Acorn Archimedes A4000 Troubleshooting Guide

This guide gives diagnosis for the Acorn Archimedes A4000 (ARM250 "Roadrunner" board, 0194,600). It follows the module-level fault-finding of the Acorn A3010, A3020 and A4000 Service Manual, supported by the Acorn A3010, A3020 and A4000 Technical Reference Manual circuit diagrams. Because the ARM250 integrates the ARM2, MEMC1a, VIDC1a and IOC into one package, processor-family faults are isolated to that single device rather than to separate chips.

⚠️ Mains warning — risk of fatal shock

The A4000 has a mains-powered switch-mode PSU inside the case. Its primary side and its reservoir and mains-suppression capacitors can hold a lethal charge even after the machine is unplugged — a shock from them can kill you. Disconnect the mains lead before removing the cover, and never work on the supply while it is plugged in. 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 (see the Acorn Archimedes A4000 Capacitor Guide). Re-apply the earth-continuity and DC-insulation safety tests after any internal work.

Acorn A4000 "Roadrunner" main PCB circuit diagram, sheet 1 of 3 (Technical Reference Manual), showing the ARM250, the battery-backed RAM/RTC, reset circuitry and the filtered +5 V supply. Sheets 2 and 3 are also on the wiki. (Source: The Centre for Computing History.)

First steps: power

Most "dead A4000" faults are in the PSU or its feed, so confirm power before anything else.

1. Switch off, unplug the mains lead.
2. Check the 5 A fuse in the mains plug. Fit a new 5 A fuse; if it blows on switch-on, the PSU is faulty — service it (see the Acorn Archimedes A4000 Capacitor Guide) or fit a known-good replacement supply.
3. Check the internal fuse FS1.
4. Measure the rails at the PSU-to-PCB connector: +5 V (logic), +12 V (video connector SK1 pin 12 and the disc motors) and −5 V. A single missing rail points to the PSU.
5. If the PSU runs briefly then cuts out, suspect thermal shutdown (check ventilation and the load); repeated shutdowns mean a faulty PSU.

A4000 supply rails

Rail	Feeds	Note
+5 V	Logic (ARM250, RAM, ROM, I/O)	Main logic supply
+12 V	Video connector SK1 pin 12; floppy/HD motors	Disc + monitor feed
−5 V	Analogue / video	Low-current rail

Dead machine with power present

If the rails are good but the machine is dead, the fault is on the main PCB. Acorn's procedure is to inspect the board, then run the functional test software:

• Check the ROMs are correctly seated with no lifted pins, and the configuration links LK10, LK11, LK12 and LK14.
• Because the ARM2, MEMC1a, VIDC1a and IOC are inside the ARM250, the bus-level checks used on the discrete-chip A3000 (clock present, reset released, DRAM RAS/CAS, address lines incrementing under a held reset, data bus not stuck) apply here at the ARM250 pins and the RAM. If the clock, reset and rails are present but the ARM250 produces no bus activity, the ARM250 itself is the suspect — it is a whole-device replacement.
• As on the A3000, do not leave a board powered with no clock: loss of DRAM refresh can damage the RAM.

Functional tests (ARM250 Dealer Test Disc)

For a machine that is partly working, Acorn's functional test software isolates the fault by subsystem. The test disc runs, and reports on:

A4000 functional tests and what they exercise

Test	Exercises
Audio test	VIDC sound output, the audio summing/filter stage and the headphone output
Battery-backed RAM test	CMOS RAM and its battery backup
Disc interface test	floppy controller and drive (needs a scratch disc)
IDE hard disc interface test	the on-board IDE interface and drive
Hard disc exerciser / initialiser	surface and seek behaviour; formatting
External port tests	parallel, serial and (if fitted) Econet
Joystick test	analogue joystick inputs (A3010)
Keyboard and mouse tests	keyboard matrix and quadrature mouse
Memory test	the 2 MB (or 4 MB) DRAM
Real-time test	the real-time clock
UHF modulator test / Video tests	RF output and the RGB/video modes

Video faults

Symptom	Probable cause	Action
No display, machine boots (disc/sound active)	Monitor, cable, or VIDC output in the ARM250	Check the analogue RGB at the 15-pin video connector; try another monitor; run the video test
Corrupt/striped display	DRAM fault	Run the memory test; a single failed device gives a repeating pattern
No RF picture but RGB works	UHF modulator	Run the UHF modulator test; check the modulator and its feed
Dead, no rails	PSU	Use the power procedure above

Storage faults

• Hard disc not detected: check the 40-way IDE cable and the drive power; reseat. RISC OS will not see a drive on a broken cable or a dead drive.
• Drive spins but won't boot / read errors: ageing Conner drive (stiction or surface faults). Run the hard-disc exerciser; replace the drive if it fails. Mind the ~512 MB usable-capacity limit of RISC OS 3.1's IDE filing system when fitting a replacement.
• Floppy won't read: clean the head and try a known-good disc before suspecting the controller; run the disc interface test.

Keyboard, mouse and ports

• Dead keyboard / mouse: the keyboard interface and quadrature mouse are handled through the I/O in the ARM250; run the keyboard and mouse tests. A dead keyboard with a working display points to the keyboard, its cable, or the I/O.
• Parallel/serial faults: run the external-port tests; check the connector and the I/O.
• Expansion only fails with a card fitted: clean the mini-podule edge connector and check for bridged contacts.

Boots to the Supervisor prompt instead of the desktop

If the machine starts to a text screen at the Supervisor * prompt (or a BASIC > prompt) instead of loading the RISC OS desktop, the desktop itself is almost certainly fine — the machine has lost its configured startup because the CMOS settings are corrupt or have been cleared.

• To reach the desktop now: type *Desktop and press Return (at the Supervisor prompt the leading * is optional, so Desktop also works). The desktop loads normally for the rest of the session.
• Why it happens: the setting that tells RISC OS to start the desktop at power-on is held in battery-backed CMOS RAM. A flat or leaked backup battery loses or corrupts that setting, so the machine falls back to the Supervisor prompt. It usually appears alongside a wrong clock, lost configuration, or the CMOS POST faults (checksum error, or "CMOS unreadable" on a battery-corroded board).
• Permanent fix: replace the backup battery (see the Acorn Archimedes A4000 Maintenance Guide) so the configuration is retained, then restore a known-good configuration — hold Delete at power-on until the screen clears to reset CMOS to its defaults (which re-enable desktop startup), and set anything machine-specific again with *Configure. With a healthy battery fitted, the machine boots straight to the desktop again.

If *Desktop reaches the desktop but the machine still drops to the Supervisor on every cold start even after a CMOS reset, the battery is not holding charge — fit a new cell.

Common fault catalogue

• Boots to the Supervisor * prompt, not the desktop — corrupt/cleared CMOS from a flat battery; type *Desktop to enter RISC OS, then replace the battery and reset CMOS.
• Completely dead — blown plug fuse or FS1, or a faulty PSU; check fuses, then service the PSU (recap) or fit a known-good supply.
• Random resets / cuts out when warm — PSU thermal shutdown or ageing PSU electrolytics; check ventilation, then recap the PSU.
• Boots but no video — monitor/cable or the VIDC section of the ARM250; run the video test.
• Corrupt display / crashes — DRAM; run the memory test.
• No hard disc — IDE cable, drive power, or a dead/stiction Conner drive.
• Lost time/config — flat or leaked CMOS battery (see the maintenance guide).
• No combination of CPU/memory/video works — the ARM250 itself; whole-device replacement.

Related pages

• Acorn Archimedes A4000
• Acorn Archimedes A4000 Maintenance Guide
• Acorn Archimedes A4000 Capacitor Guide
• Acorn Archimedes A3000 Troubleshooting Guide — bus-level ARM/MEMC/VIDC/IOC checks
• Acorn A3010, A3020 and A4000 Service Manual · Acorn A3010, A3020 and A4000 Technical Reference Manual