Josh: Create: detailed troubleshooting (PSU rails/fuses, functional test suite, ARM250; cited)

2026-06-17T20:07:10Z

Create: detailed troubleshooting (PSU rails/fuses, functional test suite, ARM250; cited)

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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.<ref name="sm">[[Acorn A3010, A3020 and A4000 Service Manual]], Acorn Computers — hosted on this wiki. Source for the dead-machine procedure, the "action if PSU fails" / "action if main PCB fails" routines, the 5 A mains fuse and internal fuse FS1, thermal shutdown, the +5 V/+12 V/−5 V rails, the ARM250 Dealer Test Disc functional tests, and the earth-continuity / DC-insulation safety tests.</ref><ref name="trm">[[Acorn A3010, A3020 and A4000 Technical Reference Manual]], Acorn Computers — hosted on this wiki. Main-PCB ("Roadrunner") circuit diagrams and hardware reference.</ref>

== ⚠️ Mains warning ==

The A4000 has a mains-powered switch-mode PSU inside the case. Disconnect the mains lead before removing the cover. Never open the PSU module; it is exchanged, not repaired, and the full earth-continuity and DC-insulation safety tests must be re-applied after any internal work.<ref name="sm" />

[[File:Acorn Archimedes A4000 circuit diagram (1 of 3).png|center|thumb|680px|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.<ref name="sm" />

# Switch off, unplug the mains lead.
# 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 — exchange it.
# Check the internal fuse '''FS1'''.
# 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.
# If the PSU runs briefly then cuts out, suspect '''thermal shutdown''' (check ventilation and the load); repeated shutdowns mean a faulty PSU.

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{| class="wikitable styled-table" style="width:90%; text-align:center;"
|+'''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:<ref name="sm" />

* 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 [[Acorn Archimedes A3000 Troubleshooting Guide|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:<ref name="sm" />

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''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 ==

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{| class="wikitable styled-table" style="width:100%; text-align:left;"
! 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.

== Common fault catalogue ==

* '''Completely dead''' — blown plug fuse or FS1, or a faulty PSU; check fuses, then exchange the PSU.
* '''Random resets / cuts out when warm''' — PSU thermal shutdown or failing PSU; check ventilation, exchange 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]]

== References ==

<references />

{{Navbox-AcornComputers|state=collapsed}}

[[Category:Acorn Computers]]
[[Category:Troubleshooting Guides]]

Acorn Archimedes A4000 Troubleshooting Guide - Revision history

Josh: Create: detailed troubleshooting (PSU rails/fuses, functional test suite, ARM250; cited)