Amstrad CPC 664 Troubleshooting Guide

This guide covers diagnostic procedures for the Amstrad CPC 664 (1985). The 664 shares the Amstrad CPC 464's diagnostic philosophy — no numeric POST codes, symptom-based diagnosis using the service-manual flowchart, plus signal-level probing for the floppy disc subsystem that is unique to the 664. This guide adapts the official Basic Hardware Analysis flowchart from the Amstrad CPC664 Service Manual and adds the field-experience disc fault catalogue.

Power-On Behaviour

A healthy CPC 664 with firmware v2 (mask ROM IC103 part 40012 for the English-language model) and AMSDOS ROM (part 40015) goes through the following at power-on:

1. +5 V and +12 V rails come up from the monitor through the DC cord.
2. The Gate Array (IC116, 40010 / 40007) generates the Z80 reset pulse, the 4 MHz Z80 clock, and the 1 MHz CRTC and AY-3-8912 clocks.
3. CRTC (IC108) generates HSYNC and VSYNC; the Gate Array generates the RGB output.
4. The Z80 reads the OS+BASIC ROM (IC103) starting at address 0000h.
5. AMSDOS ROM is initialised: a | DIR command tests the floppy controller; if successful, the system advertises disc support.
6. The boot screen appears:

   Amstrad 64K Microcomputer (v2)
   "© 1985 Amstrad Consumer Electronics plc
   "and Locomotive Software Ltd."
   "BASIC 1.1"
   "Ready"

7. The cursor appears.

For Schneider-branded units the first line reads "Schneider 64K Microcomputer (v2)".

If the disc drive is present but has a fault, the system may take longer to reach "Ready" because AMSDOS retries the drive initialisation several times before giving up. A 4–6 second delay between video and prompt is a clue that the drive is being polled but is not responding.

Symptom Flowchart

Same three branches as the CPC 464:

Branch A — No display

1. Verify +5 V at the system unit DC input with a multimeter. If absent, suspect the monitor 5 V supply or the DC cord.
2. Verify +12 V at the drive power input (or the disc-controller area, depending on the cable layout). If absent, the drive will not spin but display should still work; if both +5 V and +12 V are missing, suspect the monitor or the DC cord.
3. If both rails are present but no display, follow the Amstrad CPC 464 Troubleshooting Guide Branch A:

   Probe X101 (16 MHz crystal) — if dead, GA dead.

   Probe the Z80 clock at IC111 pin 6 — if dead, GA clock divider dead.

   Probe CRTC IC108 pin 40 (HSYNC) — if dead, CRTC dead.

   Suspect FRUs (in order): Gate Array IC116, RAM bank IC117–IC124, CRTC IC108, OS ROM IC103.

Branch B — Garbled display, no Ready prompt

• Patterned garbage in video: RAM failure. Same as 464 — a regular vertical stripe identifies the bit position. The 664 uses the same eight 4164-family chips at IC117–IC124.
• "Amstrad 64K Microcomputer" displayed but then hangs before "Ready" with the floppy drive making a sequence of seek-stepper sounds: AMSDOS ROM corrupted, or the FDC is unable to communicate with the drive. Reseat the AMSDOS ROM and the drive ribbon.
• "BASIC 1.1" displayed then immediate freeze: more likely the FDC reset sequence is stuck. Disconnect the floppy drive ribbon and re-test — if the system reaches Ready without the drive, the drive is jamming the FDC.

Branch C — Display OK, system runs, but specific function fails

This branch covers the floppy-drive-specific failures unique to the 664.

Floppy Drive Symptom Catalogue

The Hitachi 3-inch drive in the CPC 664 has well-known wear failures. The community-documented symptoms are:

Drive motor does not run

• Verify +12 V at the drive motor power pin with a multimeter.
• If +12 V is present but motor does not run: motor brushes worn (rare on Hitachi 3-inch), or motor relay (where used) failed.
• If +12 V is absent: drive power lead detached from the system unit's PSU input.

Drive motor runs but disc does not spin

Most common 664 fault. The rubber drive belt (54 × 1 mm flat) connecting the motor to the spindle has perished, liquefied or stretched.

Symptom: system reports "Disc missing" or "Read fail a" even with a known-good disc inserted, because the spindle is not spinning so no tracks can be read.

Fix: Replace the drive belt. Procedure is in Amstrad CPC 664 Maintenance Guide.

Disc spins but reads fail with "Disc error"

• Head dirty: clean with IPA on a foam swab.
• Disc itself faulty: try a known-good disc. 3-inch discs from the 1980s sometimes develop binder failures and shed oxide.
• Head alignment off: rare; the drive needs alignment-disc service.
• AMSDOS ROM: reseat or replace.

"Track 0" error or stepper stuck

The stepper motor cannot find track 0. Causes:

• Dried lubrication on the worm-drive lead screw — apply a single drop of clock oil to the lead screw (per Amstrad CPC 664 Maintenance Guide).
• Track 0 sensor (optical interrupter at the inner end of the head travel) blocked by dust.
• Stepper motor coil failed (rare).

Disc reads but writes fail

• Disc write-protect tab in the protected position.
• Drive head failing on the write side (rare, usually paired with read failures eventually).
• AMSDOS reports "Disc is read only" — verify the disc and not the drive.

Disc detected but CAT shows random / corrupt directory

• Disc not formatted with AMSDOS / CP/M file system — try formatting first.
• Drive belt is intermittently slipping — the disc spins at the wrong speed and the FDC decodes wrong bytes. Replace the belt.

Probing the FDC (NEC uPD765A)

If the drive is mechanically sound (verified by ear — clean seek sounds, disc spinning) but the system still reports disc errors, the next suspect is the FDC chip (NEC uPD765A) or the drive cable.

Test points:

• Probe pin 30 of the FDC (RDY/READY signal coming from the drive). If LOW, the drive is reporting ready; if HIGH, the drive is reporting not ready.
• Probe pins 32–33 of the FDC (RD/WR data lines). Should see digital activity during a read or write operation.
• Probe the FDC clock (4 MHz from the GA) at FDC pin 19. If absent, the FDC has lost its clock from the GA.

The FDC is socketed on most 664 boards; reseating is the first step before suspecting failure. New uPD765A replacements (or the compatible Intel 8272A, NEC D72065B) are widely available.

AMSDOS Error Codes

AMSDOS reports a small set of error codes when a disc operation fails. These are returned at the BASIC prompt and are listed below.

AMSDOS error reports

Message	Likely cause
Drive A: disc missing	Drive door not closed, disc absent, or disc-present microswitch dirty
Drive A: read fail	Drive belt slipped, head dirty, or disc damaged
Drive A: write fail	Write protected, or drive head failing
Drive A: disc is read only	Write-protect tab in protect position
Bad command	AMSDOS RSX (bar command) not recognised — verify spelling
Bad file name	File name does not match 8.3 AMSDOS pattern
File not found	File not in directory; verify with CAT
Disc full	No free directory entries or no free clusters
Cannot write to file	File already exists and write-permission is set deny
Press a key	AMSDOS retry prompt — press a key to retry the operation, or Esc to abort

For "Drive A: read fail" repeated multiple times on different discs, the drive itself is the suspect — verify the drive belt and head condition before the disc.

Keyboard, Audio, Printer Port, Expansion Port

These subsystems are identical to the CPC 464. See Amstrad CPC 464 Troubleshooting Guide for:

• Keyboard fault diagnosis (row dead, key dead, stuck key).
• AY-3-8912 audio fault diagnosis.
• Printer port D0–D6 probing.
• Expansion port edge-connector contact maintenance.

Diagnosing the Gate Array

Same as the CPC 464. The 664 may have either the 40007 (Ferranti, heatsinked, hot-running) or the 40010 (CMOS, cooler) Gate Array. Symptoms of a failing GA:

• No clock, no display, system completely dead but +5 V and +12 V present and crystal oscillating.
• Display present with vertical colour bands — GA video output intermittently failing.
• System runs but hard locks after a few minutes — GA overheating. Fit or refit a heatsink with thermal compound, or replace with a CMOS 40010.

Diagnosing the CRTC

The 664 ships with the HD6845SP (Type 0) on most production runs and the UM6845R (Type 1) on some late runs. Type differences are mostly invisible to standard software; obscure demos that rely on undocumented CRTC behaviour may behave differently between types. Failure modes:

• No HSYNC at CRTC pin 40 — CRTC dead.
• No VSYNC at CRTC pin 39 — CRTC dead or specific output stage failed.
• Rolling display — VSYNC missing or out of spec.

Diagnosing the AY-3-8912

Same as CPC 464. The 664 cassette interface is removed but the AY chip's three sound channels and the audio amplifier (IC301, LA4140) remain. Diagnostic procedure is identical to the Amstrad CPC 464 Troubleshooting Guide audio section.

No-Disc-Activity Diagnostic Procedure

If the system boots to "Ready" but disc commands hang or return immediate errors:

1. Insert a known-good test disc.
2. At the BASIC prompt, type | DIR (or CAT) and watch the drive LED.
3. If the LED illuminates: drive is being addressed by the FDC. Listen for the seek sound; if seek is clean and disc spins, the issue is likely software-level (disc not formatted, wrong format).
4. If the LED illuminates but the seek sound is chattering or absent: stepper lubrication, drive belt, or stepper motor failure.
5. If the LED does not illuminate: FDC is not driving the drive, or the drive cable is detached / reversed. Verify the cable pin-1 orientation and reseat the cable.

If | DIR returns immediately with "Bad command" then AMSDOS ROM is not present or not initialising. Reseat the AMSDOS ROM at its socket.

Drive Belt Failure Quick Test

The classic non-destructive test for drive belt failure:

1. Open the system unit (CRT discharged if monitor is part of the test setup).
2. Insert a 3-inch diskette.
3. Power on. The drive motor should start running shortly after the system reaches "Ready".
4. Look through the cover at the disc spindle. If the disc is visibly spinning, the belt is OK. If the disc is stationary while the motor is running, the belt has failed.
5. Audible diagnostic: a healthy drive produces a steady whirring sound from the spinning disc. A failed-belt drive is silent or makes only the chattering of the stepper motor.

Common Fault Catalogue

• Drive belt failure — 80%+ of 664 floppy faults. Replace the 54 × 1 mm flat rubber belt.
• Stepper lubrication dried — "Track 0" errors. Single drop of clock oil.
• Disc head dirty — CRC errors on read. IPA on foam swab.
• AMSDOS ROM socket loose — "Bad command" on | commands. Reseat.
• Gate Array overheating — system runs then locks. Fit heatsink or swap to CMOS 40010.
• RAM failure — vertical stripe at boot. Identify bit, swap chip.
• Power supply +12 V failing — drive does not spin even with new belt. Verify +12 V at drive connector.
• Volume control noisy — VR301 needs cleaning.
• Speaker quiet — aged C309/C311/C314 (1 µF/50 V on AY output). Recap.

Related Pages

• Amstrad CPC 664
• Amstrad CPC 664 Maintenance Guide
• Amstrad CPC 664 Capacitor Replacement Guide
• Amstrad CPC 464 Troubleshooting Guide — identical for the non-disc subsystems
• Capacitor Failure Symptoms

References

• Amstrad CPC664 Service Manual (1985, Amstrad Consumer Electronics). Source for the disc drive schematic, disc-area circuit diagram, the FDC connections to the Z80 bus, and the electrical parts list.
• Replacing the FDD drive belt on the new CPC 664, Amstrad noob. Field-experience source for the drive-belt failure mode and replacement.
• NEC uPD765A FDC, CPCWiki. Reference for the FDC pinout and the CPC-specific Z80 interface.
• CPC CRTC types, CPCWiki. Reference for the Type 0/1/2/3/4 CRTC family and the implications for software compatibility.
• Amstrad CPC hardware documentation, Grimware. Reference for the Gate Array, the connector pinouts and the shared 464/664/6128 architecture.