Acorn Risc PC 600 Maintenance Guide

Acorn Risc PC 600 motherboard (board 0197,000): the two CPU-card sockets and RISC OS ROMs (left), IOMD and VIDC20, the VRAM and DRAM SIMM sockets, the on-board IDE/floppy connectors and the backplane socket.

This guide covers preventive maintenance for the Acorn Risc PC 600 (launched April 1994). The Risc PC is an ARM610 machine in a slice-based case with an internal mains switch-mode PSU and an internal IDE hard disc. The maintenance priorities are the mains PSU, the NiMH backup battery, and the surface-mount electrolytic capacitors, which leak and corrode the board.

Safety: mains PSU

The Risc PC has a fan-cooled mains switch-mode PSU in the base slice. Disconnect the mains lead before removing the cover, and never work on the supply while it is plugged in. Its primary side and its reservoir and mains-suppression capacitors can hold a lethal charge after the machine is unplugged — a shock can be fatal. The PSU can be opened and recapped as part of a restoration, but only if you are competent with mains switch-mode supplies: discharge the reservoir capacitor and confirm 0 V before touching the primary side. Observe anti-static precautions for the CPU card, IOMD, VIDC20, ROMs, SIMMs and VRAM.

Opening the slice case

1. Unplug the mains lead and peripherals.
2. Release the cover (the slices are held by twist-lock pins) and lift it off. The base slice holds the motherboard and PSU; centre slices hold drives and podules.
3. To reach the motherboard, lift the CPU card(s) from the CPU sockets, and remove the drive/podule slice. Note the CPU-card orientation before unplugging.
4. The TRM notes the PSU may need uprating if more than two case sections are fitted.

A common cosmetic fault is a broken front-flap spring, which lets the front flap sag; it does not affect operation.

NiMH backup battery

The Risc PC keeps its configuration and clock in CMOS RAM, backed by an on-board NiMH cell (BT1, 1.2 V, 280 mAh). These cells leak with age, and the electrolyte corrodes the nearby tracks. Inspect BT1 on any unrestored machine; if it has leaked, remove it, neutralise and clean the residue with isopropyl alcohol, check the tracks for continuity, and fit a replacement. A flat cell shows up as lost configuration and a wrong clock at each power-on.

Surface-mount capacitor leakage

The Risc PC main board uses surface-mount aluminium electrolytic capacitors, and like the Acorn Archimedes A5000 and the Acorn A4 laptop it suffers from electrolyte leakage as they age. The leaked electrolyte is corrosive and attacks the pads and tracks, which causes progressive and intermittent faults. On any un-recapped Risc PC, plan to replace the surface-mount electrolytics and clean the board; full verified list and procedure: Acorn Risc PC 600 Capacitor Guide.

On-board fuses

The motherboard carries two surface-mount fuses: FS1 (2 A) and FS2 (800 mA). A dead subsystem with otherwise-healthy rails can be a blown on-board fuse rather than a failed chip; check FS1/FS2 for continuity.

Cleaning and reseating

• Brush and air-blow dust from the board, the PSU fan and vents, and the drives.
• Reseat the CPU card(s) in their sockets; reseat the DRAM SIMMs and the VRAM. Intermittent faults on a 30-year-old machine are often a contact issue.
• Clean the podule backplane connector and the podule/network-card fingers with a soft eraser or contact cleaner.
• Clean the rear ports (VGA, keyboard, mouse, parallel, serial, audio).

Storage

The Risc PC has an on-board IDE interface and 3.5" floppy (and an optional CD-ROM). The IDE drives are now decades old; mechanical failure (failure to spin up, noise) is the usual fault. RISC OS 3.50 limits ADFS hard discs to about 512 MB; RISC OS 3.60 raised this to 128 GB, so a 3.60 upgrade helps when fitting a larger replacement (or a CompactFlash/SD adapter).

Common failure points

Acorn Risc PC 600 — failure-prone items

Item	Symptom	Action
Surface-mount electrolytics	Progressive/intermittent faults, corrosion	Recap; clean residue; repair tracks
NiMH battery (BT1)	Lost config/clock; corrosion	Clean, repair tracks, replace cell
Mains PSU module	Dead, random resets, fan/thermal issues	Check rails and fan; recap the PSU (discharge first) or fit a known-good supply
On-board fuses FS1/FS2	A subsystem dead with rails present	Check FS1 (2 A) and FS2 (800 mA) continuity
CPU card / SIMM / VRAM seating	Won't start, RAM errors	Reseat the CPU card, SIMMs and VRAM
IDE hard disc	Won't spin up, boot errors	Replace drive (mind the RISC OS 3.5 ~512 MB limit)
Podule backplane / contacts	Expansion unreliable	Clean contacts; reseat

Recommended tools

• Posidrive/Philips screwdrivers and an anti-static strap.
• DC multimeter (rail and fuse checks); oscilloscope for bus-level work.
• Hot-air or fine-tip soldering equipment, solder wick, flux and isopropyl alcohol for the surface-mount recap and battery/track repair.
• Replacement NiMH cell (or a modern coin-cell + diode modification) and a replacement IDE drive or solid-state adapter.

See Recommended Tools for the general toolkit.

Preventive maintenance checklist

1. Inspect/replace the NiMH battery (BT1); clean any corrosion.
2. Recap the surface-mount electrolytics and clean the board (Acorn Risc PC 600 Capacitor Guide).
3. Verify the PSU rails and that the fan runs; check FS1/FS2.
4. Reseat the CPU card, SIMMs and VRAM; clean the backplane contacts.
5. Back up and assess the IDE drive; consider a RISC OS 3.60 upgrade for larger discs.

Related pages

• Acorn Risc PC 600
• Acorn Risc PC 600 Troubleshooting Guide
• Acorn Risc PC 600 Capacitor Guide
• Acorn Risc PC 700 — the 1995 ARM710 model
• Acorn Risc PC Technical Reference Manual
• Capacitor Failure Symptoms · Recommended Tools