2 A) and FS2 (800 mA);[1] confirm the positions on a 1208,000 board before replacing anything.

"CMOS unreadable" and lost settings

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The stardot user philpem worked through this fault on a 1208,000 board with a good 1.3 V battery and only 0.13 V at the PCF8583 clock chip (IC20):[3]

  1. Measure the cell, then the voltage at IC20 pin 8 (supply) to pin 4 (0 V).
  2. Measure the drops across R212 (180 Ω, in the battery's negative lead) and R130 and R133. A large drop means current is leaking somewhere on the clock chip's supply.
  3. Test D2. On philpem's board it read 0.6 V in both directions.
  4. Check C70 (4.7 µF) and the via under its positive pad, which had failed open.
  5. Check the small decoupling capacitor beside R133, between IC20 pins 8 and 4. It is not on Acorn's diagram, and on this board it had leaked.

All of these were found on one board.[3] Before working on the battery circuit, check that the CMOS write-protection link LK5 is fitted across pins 1 and 2 (its position on the 0197,000 board).[1]

Leaking capacitors

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On philpem's board all the 10 µF, 4.7 µF and 1 µF surface-mount electrolytics had leaked, and the 47 µF parts had not.[3] An unstable picture in high-resolution modes on another Risc PC board was traced to C122 (4.7 µF).[4] See Acorn Risc PC 700 Capacitor Guide.

Other symptoms

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Common symptoms
Symptom Probable cause Action
Completely dead, no fan Mains fuse or power supply Check the plug fuse, then the supply
Red screen, floppy light flashing POST fault Read the code (table above)
Stops on a purple screen Major system fault Reseat the processor card, SIMMs and VRAM; inspect the board for corrosion
Hard disc not found IDE cable, power lead or drive Reseat the 40-way IDE cable and the power lead; try another drive. RISC OS 3.60 supports discs above 512 MB[5]
Fails only with a PC card or second processor fitted Card or its seating Test with the ARM710 card alone
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Acorn Computers, Risc PC Technical Reference Manual, part no. 0497,057, issue 1, September 1994, hosted on this wiki. Covers the 0197,000 board. Power On Self Test pp. A-1 and A-2 (the version in the RISC OS 3.50 ROMs, release 2.04); power supply data p. E-1.
  2. ↑ 2.0 2.1 Whytehead, Chris. "Acorn Risc PC 700", Chris's Acorns, archived by The Centre for Computing History (retrieved 2026-10-01). 40 MHz ARM710 (VY86C710A), RISC OS 3.60, VRAM and 16-bit sound standard, main board 1208,000.
  3. ↑ 3.0 3.1 3.2 "(Repair) RISC PC Series 3 (1208,000) motherboard", stardot.org.uk forum thread, July 2020 to November 2022. philpem's "CMOS unreadable" repair on a 1208,000 board: 1.3 V across the battery and 0.13 V at the clock chip; voltage dropped across R212 and across R130/R133; D2 reading 0.6 V both ways; the via under C70 open circuit; a leaking decoupling capacitor beside R133 between IC20 pins 8 and 4 that is not on Acorn's diagram.
  4. ↑ "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; danielj found C122 leaking on an issue 1 board that had become unstable in high-resolution modes.
  5. ↑ "The Acorn Risc PC and A7000", 4corn Computers (retrieved 2026-10-01). RISC OS 3.60 supports ADFS hard discs larger than 512 MB, up to 128 GB.


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