Amstrad 464 Plus Maintenance Guide

The Amstrad 464 Plus is a CPC 464 rebuilt around a single ASIC, with a cartridge slot on the left and a cassette deck on the right. Its 5 V supply comes from the monitor. The main board, keyboard and cartridge board are the same assemblies as in the Amstrad 6128 Plus, so the board, cartridge and keyboard sections below apply to both machines; the 6128 Plus pages cover its disc drive. Part numbers and adjustments are from Amstrad's Plus range service manual. Faults are in the Amstrad 464 Plus Troubleshooting Guide and capacitor values in the Amstrad 464 Plus Capacitor Replacement Guide.

The 464 Plus main board MC0122, top view (Amstrad service manual p. 5). The 6128 Plus uses the same assembly.

Power and safety

The computer takes 5 V DC through a 5 mm socket (J104) from the MM12 or CM14 monitor and carries no mains voltage. The monitors contain the mains supply and a CRT and hold a dangerous charge after switch-off; read CRT Discharge Procedure before opening one.

Amstrad's safety tests for the monitors, to be repeated after any work, are a flash test at 1.5 kV RMS for 3 seconds between the live and neutral poles of the mains lead and all accessible metal, an insulation test at the same voltage requiring more than 4 MΩ, and a mains-lead earth resistance of no more than 0.5 Ω. Parts marked with a warning triangle in the parts lists are safety items and must be replaced with parts of identical safety specification.

Cartridge slot

The cartridge holds the firmware and BASIC, so a machine with a dirty or damaged cartridge connection will not start at all. Amstrad's user instructions say the cartridge may be fitted or removed only with the computer's slide switch off, and that the computer must not be operated without a cartridge. The slot is on its own small board, PCB401 (PCB ASSY MC0121), under the cartridge opening; the exploded view shows it fitted to the top cabinet above the main board.

• Switch off before touching the cartridge.
• Clean the cartridge edge contacts with isopropyl alcohol on a lint-free swab and let them dry.
• Inspect the slot contacts on PCB401 for bent or dirty fingers with the case open.

Opening the unit

Amstrad's exploded view shows the bottom cabinet assembly (part 270774) held to the top cabinet by screws marked 205, the main board PCB101 fixed to the top cabinet by screws marked 204, the keyboard SW101 lying between the board and the bottom cabinet, the cassette board PCB301 on the right and the cartridge board PCB401 at the top left.

1. Switch off the computer and the monitor, unplug the monitor from the mains, and disconnect the 5 V and DIN leads.
2. Remove the cartridge.
3. Turn the computer over on a soft cloth and remove the screws from the bottom cabinet.
4. Lift the bottom away carefully: the keyboard and the board connectors sit between the two halves.
5. Photograph the cable routing before you unplug anything.

Luminance output adjustment

The only adjustment on the main board sets the luminance signal for the monochrome monitor.

1. Connect an oscilloscope to pin 3 of J102, the 8-pin DIN socket.
2. Adjust VR101 for a signal of 1.65 V, measured as shown in Amstrad's Fig. 1.

The same procedure, on the same VR101, appears in the 6128 Plus section on p. 15.

Cassette deck

The deck is a TN-21ZH-754 assembly (part 270775). The parts list gives the main belt (270807), an "RF belt" (270805) and the motor (270858) as spares; the deck exploded view on p. 13 shows them in place. Amstrad gives part numbers only, not belt dimensions, so measure the old belt or the pulleys before ordering a generic one.

Azimuth adjustment

Amstrad's procedure:

1. Connect the oscilloscope probe to jumper line A on the cassette board.
2. Insert the MTT-113 test tape (6.34 kHz) in the deck.
3. Press PLAY.
4. Adjust screw 1 on the head mounting for the largest waveform.

Cleaning

• Clean the head, capstan and pinch roller with isopropyl alcohol on a foam swab.
• Clean any residue of a perished belt off the pulleys before fitting a new belt.

Amstrad's user instructions warn that switching the computer off or on with PLAY or REC held down can put a pulse through the head and scramble that part of the tape, and that a tape left clamped between the capstan and pinch roller for long periods can crease. Release the deck keys before switching off.

Keyboard

The keyboard is a single assembly, SW101, with country variants ESU44LN027AA (UK, part 270779), ESU44LN027DA (Spain, 270795) and ESU44LN027CA (France, 270794); the 6128 Plus parts list gives the same three. The key matrix on p. 11 applies to both computers.

• Remove loose dirt with a soft brush and low-pressure air.
• Clean keycaps with a damp cloth and a little mild detergent; keep liquid out of the switch assembly.

Board and connectors

• Brush and low-pressure air for the main board.
• Clean the joystick, printer and expansion connectors with a little contact cleaner.
• Inspect the electrolytic capacitors; the values and positions are in the Amstrad 464 Plus Capacitor Replacement Guide.

Tools

• Cross-head screwdrivers and an anti-static strap
• Digital multimeter
• Oscilloscope, for the luminance and azimuth adjustments
• MTT-113 test tape or another 6 kHz azimuth tape
• Isopropyl alcohol, foam swabs and lint-free cloth
• See Recommended Tools.

Related pages

• Amstrad 464 Plus · Amstrad 464 Plus Troubleshooting Guide · Amstrad 464 Plus Capacitor Replacement Guide
• Amstrad 6128 Plus Maintenance Guide – the disc drive
• Amstrad CPC 464 Maintenance Guide – the original CPC 464
• Amstrad 464 Plus / 6128 Plus / GX4000 Service Manual
• CRT Discharge Procedure · Capacitor Failure Symptoms