Acorn Archimedes A4000 Maintenance Guide: Difference between revisions
Create: detailed maintenance guide (ARM250 Roadrunner; PSU/battery/IDE; cited) |
PSU: permit safe servicing/recap of the mains PSU for restoration; strengthen lethal-shock warnings and discharge procedure (replaces exchange-only/do-not-open wording) |
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== ⚠️ Safety: mains PSU == | == ⚠️ Safety: mains PSU == | ||
The A4000 is powered by a separate cased switch-mode PSU inside the unit. | The A4000 is powered by a separate cased switch-mode PSU inside the unit. It runs directly from the mains, and its primary side, reservoir capacitor and mains-suppression capacitors '''can hold a lethal charge even after the machine is unplugged''' — a shock from the primary side '''can kill you'''. '''Disconnect the mains lead before opening the case,''' and never work on the supply while it is plugged in. | ||
Acorn's service manual told dealers to exchange the whole PSU module rather than repair it, but that assumed replacement modules were available; none have been made for decades, so on a surviving machine the supply is '''recapped, not discarded'''. This is a legitimate restoration job, but only for someone competent with mains switch-mode supplies: discharge the reservoir capacitor and confirm 0 V before touching the primary side (procedure in the [[Acorn Archimedes A4000 Capacitor Guide]]). After any work, re-apply the full earth-continuity and DC-insulation (class 1) safety tests, because the A4000 is mains-earthed equipment.<ref name="sm">[[Acorn A3010, A3020 and A4000 Service Manual]], Acorn Computers — hosted on this wiki. Source for the PSU-fail and main-PCB-fail procedures, the 5 A mains fuse and internal fuse FS1, the thermal-shutdown behaviour, the +5 V/+12 V/−5 V rails (+12 V feeds the video connector SK1 pin 12), the functional test suite (ARM250 Dealer Test Disc) and the earth-continuity / DC-insulation safety tests. Its exchange-only PSU guidance dates from when spare modules were stocked.</ref> | |||
* Disconnect the mains lead before opening. The A4000 is unusually easy to open — two screws release the cover.<ref name="chris">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/A4000.html "Acorn A4000"], Chris's Acorns / The Centre for Computing History. Source for the ARM250 integration (ARM2, MEMC1a, VIDC1a, IOC), the "Roadrunner" board (0194,600 Issue 1) being almost entirely surface-mount, the model list (AGC10–AGC82), the Conner CFS105A/CFS210A drives and Citizen OSDA-75G-U floppy, and the easy two-screw disassembly.</ref> | * Disconnect the mains lead before opening. The A4000 is unusually easy to open — two screws release the cover.<ref name="chris">Whytehead, Chris. [https://chrisacorns.computinghistory.org.uk/Computers/A4000.html "Acorn A4000"], Chris's Acorns / The Centre for Computing History. Source for the ARM250 integration (ARM2, MEMC1a, VIDC1a, IOC), the "Roadrunner" board (0194,600 Issue 1) being almost entirely surface-mount, the model list (AGC10–AGC82), the Conner CFS105A/CFS210A drives and Citizen OSDA-75G-U floppy, and the easy two-screw disassembly.</ref> | ||
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# Switch off and unplug the mains lead. | # Switch off and unplug the mains lead. | ||
# Check the '''5 A fuse in the mains plug'''. If a replacement blows on switch-on, the PSU is faulty — | # Check the '''5 A fuse in the mains plug'''. If a replacement blows on switch-on, the PSU is faulty — service it (recap, see the [[Acorn Archimedes A4000 Capacitor Guide]]) or fit a known-good supply. | ||
# Check the internal fuse '''FS1'''. | # Check the internal fuse '''FS1'''. | ||
# If the PSU runs then shuts down, suspect thermal shutdown; if it keeps shutting down, the PSU is faulty. | # If the PSU runs then shuts down, suspect thermal shutdown; if it keeps shutting down, the PSU is faulty. | ||
A repeatedly-failing or ripple-prone supply is usually cured by recapping the PSU (see the [[Acorn Archimedes A4000 Capacitor Guide]]); observe the mains-shock warning above and discharge the reservoir capacitor before working on the primary side.<ref name="sm" /> | |||
== Capacitor health == | == Capacitor health == | ||
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! Item !! Symptom !! Action | ! Item !! Symptom !! Action | ||
|- | |- | ||
| Mains PSU module || Dead, random resets, thermal shutdown || Check plug fuse and FS1; | | Mains PSU module || Dead, random resets, thermal shutdown || Check plug fuse and FS1; recap the PSU (discharge first — see Capacitor Guide) or fit a known-good supply | ||
|- | |- | ||
| CMOS battery (top-right) || Lost config/clock; corrosion || Clean, repair tracks, replace cell | | CMOS battery (top-right) || Lost config/clock; corrosion || Clean, repair tracks, replace cell | ||
Revision as of 09:56, 19 July 2026
This guide covers preventive maintenance for the Acorn Archimedes A4000 (launched September 1992). The A4000 is a three-box ARM250 machine sharing its "Roadrunner" main board with the A3010 and A3020. It has an internal IDE hard disc, a separate cased mains switch-mode PSU, and a highly integrated board with few serviceable parts. The maintenance priorities are the mains PSU, the CMOS backup battery, and the ageing IDE hard disc.
⚠️ Safety: mains PSU
The A4000 is powered by a separate cased switch-mode PSU inside the unit. It runs directly from the mains, and its primary side, reservoir capacitor and mains-suppression capacitors can hold a lethal charge even after the machine is unplugged — a shock from the primary side can kill you. Disconnect the mains lead before opening the case, and never work on the supply while it is plugged in.
Acorn's service manual told dealers to exchange the whole PSU module rather than repair it, but that assumed replacement modules were available; none have been made for decades, so on a surviving machine the supply is recapped, not discarded. This is a legitimate restoration job, but only for someone competent with mains switch-mode supplies: discharge the reservoir capacitor and confirm 0 V before touching the primary side (procedure in the Acorn Archimedes A4000 Capacitor Guide). After any work, re-apply the full earth-continuity and DC-insulation (class 1) safety tests, because the A4000 is mains-earthed equipment.[1]
- Disconnect the mains lead before opening. The A4000 is unusually easy to open — two screws release the cover.[2]
- Observe anti-static precautions for the ARM250 and the RAM.
The machine and its board
The ARM250 integrates the four chips that were separate on the Acorn Archimedes A3000 — the ARM2 core, MEMC1a, VIDC1a and IOC — into one package running at 12 MHz.[2] Because of this, there are no socketed processor-family chips: a failure inside the ARM250 means replacing the whole device. The board is part 0194,600 ("Roadrunner"), the same board used by the A3020.
| Item | Detail |
|---|---|
| Processor | ARM250 @ 12 MHz (integrates ARM2 + MEMC1a + VIDC1a + IOC) |
| RAM | 2 MB standard, expandable to 4 MB |
| ROM | RISC OS 3.11 (2 ROMs) |
| Storage | 3.5" floppy (Citizen OSDA-75G-U) + internal IDE hard disc |
| Hard disc (by model) | AGC20 80 MB, AGC40 105 MB (Conner CFS105A), AGC50 210 MB (Conner CFS210A) |
| Expansion | one mini-podule slot; optional Econet |
| PSU | separate cased switch-mode module |
⚠️ CMOS backup battery
The A4000 keeps its configuration and clock in battery-backed CMOS RAM, with the cell at the top-right of the board.[2] As on every Acorn machine of this era, an ageing backup battery can leak and corrode the surrounding tracks. Inspect it on any unrestored machine, clean any residue with isopropyl alcohol, check the nearby tracks for continuity, and replace the cell. Lost configuration, a wrong clock or boot problems are the usual symptoms of a failed cell.
Opening and cleaning
- Unplug the mains lead and peripherals.
- Remove the two cover screws and slide the cover off. Disconnect the floppy and hard-disc data and power cables and the PSU connector to lift the board.[2]
- Brush and air-blow dust from the board, the PSU vents, the hard disc and the floppy mechanism. Good airflow matters because the hard disc and PSU share the case.
- Clean the mini-podule edge connector with a soft eraser; clean the keyboard, parallel, serial and video sockets with contact cleaner on a foam swab.
Hard disc
The internal IDE drives are Conner units (CFS105A / CFS210A). After decades, the common faults are mechanical: a drive that will not spin up (stiction) or that has become noisy. The IDE interface and the floppy share the on-board I/O. If replacing the drive, note that RISC OS 3.1's IDE filing system limits usable capacity to roughly 512 MB; larger drives, CompactFlash or SD adapters need third-party software or a podule. Keep a backup of the original drive's contents where possible.
Power supply checks
The PSU delivers +5 V (logic), +12 V (the video connector SK1 pin 12 and the disc motors) and −5 V. The Service Manual's "action if PSU fails" routine is the safe way to diagnose supply problems:[1]
- Switch off and unplug the mains lead.
- Check the 5 A fuse in the mains plug. If a replacement blows on switch-on, the PSU is faulty — service it (recap, see the Acorn Archimedes A4000 Capacitor Guide) or fit a known-good supply.
- Check the internal fuse FS1.
- If the PSU runs then shuts down, suspect thermal shutdown; if it keeps shutting down, the PSU is faulty.
A repeatedly-failing or ripple-prone supply is usually cured by recapping the PSU (see the Acorn Archimedes A4000 Capacitor Guide); observe the mains-shock warning above and discharge the reservoir capacitor before working on the primary side.[1]
Capacitor health
The A4000 board uses conventional leaded electrolytics plus surface-mount ceramic decouplers, and is not one of the Acorn machines that suffer leaking surface-mount electrolytics (those are the Acorn Archimedes A5000, the A4 laptop and the Risc PC).[3] The recap priorities are therefore the PSU module and any visibly aged board electrolytic. Full detail: Acorn Archimedes A4000 Capacitor Guide.
Functional and safety testing
Acorn's service procedure uses the ARM250 Dealer Test Disc, which runs audio, battery-backed RAM, disc-interface, external-port, hard-disc, IDE, joystick, keyboard/mouse, memory, real-time-clock, UHF-modulator and video tests, and produces a test report. After any repair the earth-continuity and DC-insulation (class 1) safety tests must be re-applied because the A4000 is mains-earthed equipment.[1]
Common failure points
| Item | Symptom | Action |
|---|---|---|
| Mains PSU module | Dead, random resets, thermal shutdown | Check plug fuse and FS1; recap the PSU (discharge first — see Capacitor Guide) or fit a known-good supply |
| CMOS battery (top-right) | Lost config/clock; corrosion | Clean, repair tracks, replace cell |
| IDE hard disc (Conner) | Won't spin up, noisy, boot errors | Mechanical failure; replace drive (mind the ~512 MB RISC OS 3.1 limit) |
| Mini-podule edge connector | Expansion unreliable | Clean fingers; reseat |
| ARM250 | No combination of CPU/MEMC/VIDC/IOC works | Whole-device replacement (no socketed sub-chips) |
Recommended tools
- Posidrive/Philips screwdrivers and an anti-static strap.
- DC multimeter (rail checks); oscilloscope for bus-level diagnosis.
- Isopropyl alcohol, foam swabs, soft eraser, contact cleaner.
- Replacement CMOS battery; replacement IDE drive or solid-state adapter.
- Earth-continuity and insulation testers if returning the machine to service (per Acorn's safety tests).
See Recommended Tools for the general toolkit.
Preventive maintenance checklist
- Inspect/replace the CMOS battery; clean any corrosion.
- Check the plug fuse and FS1; verify the +5 V, +12 V and −5 V rails.
- Clean the mini-podule edge connector and the floppy head.
- Back up and assess the IDE hard disc.
- Re-apply earth-continuity / insulation safety tests after any internal work.
Related pages
- Acorn Archimedes A4000
- Acorn Archimedes A4000 Troubleshooting Guide
- Acorn Archimedes A4000 Capacitor Guide
- Acorn Archimedes A3000 — the earlier discrete-chip machine
- Acorn A3010, A3020 and A4000 Service Manual · Acorn A3010, A3020 and A4000 Technical Reference Manual · Acorn A3010, A3020 and A4000 Training Reference Manual
- Capacitor Failure Symptoms · Recommended Tools
References
- ↑ 1.0 1.1 1.2 1.3 Acorn A3010, A3020 and A4000 Service Manual, Acorn Computers — hosted on this wiki. Source for the PSU-fail and main-PCB-fail procedures, the 5 A mains fuse and internal fuse FS1, the thermal-shutdown behaviour, the +5 V/+12 V/−5 V rails (+12 V feeds the video connector SK1 pin 12), the functional test suite (ARM250 Dealer Test Disc) and the earth-continuity / DC-insulation safety tests. Its exchange-only PSU guidance dates from when spare modules were stocked.
- ↑ 2.0 2.1 2.2 2.3 Whytehead, Chris. "Acorn A4000", Chris's Acorns / The Centre for Computing History. Source for the ARM250 integration (ARM2, MEMC1a, VIDC1a, IOC), the "Roadrunner" board (0194,600 Issue 1) being almost entirely surface-mount, the model list (AGC10–AGC82), the Conner CFS105A/CFS210A drives and Citizen OSDA-75G-U floppy, and the easy two-screw disassembly.
- ↑ Stardot forums (Acorn restoration community). The surface-mount electrolyte-leakage problem affects the Acorn machines fitted with SMD electrolytics; the through-hole-capacitor machines such as the A3020/A4000 are generally unaffected.