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Amstrad CPC (464 / 664 / 6128)

Amstrad's all-in-one 8-bit (1984), huge in France and Spain: a Z80, a house Gate Array, a CRTC 6845, AY-3-8912 sound, and a bundled monitor that powers the computer. Standard CPC repair: 3″ drive belt, membrane, monitor PSU, recap. The cartridge/console version is the GX4000.

Amstrad CPC 464

Amstrad CPC 464 — photo by Bill Bertram, CC BY-SA 2.5 (Wikimedia Commons).

Identification / models

Model Year Media RAM Note
CPC 464 1984 built-in tape 64 KB the best-seller
CPC 664 1985 3″ disk 64 KB short-lived (replaced by the 6128)
CPC 6128 1985 3″ disk 128 KB the flagship

Bundled with a monitor: CTM640 (colour RGB) or GT65 (green). The range evolves into the CPC Plus (1990, ASIC + cartridge).

Architecture

Block Chip Role
CPU Zilog Z80 @ 4 MHz logic
Video Gate Array + CRTC 6845 3 modes (160/320/640×200), 27-colour palette
Sound AY-3-8912 (3-channel PSG) music/SFX
Power from the monitor (5 V + 12 V for the drive) no internal PSU

Repair (technician)

3″ disk drive (664/6128) — the No. 1 fault

The rubber belt perishes over time (sticky residue) → disk won't spin / errors. Replace the belt (square section), clean the pulley and the head (IPA). See traces & vias / mechanics.

  • 3″ media (Amsoft/Maxell) rare and expensive → Gotek (Amstrad firmware) / HxC for preservation.
  • Power = the monitor: no boot? Check the voltage coming from the monitor (the CTM has its own caps to recap) before suspecting the computer.
  • Membrane keyboard: dead keys = tired membrane / cut trace (conductive repair).
  • Gate Array / CRTC: corrupt video = Gate Array / CRTC / RAM → substitution.
  • Targeted recap if picture/sound degrade → Recap.

Original service manual ✅

Manufacturer manual archived locally. It typically carries the schematics, the block diagram, the expected voltages and waveforms, the parts list and the disassembly procedures — the reference when a symptom matches no known fault.

  • Amstrad CPC664 — service manual (1985) — PDF
  • CPC 6128 — PDF
  • CPC 464 — PDF

Status

A manufacturer document with no explicit licence, kept for preservation (machine long out of production). Source: Internet Archive.

Procedure — a dead computer, step by step ✅

The power supply is in the monitor

Which means the machine will not start without it — and that a fault that looks like a dead computer may be in a part you have not even plugged in.

What this adds

Ordered for a home computer: what kills these is its power supply, its battery, its capacitors, its socketed RAM and its keyboard — roughly in that order, and well before the CPU. → PCB diagnosis

1. The power supply — measured before it is plugged in. This is the one step not to swap around. On a machine of this age a supply does not simply stop: a 5 V regulator that fails high puts 7, 9, 12 V onto the bus and takes the RAM and the custom chips with it. Measure every rail off the machine, under a dummy load if you can, and only then plug it in.

2. The battery, if it has one. Any RTC or backup cell soldered to a board is a future leak. If yours has one and it has never been changed, take it out before it takes the tracks with it. → Backup batteries

3. Capacitors. Recap is the default on these machines, not a last resort — and on the supply first, since that is what protects everything else. → Recap

4. RAM, on its sockets. These machines were built to be repaired: the RAM, and often the logic, sit in sockets. Reseat everything first — thirty years of oxide is a classic no-boot. Then swap chips: a bad RAM chip usually gives a repeatable pattern of garbage or a screen frozen at a fixed character.

5. The keyboard. A console does not have one; a computer's is a wear part. A membrane keyboard fails as whole dead rows or columns — that is the membrane, not the machine. A mechanical one wants its contacts cleaning, not replacing. Check this page: the failure mode is often documented per model.

6. Storage — tape or disk. Both often come down to the same things: a dirty head, and for tape the azimuth and a belt turned to tar. ⚠️ On the floppy side, check whether the mechanism actually has a belt before looking for one: many do, but some drives are direct drive, and hunting a belt that isn't there means stripping a drive for nothing. A machine that boots but loads nothing is a drive fault, not a logic fault — do not go looking on the board for it.

7. Video output. RF, composite, RGB or a dedicated monitor, depending on the machine (see Identification). Test with something else before you conclude: a period modulator and a modern TV disagree far more often than either is broken.

8. Still nothing. Now the logic: rails on the board, reset, clock, bus. → PCB diagnosis

Common faults

Symptom Likely cause Lead
Disk not read (664/6128) perished belt / head Replace belt, clean
Nothing at power-up monitor PSU / fuse Check monitor→board voltage
Dead keys keyboard membrane Replace the membrane
Corrupt video Gate Array / CRTC / RAM Substitute, RAM test
Wrong/no sound AY-3-8912 Sound-stage diagnosis

Modernisation & mods — what to buy, how

CTM monitor = CRT high voltage + 3″ belt

The original CTM monitor is a CRT (the CPC's power comes through the monitor): high voltage, discharge the tube before any work → CRT safety. The recap covers the base unit and the CTM monitor/PSU. On the 3″ drive, a dried belt is the #1 cause of unreadable disks → replace it.

Modern storage - Gotek + FlashFloppy or HxC 🟡: replace the 3″ drive (rare CF2 disks) — internal drive connector, DSK images from USB. - M4 board 🟢 (ROM/SD + Wi-Fi), Dandanator 🟢, X-MEM 🟡 (RAM+ROM) on the expansion port; C4CPC 🟢 = SD cartridges for the CPC Plus / GX4000.

Video - RGB output on the monitor connector 🟢 → SCART (Amstrad pinout) then OSSC / RetroTINK or RGB2HDMI.

Reliability: clean the keyboard/contacts.

⚠️ Do not host ROMs/disks — method only.

Peripheral and monitor schematics ✅

CTM640 and CTM644 colour monitors, GT64 and GT65 monochrome, the DDI disc interface, and the disc/cassette stage. Since the CPC draws its power from the monitor, these schematics serve the computer as much as the screen.

Provenance

Undocumented origin in the collection these schematics came from: no author and no licence is stated there.

Peripheral and monitor schematics — cpc disc cassette schematic
Peripheral and monitor schematics — ctm640 schematic
Peripheral and monitor schematics — ctm644 schematic
Peripheral and monitor schematics — ddi schematic
Peripheral and monitor schematics — gt64 schematic
Peripheral and monitor schematics — gt65 schematic

Additional schematics ✅

Plates grouped by machine. Expand to view.

Provenance

Undocumented origin in the collection: no author and no licence stated.

CPC miscellaneous — 5 planche(s)

CPC miscellaneous — cpc6128 disk interface schematic

CPC miscellaneous — cpc6128 schematic

CPC miscellaneous — cpc664 disk interface schematic

CPC miscellaneous — cpc664 schematic

CPC miscellaneous — schaltplan cpc 464

See also

Sources & attribution

Review & corrections

How this space is used

Spotted a wrong value, an outdated procedure, a chip reference that does not match your board? Say so here, with what you observed (model, board revision, serial number, measurement). Every report is cross-checked against a source before anything changes — an unverifiable correction is published as “reported by …, not cross-checked” rather than silently applied.

Reading is open to everyone; posting requires signing in with Discord. Reports from the wiki's declared reviewers are handled first; anyone else's are read too, but go through a human before anything is changed.