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Acorn Risc PC 600 Troubleshooting Guide

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Revision as of 08:57, 1 October 2026 by Josh (talk | contribs) (Rewrite POST section from TRM App. A in prose; add supply limits, LK4/LK5, ROM and RTC designators from the circuit diagram; remove uncited claim that code 00020000 is a leaked-capacitor symptom; cite board damage to stardot)
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Acorn Risc PC 600 (photo: Christof Efkemann, CC BY-SA 2.5)

The Acorn Risc PC 600 (ARM610, main board 0197,000) tests itself at every power-on and reports faults on screen and by flashing the floppy drive light. The test, the codes and the supply limits are documented in the Acorn Risc PC Technical Reference Manual.[1] Acorn's circuit diagrams for the board are hosted as File:Acorn Risc PC Technical Reference Manual drawings.pdf.

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.

Power-on self test

The POST in the RISC OS 3.50 ROMs is release 2.04. The screen colour shows its progress:[1]

  1. Purple: testing has started. Brief ROM and RAM tests run, then IOMD and VIDC20 are set up and tested. This phase is quick. If the machine crashes or stops here, Acorn says the fault is a major one, most probably in the I/O system.
  2. The screen mode is then set from the monitor ID pin: a VGA-type mode if a VGA, SVGA or multiscan monitor pulls ID[0] low, otherwise a 15 kHz TV mode. No stable picture at this point means a monitor connection fault, a video fault, or a fault that stops the test software running.
  3. Blue: a longer memory test, then short video and sound tests.
  4. Purple again: the processor ID is read. This needs working RAM. A purple screen that stays means a major system fault.
  5. Green for a pass or red for a fail. A pass is not normally seen, because RISC OS starts at once and shows a black screen with the memory size. On a fail the screen stays red and the floppy light flashes the fault code.

Hold any power-on key combination until the memory size message appears; keys released earlier are ignored.[1]

Reading the floppy light

The code is eight hexadecimal digits, flashed as eight groups of four flashes. A long flash is binary 1 and a short flash is 0. Acorn's example is a ROM failure on an ARM610 machine, code 00000219.[1]

POST fault bits (Technical Reference Manual p. A-1)
Bit Meaning Where to look
00000200 ROM failed checksum test The two RISC OS ROMs (IC14, IC15) and their sockets
00001000 IOC register test failed IOMD and its supply and clock
00004000 VIDC video (Virq interrupt) timing failed VIDC20 and its clock
00008000 VIDC sound (Sirq interrupt) timing failed VIDC20 and the sound DMA path
00010000 CMOS unreadable PCF8583 (IC20), its supply from the battery circuit, and the I²C lines
00020000 RAM control line failure SIMM seating, the SIMM sockets and the tracks around them
00040000 Long RAM test failure The SIMMs: try each alone, or a known-good SIMM

Only bits 9 to 31 are faults. Bits 0 to 8 are status: 00000001 power-on test, 00000040 integrated I/O controller fitted, 00000080 VRAM present, 00000100 CMOS RAM checksum error.[1] A POST interface box on the POST test connector (LK4, six-way) shows each test and the PASS or FAIL code on an LCD.[1][2]

If bit 7 of CMOS byte &BC is set, the POST skips the long memory test. Acorn advises against it, because it also disables the VIDC tests.[1]

Power

  1. Check the fuse in the mains plug and the mains lead.
  2. With the machine running, measure the supply at the main board: +5 V between 4.90 and 5.20 V, +12 V and −12 V within 7 %. Ripple should be under 50 mV on +5 V.[1] The supply needs at least 0.5 A on +5 V to regulate, so test it under load.[1]
  3. Check that the supply fan runs.
  4. Check the board fuses FS1 (2 A) and FS2 (800 mA).[1]
  5. A 4-slot backplane needs the 103 W supply; the 70 W supply is for machines with no backplane or the 2-slot one.[1]

Symptoms

Common symptoms
Symptom Probable cause Action
Completely dead, no fan Mains fuse or power supply Check the plug fuse, then the supply
Fan runs, nothing on screen, no POST colours Supply rail out of limits, processor card, or monitor Measure the rails; reseat the processor card; try another monitor
Stops on the first purple screen Major fault in the I/O system Check IOMD, the ROMs and the board for corrosion
Red screen, floppy light flashing POST fault Read the code and use the table above
Settings and clock lost at every power-on Battery circuit, or LK5 Check BT1 and its charging path; check that LK5 (CMOS write protection) is on pins 1 and 2[1]
Unstable picture in high-resolution modes Leaking surface-mount capacitor One owner traced this to C122 (4.7 µF) leaking[3]
Hard disc not found IDE cable, power lead or drive Reseat the 40-way IDE cable on PL3 and the drive power; RISC OS 3.50 cannot use ADFS discs over 512 MB[4]
Fails only with a second processor or PC card fitted Card or its seating Test with the ARM610 card alone in its slot

Board damage

Two kinds of leakage damage the 0197,000 board: electrolyte from the surface-mount capacitors and alkaline leakage from the battery. Inspect the board under magnification around BT1, the clock chip and the SIMM sockets before replacing chips.[3][5] Cleaning and replacement are covered on Acorn Risc PC 600 Maintenance Guide and Acorn Risc PC 600 Capacitor Guide.

References

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 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; links, plugs and sockets p. 2-44; parts lists pp. 3-1 to 3-6; Power On Self Test pp. A-1 and A-2; power supply data p. E-1.
  2. ↑ Acorn Computers, "Medusa" main PCB circuit diagram, drawing 0197,000/C, sheets 1–7, 1994. Sheet 1: processor slots, IOMD, ROMs IC14 and IC15, POST test connector LK4, battery-backed RAM/RTC (BT1, D2, R130, R133, R212, C70, PCF8583 IC20) and reset. Part of File:Acorn Risc PC Technical Reference Manual drawings.pdf.
  3. ↑ 3.0 3.1 "RISC PC motherboard refurbish", stardot.org.uk forum thread, July 2020 to March 2022. danielj found C122 had leaked on an issue 1 board that had become unstable in high-resolution modes; sP1d3r found leakage under most of the surface-mount electrolytics on another board.
  4. ↑ "The Acorn Risc PC and A7000", 4corn Computers (retrieved 2026-10-01). RISC OS 3.60 added support for ADFS hard discs larger than 512 MB, up to 128 GB.
  5. ↑ "Dissolving Risc PC 600", stardot.org.uk forum thread, January 2016. Heavy corrosion around the CMOS chip and the edge of the RAM connectors on a Risc PC 600; a 47 µF 16 V surface-mount electrolytic had come off with its pads; vinegar cleaning; using a leaded capacitor wired to nearby joints where pads are lost.


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