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Amstrad CPC 464 Troubleshooting Guide

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Amstrad CPC 464 (photo)
Amstrad CPC 464 with monitor

The Amstrad CPC 464 has no numbered power-on test codes. Faults are found by symptom, following the flow charts in Amstrad's service manual, and by measurement against the parts list and circuit diagram.[1] Amstrad's manual refers full CPU diagnostics to its RP1 Test Pack.[1] Routine care is in the Amstrad CPC 464 Maintenance Guide and capacitor values in the Amstrad CPC 464 Capacitor Replacement Guide.

Power and safety

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The CPC 464 takes 5 V DC from its monitor and has no supply of its own; mains enters the monitor.[1] A machine that appears dead may have a monitor fault, a damaged DC lead or a bad joint at the computer's power socket. The monitors contain the mains supply and a CRT; read CRT Discharge Procedure before opening one.

Amstrad's basic hardware analysis

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Amstrad's flow chart on page 9 starts with the system connected to the mains and both units switched on.[1]

CPC 464 basic hardware analysis (service manual p. 9)
Symptom Check Amstrad's suspects
Red LED does not light Monitor mains fuse; 5 V DC present on the cable from the monitor Mains fuse; cable or plug assembly from the monitor. With 5 V present: internal connectors and soldered joints (faulty LED, broken PCB track, faulty connector), then the switch on the keyboard unit
LED lights, no start-up message RGB video connector inserted; brightness turned up; any picture at all? With a picture: faulty video connector, cable assembly, ULA or PCB. No picture: try another monitor. Works on another monitor: the monitor's cable termination, internal connectors or brightness control. Still nothing: ULA, Z80A, 6845, crystal or RAM
Start-up message, no reaction to the keyboard Keyboard connector Keyboard connector
Some keys do not work Keyboard PCB Cracks, liquid spillage or distorted mounting; connector lead and termination
Program tape does not load Try another tape If another tape loads, replace the tape; if not, see the cassette chart below
No sound – Audio amplifier or speaker, volume control, audio connector plug or socket, AY-3-8912 sound IC

The ULA in Amstrad's chart is the gate array, IC116.[1] If the unit is still faulty after these checks, Amstrad's chart says to seek specialist advice.

Cassette faults

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Amstrad's cassette and software chart (p. 10) runs in this order:[1]

  1. Check the connections to the tape deck (the 8-way connector CD001 on the cassette PCB).
  2. Clean the tape head.
  3. Check the head alignment: with a 6 kHz test tape and the volume at maximum, set the azimuth screw for 330–520 mV at pin 7 of IC302 (p. 6).
  4. Check the switches on the cassette keys.
  5. Check the tape motor control relay, RY301.

If software loads but behaves strangely, Amstrad points to the computer board: a RAM or ULA failure. A tape with physical damage will not load at all; Amstrad notes there are no "half" working programs.[1] A speed variation of ±4% does not affect loading.[1]

Restorers report two mechanical faults in the deck. The motor and counter belts stretch and slip, and the rubber tyre on the idler wheel, which sits between the drive spindle and the right-hand tape spool, hardens and slips; replacement idler wheels are sold by retro parts suppliers.[2] Amstrad's parts list gives the main belt as 170204 and the idler assembly as 809155.[1]

Keyboard faults

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Each CPC 464 key has two springs: a large return spring and a small one that presses on the membrane. On machines kept in damp conditions the small springs rust and can break when removed, and a key with a short or missing spring will not close the membrane contacts.[2][3] A space bar that rocks instead of moving down evenly usually has broken plastic lugs holding its stabiliser bar.[2] Do not clean the keycaps in sodium hydroxide: the plastic turns white.[3]

The membrane is two plastic sheets with printed traces. Clean only the outer faces and keep liquid out from between the layers.[2]

Power and connector faults

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  • A broken solder joint in the power input circuit has been found as the whole cause of a machine that did not start.[3] Inspect and reflow the joints at the DC socket J104.
  • The area around the video output connector, at the upper right of the board, corrodes more than the rest; on one board the corrosion was hidden under the connector itself.[3]
  • Measure 5 V at J104 before suspecting anything on the board.

Board-level faults

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Amstrad's chart names the gate array (ULA), the Z80A, the 6845, the 16 MHz crystal and the RAM as the suspects for a machine that gives no picture on a known-good monitor.[1] On the original board these are IC116 (20RA043), IC111 (Z8400APS), IC108 (HK6845SP), X101 and IC117–124 (eight HM4864U-2 DRAMs).[1] Each DRAM is 64K × 1, so each chip holds one data bit of every address.

The gate array is not one part throughout production. CPCWiki lists 40007 and 40008 as the heat-sinked versions and 40010 as the later version with different pinouts; version 2 and 3 boards have pads for both kinds.[4][5] Check which footprint your board uses before fitting a replacement. The cost-down board (Z80329) uses the 40226 ASIC instead.[4]

The gate array arbitrates access to RAM between the CPU and the video hardware, refreshes the DRAM and generates the interrupts.[6]

Printer and expansion ports

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The printer port outputs only seven data bits; pin 9, D7 on a standard Centronics port, is ground. Some printers and interfaces need an adapter.[7] The expansion port is a 50-way edge connector carrying the Z80 bus, with sound on pin 1, +5 V on pin 27 and the 4 MHz CPU clock on pin 50.[7] Both are bare edge connectors; clean the fingers if a connected device is unreliable.

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References

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  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 Amstrad Consumer Electronics plc, CPC464 Micro Computer / CTM640 Colour Monitor / GT64 Green Monitor Service Manual. Described at Amstrad CPC 464 Service Manual. Technical specification p. 2; cassette alignment p. 6; cassette PCB p. 7; basic hardware analysis p. 9; cassette and software analysis p. 10; electrical parts list p. 17.
  2. ↑ 2.0 2.1 2.2 2.3 Andrew Dorey, "Amstrad CPC 464 Repair and Restoration", 24 March 2025. Restoration write-up: stretched motor and counter belts, slipping idler wheel and its replacement, rusted keyboard springs, broken space bar lugs.
  3. ↑ 3.0 3.1 3.2 3.3 Retrohax, "CPC 464 repairs and restoration". Restoration write-up of a Schneider CPC 464: a broken solder joint in the power input circuit, corrosion under the monitor connector, the two-spring key design, and keycaps damaged by sodium hydroxide cleaning.
  4. ↑ 4.0 4.1 CPCWiki, "Amstrad part numbers". Retrieved 2026-10-01. Community list of Amstrad custom-chip and PCB numbers.
  5. ↑ CPCWiki, "Mainboard Versions". Retrieved 2026-10-01.
  6. ↑ CPCWiki, "Gate Array". Retrieved 2026-10-01. Describes the gate array's display, bus arbitration, DRAM refresh and interrupt functions.
  7. ↑ 7.0 7.1 Grimware, "Amstrad CPC 464". Retrieved 2026-10-01. Pinouts of the printer, expansion, joystick and monitor connectors.


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