Acorn Risc PC 600 Maintenance Guide
Maintenance of the Acorn Risc PC 600 (1994, main board 0197,000) centres on three things: the rechargeable backup cell on the main board, the surface-mount electrolytic capacitors, and the mains power supply in the base of the case.[1]
Safety
[edit | edit source]The power supply is a mains unit in the base of the case, with an IEC mains inlet and a switched IEC outlet on the back.[1] Unplug the mains lead before removing the top. Warning: the primary side of any mains switch-mode supply holds a dangerous charge in its reservoir capacitor for some time after unplugging. If you open the supply itself, discharge that capacitor and measure it before touching the primary side. The rest of the machine runs at 5 and 12 V. Use an earthed wrist strap when handling the processor card, SIMMs, VRAM and main board.
Opening the case
[edit | edit source]Acorn's description of the case, from the Technical Reference Manual:[1]
- The top is held on by two glass-reinforced nylon twist-lock pins. Turn them and lift the top off.
- The centre section is held by two more twist-lock pins. Unplug the drives and lift the centre section out to reach the main board and power supply.
- The floppy and hard disc drives are held by spring clips (some early machines also have screws on the floppy).
- The main board clips into place and is held by one screw, which also earths it to the case. The power supply is held by one screw.
Each extra centre section takes one 3.5-inch drive, one 5.25-inch drive and up to two expansion cards, and needs longer twist-lock pins. Acorn notes that the power supply may need upgrading if more than two sections are used.[1]
Chris Whytehead notes that the spring holding up the front flap has broken on many Risc PCs, leaving the flap sagging.[2]
Backup battery
[edit | edit source]The clock and CMOS RAM (a PCF8583, IC20) are backed by BT1, a 1.2 V 280 mAh nickel-metal hydride cell (Acorn 0817,016). It is trickle charged from +5 V while the machine is on.[1] On Acorn's circuit diagram the charging path runs from +5 V through diode D2 and R130 (180 Ω) to the cell, with R212 (180 Ω) in its negative lead. The clock chip is fed through R133 (47 Ω) and decoupled by C70 (4.7 µF).[3]
Check BT1 on any machine that has been stored. If it has leaked, remove it before doing anything else. Stardot members clean the leakage from these cells with white vinegar, scrubbing with a toothbrush, then rinse with water (distilled in hard-water areas) and dry the board for a day or two before powering it.[4] Leakage makes the solder hard to flow, so clean the area before trying to desolder parts.[4] Then check the tracks around the battery, D2, C70 and the PCF8583 for continuity. A replacement should be a 1.2 V rechargeable cell, because the board charges it. More on rebuilding and replacing cells is on Battery Refurbishment.
Link LK5 is the CMOS write-protection link and is fitted on pins 1 and 2 as standard.[1] If configuration changes will not save, check that link before suspecting the battery.
Surface-mount capacitors
[edit | edit source]The main board has 24 surface-mount aluminium electrolytic capacitors (10 µF, 47 µF, 4.7 µF and 1 µF), as well as three surface-mount tantalums and eight radial electrolytics.[1] Owners on stardot report electrolyte leaking from the surface-mount parts, sometimes hidden under the capacitor until it is removed. One owner found an unstable picture in high-resolution modes caused by C122 (4.7 µF) leaking, the first leaking capacitor he had seen; his four other Risc PC boards showed none.[5] The full list and the recap procedure are on Acorn Risc PC 600 Capacitor Guide.
Fuses
[edit | edit source]The main board has two surface-mount fuses, FS1 (2 A) and FS2 (800 mA).[1] If one function is dead while the supply rails are good, check them for continuity.
Power supply checks
[edit | edit source]The supply's main board connector, PL4, is six-way: pins 1 and 4 red (+5 V), pins 2 and 5 black (0 V), pin 3 yellow (+12 V), pin 6 blue (−12 V).[1]
| Rail | Nominal | Limits | Ripple and noise |
|---|---|---|---|
| +5 V | +5.1 V | 4.90 to 5.20 V (+2 %, −4 %) | 50 mV |
| +12 V | +12.0 V | ±7 % (11.16 to 12.84 V) | 100 mV |
| −12 V | −12.0 V | ±7 % | 50 mV |
The supply needs a minimum load of 0.5 A on +5 V and 0.1 A on +12 V, so measure it connected to the machine, not on the bench with no load.[1] Expansion cards get −5 V from a 7905 regulator on the backplane, not from the supply.[1] Check that the supply fan turns and the vents are clear.
Cleaning and reseating
[edit | edit source]- Brush and blow dust from the main board, the power supply vents and fan, and the drives.
- Reseat the processor card, the DRAM SIMMs and the VRAM card.
- Clean the edge connector for the backplane, the expansion card contacts and the network card slot.
Hard disc
[edit | edit source]The hard disc is IDE, on the 40-way connector PL3.[1] RISC OS 3.50 cannot use ADFS hard discs larger than 512 MB; RISC OS 3.60 raised the limit to 128 GB.[6] A larger replacement drive therefore needs RISC OS 3.60 or later.
Preventive maintenance checklist
[edit | edit source]- Inspect BT1; remove it at once if it has leaked, and clean the board.
- Inspect the surface-mount electrolytics, especially the small values, for leakage.
- Measure the supply rails with the machine running, and check the fan.
- Check FS1 and FS2.
- Reseat the processor card, SIMMs and VRAM; clean the backplane connector.
- Back up the hard disc.
Related pages
[edit | edit source]- Acorn Risc PC 600
- Acorn Risc PC 600 Troubleshooting Guide
- Acorn Risc PC 600 Capacitor Guide
- Acorn Risc PC 700 Maintenance Guide
- Acorn Risc PC Technical Reference Manual
- Battery Refurbishment
References
[edit | edit source]- ↑ 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; backplanes and power requirements p. 1-21; links, plugs and sockets p. 2-44; main PCB parts list pp. 3-1 to 3-5; backplane parts lists p. 3-6; power supply, mechanical data and case construction pp. E-1 and E-2.
- ↑ Whytehead, Chris. "Acorn Risc PC 600", Chris's Acorns, archived by The Centre for Computing History (retrieved 2026-10-01). Broken front-flap spring; main boards 0197,000 issue C and issue D.
- ↑ Acorn Computers, "Medusa" main PCB circuit diagram, drawing 0197,000/C, sheet 1 of 7 (battery-backed RAM/RTC), 1994. Part of File:Acorn Risc PC Technical Reference Manual drawings.pdf.
- ↑ 4.0 4.1 "Where to start resurrecting a RiscPC 700", stardot.org.uk forum thread, April–May 2018. A corroded battery whose leakage had spread down the board to the reset switch; 1024MAK and danielj on cleaning with white vinegar and a toothbrush, rinsing, using distilled or demineralised water in hard-water areas, and drying for 24–48 hours before power-up; leakage makes solder hard to flow, so clean before desoldering.
- ↑ "RISC PC motherboard refurbish", stardot.org.uk forum thread, July 2020 to March 2022. sP1d3r found visible electrolyte leakage, none of the surface-mount electrolytics passing an in-circuit ESR test, and leakage under most of them that was not visible until they were removed; danielj on using 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.
- ↑ "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.