Acorn Archimedes A4000 Troubleshooting Guide

Fault diagnosis for the Acorn Archimedes A4000 follows Acorn's module-level procedure in the Acorn A3010, A3020 and A4000 Service Manual, with designators and links from the Acorn A3010, A3020 and A4000 Technical Reference Manual. The ARM250 contains the ARM2, MEMC1a, VIDC1a and IOC functions, so a fault in any of them is a fault in that one device.

Warning: the power supply inside the case runs from the mains. Disconnect the mains lead before removing the cover. The supply is a primary switched-mode unit; see the Acorn Archimedes A4000 Capacitor Guide before opening it, and repeat the earth continuity and DC insulation tests after any work on it.

Acorn A4000 main PCB circuit diagram, sheet 1 of 3. Sheets 2 and 3 are also on the wiki.

Power-on self-test

After a power-on reset the machine runs a short self-test, shown by the screen colour. A soft reset (the Reset key) skips it and starts RISC OS straight away.

1. Purple: a brief ROM and RAM test, and the I/O and video controllers are set up. A crash here usually means a major fault, probably in the I/O system.
2. Blue: the simple memory test passed and a longer test is running (up to 12 seconds on a large memory).
3. Brief tests of the video and sound controller.
4. Purple again: a test for an ARM3 (not fitted on an A4000).
5. Black, with a memory-size message: the test has finished and RISC OS starts.

During the test the screen is in a 15 kHz mode, so a VGA monitor shows an unsynchronised picture until RISC OS starts. If the test finds a fault, the screen turns red and the floppy drive light flashes the fault code as eight groups of four flashes, most significant first, a long flash for 1 and a short flash for 0.

Self-test status and fault bits (Acorn Service Manual p. 1-4)

Bit (hex)	Meaning
00000001	Self-test due to power-on
00000002	Self-test due to interface hardware
00000008	Long memory test performed
00000010	ARM3 fitted
00000040	Integrated I/O controller fitted
00000100	CMOS RAM checksum error
00000200	ROM failed checksum test
00000400	MEMC CAM mapping failed
00000800	MEMC protection failed
00001000	IOC register test failed
00004000	VIDC (Virq interrupt) timing failed
00008000	Sound (Sirq interrupt) timing failed
00010000	CMOS RAM (clock/calendar chip) unreadable
00020000	RAM control line failure
00040000	Long RAM test failure

Retro Kit lists the codes most often seen on the A30x0, A4000 and A5000: &00109 (CMOS checksum invalid, usually a flat cell leaving random data), &10049 (CMOS unreadable) and &20141 (RAM control line failure). The last two are usually battery leakage damage to the board or to the clock chip. To reset the CMOS RAM to defaults, hold Delete while switching on until the screen shows a red border.

No power

Acorn describes three ways the supply fails: complete failure, failure of one rail, and thermal shutdown.

A4000 power faults

Symptom	Probable cause	Action
No green LED, Caps Lock does not respond	Mains plug fuse, or the supply	Check the 5 A plug fuse. If a new fuse blows at switch-on, the supply is faulty.
Machine on but floppy, hard disc or board dead	+5 V missing	Measure +5 V at PL26. Check the supply connector on the board and the drive power leads.
No picture on a SCART television, hard disc does not spin	+12 V missing	Measure +12 V at PL26; it feeds SK1 pin 12 and the hard disc.
Works for a while, then switches off	Thermal shutdown from a failed part inside the supply	Repair or replace the supply.

PL26 carries +5 V, 0 V, −12 V and +12 V from the supply. The board also has its own fuses, FS1 (2 A leaded) and FS4 (250 mA surface-mount).

Dead machine with power present

If the rails are present but the machine does not start, Acorn's checks on the main PCB are:

• the ROMs are in the right sockets with no bent or lifted pins;
• other socketed devices are in the right place and the right way round;
• the memory size links are set correctly: LK31 and LK32, 1–2 and 2–3 for 2 MB, 2–3 and 1–2 for 4 MB;
• there is no loose debris on the board.

The service manual also tells the engineer to check ROM links LK10, LK11, LK12 and LK14 and memory links LK23 and LK24. Those are the A5000's link numbers; the Technical Reference Manual's link table for the A3010, A3020 and A4000 has no ROM size links and gives LK31 and LK32 for memory size.

Then check the board around the backup cell (BT2, top right) for leakage damage; a RAM control line failure or an unreadable CMOS code points there. If the clock, reset and rails are present and the ARM250 (IC13) still shows no bus activity, the ARM250 is the suspect.

Functional tests

On a machine that starts, Acorn's ARM250 dealer test disc (part 0294,825) isolates the fault. Insert it and switch on with Shift and Delete held; this also clears the CMOS RAM, so save the owner's settings first. The tests that apply to the A4000 are audio, battery-backed RAM, disc interface, external ports (printer and serial), hard disc exerciser and initialiser, hardware check, IDE interface, keyboard and mouse, memory, real time and video. The joystick and UHF modulator tests are for the A3010.

Acorn's notes for some failures:

• Battery-backed RAM test: check that the CMOS test data file is right before suspecting the board. If this test fails, the later tests cannot be trusted, because they use the CMOS RAM.
• IDE test: the configuration must be IDEdiscs 1; check the 40-way cable at both ends.
• Floppy: the configuration must be floppies 1. On the drive power cable, pin 1 (the conductor marked white) carries +5 V and pins 2 and 3 are 0 V.
• Audio: if only the speaker fails, check the speaker lead to LK19, the solder joints on the speaker tags and the crimps.
• Keyboard: try a known good keyboard. If that also fails, the fault is on the main PCB.

Acorn's hard disc tests may destroy the data on the disc, so its procedure replaces the owner's drive with a test drive before running them.

No picture

During the self-test the picture on a VGA monitor is unsynchronised, so judge the machine by what happens after RISC OS starts. According to members of the Stardot forum, the A3010, A3020 and A4000 read the monitor type and sync from the monitor's ID pins through a 15-pin VGA lead, if both are configured to auto and the monitor is switched on before the computer. They suggest switching on with T held to select the alternate sync, or with keypad 4 held to force a VGA monitor type.

Starts at the Supervisor prompt

A machine that starts at the Supervisor * prompt instead of the desktop has usually lost its CMOS settings to a flat or leaking cell. Type *Desktop to reach the desktop for this session. Running *Desktop with corrupt settings can give "Not enough memory in module area". Replace the cell, then reset the CMOS RAM by holding Delete at power-on and set the configuration again with *Configure. If the machine still loses its settings overnight with a new cell fitted, check the cell's charging circuit and the tracks around it.

Related pages

• Acorn Archimedes A4000
• Acorn Archimedes A4000 Maintenance Guide
• Acorn Archimedes A4000 Capacitor Guide
• Acorn Archimedes A3020 Troubleshooting Guide
• Acorn Archimedes A3000 Troubleshooting Guide