Amstrad 464 Plus Troubleshooting Guide

464 Plus with the CM14 colour monitor (Amstrad service manual cover)

Amstrad's service manual for the Amstrad 464 Plus has no fault-finding charts. A note to engineers on its second page says an RP11 diagnostic ROM cartridge was available for the range, with full instructions and analysis in its own manual. Procedures are in the Amstrad 464 Plus Maintenance Guide. The Amstrad 6128 Plus Troubleshooting Guide covers the same board with a disc drive.

Power and start-up

The 464 Plus takes 5 V from the MM12 or CM14 monitor through a 5 mm socket, J104, and needs its cartridge to start: the board has no firmware ROM, and the BASIC and operating system are on the cartridge. With the cartridge in place, a working machine shows "Amstrad Microcomputer (v4)" and the 1985 Amstrad and Locomotive Software copyright line, then a menu to start BASIC or the game.

Power and start-up faults

Symptom	Probable cause	Action
No power LED	No 5 V from the monitor	Check that the monitor is on (its Power button in), then measure 5 V on the monitor's DC plug and at J104
LED on, blank screen	Cartridge missing, dirty or not seated	Switch off, reseat the cartridge, clean its contacts
LED on, blank screen, cartridge good	DIN lead, monitor brightness, ASIC	Check the 8-pin DIN lead and the monitor controls, then try another monitor
Mono picture too dark or too light	Luminance level	Set VR101 for 1.65 V at pin 3 of J102 (p. 4)

Amstrad's user instructions say the cartridge must be fitted or removed only with the computer switched off, and the computer must not be run without one.

Amstrad sets the monitors' 5 V output with VR501 to 4.80 V under a 2.4 A load, and no more than 5.25 V with no load; the MM12 page adds that a load of 3.6 ±0.4 A should bring the output to 0 V, which is the supply's overload protection. A reading of about 4.8 V at J104 with the computer running is normal. If the 5 V reads zero with the computer connected, unplug the computer's lead and measure the monitor output on its own before condemning the monitor.

Monitor and sound

The 464 Plus parts list includes no loudspeaker; the sound goes to the monitor on the 8-pin DIN lead, and the user instructions describe the volume control and built-in loudspeakers on the monitor. With no sound, check the monitor's volume control and the DIN lead before the computer. The 3.5 mm socket gives a stereo output for an external amplifier.

The monitors contain the mains supply and a CRT. See CRT Discharge Procedure before opening one.

Cassette loading

The user instructions list the usual causes of read errors: a creased or stretched tape, switching the computer off or on with PLAY or REC pressed (the power pulse through the head can scramble the tape), recordings that were paused, and tapes recorded on a machine whose head was misaligned. A tape that fails once may load at the next attempt.

Cassette faults

Symptom	Probable cause	Action
Motor runs, tape does not move	Perished main belt	Replace the main belt (part 270807); see the Amstrad 464 Plus Maintenance Guide
Motor does not run	Relay RY301, motor, deck switches	Check the relay (G4S-1112P-1-B-19) and the motor (part 270858)
Commercial tapes fail, own recordings load	Head azimuth	Set azimuth with the MTT-113 test tape (6.34 kHz) and an oscilloscope on jumper line A (p. 4)
All tapes fail	Dirty head, deck connections	Clean the head, capstan and pinch roller; check the cassette board PCB301 and its op-amp IC301 (LA6324)

Software that does not run

Not every CPC program runs on the Plus. Amstrad's user instructions warn that some software written for the CPC 464 and CPC 6128 may not operate correctly. Wikipedia attributes this to structural differences in the v4 firmware, which break programs that call firmware routines at fixed addresses, and records that Amstrad sold a separate cartridge with the CPC 6128's v3 firmware. The Plus hardware features (sprites, the 4,096-colour palette, DMA sound) are only available after an unlocking sequence and were used mainly by cartridge games. A CPC game that crashes on a 464 Plus but runs on a CPC 464 is not a hardware fault.

Peripherals

The Plus replaced the CPC's edge connectors with a 50-way Amphenol-style expansion connector, a 25-way D-type printer port and other new sockets, so CPC 464 peripherals need an adapter. Some cartridge games use the 15-pin analogue joystick socket or the RJ11 light-gun socket.

Keyboard

The key matrix for both Plus computers is on p. 11 of the service manual. Use it to see whether a group of dead keys shares a row or column. The keyboard is replaced as one assembly (UK part 270779).

Chips

The ASIC (IC101, AMS40489), the Z80A (IC102), the two KM41464AP-12 DRAMs (IC110, IC111) and the AY-3-8912 (IC114) are the main logic parts. Amstrad expected full diagnosis to be done with the RP11 cartridge; neither the cartridge nor its manual is hosted here.

Related pages

• Amstrad 464 Plus · Amstrad 464 Plus Maintenance Guide · Amstrad 464 Plus Capacitor Replacement Guide
• Amstrad 6128 Plus Troubleshooting Guide
• Amstrad CPC 464 Troubleshooting Guide
• Capacitor Failure Symptoms · CRT Discharge Procedure