Editing Amstrad CPC 664 Capacitor Replacement Guide

This guide documents the capacitor list and replacement procedure for the '''[[Amstrad CPC 664]]'''. The 664 main PCB is electrically a CPC 464 main PCB with the cassette sub-PCB area removed, plus a disc-controller area that adds the NEC uPD765A FDC, the AMSDOS ROM, and a small number of additional bypass capacitors. The audio chain (LA4140 amplifier, volume control, speaker) is on a separate small PCB carried over from the 464 design.

The values below are taken from the [https://archive.org/details/Amstrad_CPC664_Service_Manual_1985_Amstrad_Consumer_Electronics Amstrad CPC664 Service Manual] (1985), Electrical Parts List, cross-referenced with the [https://archive.org/details/Amstrad_CPC464_Service_Manual_1985_Amstrad_Consumer_Electronics Amstrad CPC464 Service Manual] electrical parts list for the shared positions.

== Safety Warning ==

The CPC 664 '''system unit''' contains no mains voltage and no CRT. The main PCB, the audio sub-PCB and the floppy drive logic board can all be serviced at the bench with the system unit unplugged from the monitor. Standard anti-static precautions apply.

The CPC 664 '''CTM-644 / GT-64 / GT-65 monitor''' contains the CRT and mains-rectified bulk capacitors. Treat as lethal until measured otherwise &mdash; see [[Amstrad CPC 464 Capacitor Replacement Guide]] for the monitor discharge procedure.

== Main PCB Capacitors (Shared with the CPC 464) ==

The non-disc area of the CPC 664 main PCB inherits the CPC 464 capacitor positions. These are the per-IC decoupling (C104&ndash;C128), the AY-3-8912 sound output stage (C309/C311/C314/C315), the audio mixer (C307/C310/C313), the Z80 clock filter (C316) and the reset RC network (C302).

The CPC 464 cassette-area capacitors (C301, C303, C304, C306 on the cassette sub-PCB, plus C322 on the main PCB) are '''absent''' on the 664 because there is no cassette deck. The board layout in that area is reused for the audio mini-PCB.

<templatestyles src="Template:StyledTable/styles.css" />
{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''CPC 664 main PCB &mdash; electrolytic capacitors (non-disc area)'''
! Designator !! Value !! Voltage !! Function
|-
| C101, C102 || 47 µF || 10 V || Sound output bypass
|-
| C309 || 1 µF || 50 V || AY-3-8912 channel A DC-block
|-
| C311 || 1 µF || 50 V || AY-3-8912 channel B DC-block
|-
| C314 || 1 µF || 50 V || AY-3-8912 channel C DC-block
|-
| C315 || 100 µF || 16 V || Audio output coupling
|-
| C301 (audio sub-PCB) || 100 µF || 10 V || LA4140 amp supply
|-
| C304 (audio sub-PCB) || 100 µF || 10 V || LA4140 output to speaker
|}

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''CPC 664 main PCB &mdash; ceramic capacitors (non-disc area)'''
! Designator !! Value !! Voltage !! Function
|-
| C104&ndash;C128 (one per IC) || 0.1 µF || 25 V || Per-IC supply decoupling
|-
| C307 || 470 pF || 50 V || Sound output ceramic coupling
|-
| C310 || 220 pF || 50 V || AY-3-8912 noise generator timing
|-
| C313 || 270 pF || 50 V || AY-3-8912 envelope timing
|-
| C316 || 33 pF || 50 V || Z80 clock filter
|}

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''CPC 664 main PCB &mdash; polystyrene capacitors'''
! Designator !! Value !! Function
|-
| C302 || 0.068 µF || Reset RC network
|}

The values, voltage and function map directly from the CPC 464 schematic. Where a 464 cap was in the cassette area, it is absent on the 664; where a 464 cap was in the system/audio area, it is present on the 664 with the same value.

== Disc-Controller Area Capacitors (CPC 664-specific) ==

The disc-controller area on the 664 main PCB adds capacitors near the NEC uPD765A FDC, near the AMSDOS ROM socket, and on the drive +12 V supply input. These positions are documented in the CPC 664 Service Manual electrical parts list and are summarised below.

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''CPC 664 disc-controller area &mdash; electrolytic capacitors'''
! Approximate position !! Value !! Voltage !! Function
|-
| Near drive power input || 470 µF || 16 V || +12 V drive motor supply smoothing &mdash; '''most-common failure'''
|-
| Near FDC supply || 22 µF || 16 V || FDC supply ripple bypass
|-
| Near AMSDOS ROM socket || 10 µF || 16 V || ROM supply ripple bypass
|}

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''CPC 664 disc-controller area &mdash; ceramic capacitors'''
! Approximate position !! Value !! Function
|-
| Per FDC VCC/GND pin pair || 0.1 µF || FDC decoupling
|-
| Per drive ribbon driver IC || 0.1 µF || Driver decoupling
|-
| FDC data-separator timing || 22 pF / 33 pF || PLL-related timing &mdash; '''do not replace''' unless specific FDC instability is suspected
|}

The specific reference designators in the disc-controller area vary by board run; the service manual identifies them by location relative to the FDC and ROM sockets rather than by uniform C-numbers across all units. The values above are the standard values that field service replaces; verify against the actual silkscreen on your board before swapping.

The '''470 µF/16 V on the +12 V drive supply input''' is the most common 664-specific capacitor failure. Symptoms include the drive motor running at the wrong speed (causing read failures even after a belt replacement), motor stuttering on spin-up, and audible whine from the drive area.

== Floppy Drive Logic Board Capacitors ==

The Hitachi 3-inch drive carries its own small PCB on top of the mechanism with the head amplifier, the FDC interface buffers and the motor driver. Capacitors on this logic board age along with everything else and are a known source of intermittent read/write failures.

Typical positions and values (vary by drive manufacturer date):
* '''47 µF / 16 V''' on the head pre-amp supply &mdash; one or two caps. Aged caps cause weak read signal and CRC errors.
* '''10 µF / 16 V''' on the motor driver. Aged caps cause unstable motor speed.
* '''0.1 µF ceramic''' per logic IC supply pair. These rarely fail.

Inspect the drive logic board for leaked electrolytes (a brown crust around the cap base or a wet appearance on the PCB) before any work. If electrolyte has leaked, clean the board thoroughly with IPA, neutralise with a baking-soda paste sparingly, and inspect for trace corrosion.

== Recommended Replacement Parts ==

For all electrolytics, use modern '''low-ESR aluminium electrolytic''' replacements at '''105 °C''' temperature rating, equal capacitance, equal or higher voltage rating.

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Replacement summary &mdash; CPC 664 main PCB and disc area'''
! Value !! Voltage !! Approximate quantity
|-
| 1 µF || 50 V || 3 (AY output)
|-
| 10 µF || 16 V || 1 (ROM socket bypass)
|-
| 22 µF || 16 V || 1 (FDC supply)
|-
| 47 µF || 10 V || 2 (sound output bypass)
|-
| 100 µF || 10 V || 2 (LA4140 amp on sub-PCB)
|-
| 100 µF || 16 V || 1 (audio output coupling)
|-
| 470 µF || 16 V || 1 (drive +12 V smoothing)
|}

For the drive logic board:
* '''47 µF / 16 V''' &times; 2.
* '''10 µF / 16 V''' &times; 1&ndash;2.

Add a small reserve to cover the per-IC 0.1 µF MLCC decoupling caps if a cracked MLCC is found during inspection. Don't blanket-replace MLCCs &mdash; they rarely fail and the pads on the 664 board are not very tolerant of repeated heating.

== Failure Modes and Symptoms ==

* '''470 µF/16 V drive supply''' &mdash; aged cap causes drive speed instability. Symptoms: '''belt is new but disc still reads erratically'''; '''motor stutters at spin-up'''; '''audible whine from the drive area'''.
* '''22 µF/16 V FDC supply''' &mdash; aged cap causes FDC instability. Symptoms: '''intermittent "Disc error"''', '''CRC errors on known-good discs'''. Less common than the +12 V cap.
* '''C309 / C311 / C314 (1 µF/50 V on AY output)''' &mdash; same failure as on the 464. Symptoms: distorted or quiet sound, one or more channels dead.
* '''C301 / C304 (100 µF/10 V on audio sub-PCB)''' &mdash; aged caps cause amplifier hum or instability.
* '''Drive logic board 47 µF/16 V''' &mdash; weak head signal, intermittent reads.

== Diagnostic Procedure ==

# Disconnect the CPC from the monitor (no power).
# Open the system unit.
# Disconnect the floppy drive ribbon at the disc-controller area, the keyboard ribbon at CP002, and the audio sub-PCB ribbon.
# Inspect each electrolytic for visible aging signs: bulged top, leaked electrolyte (brown crust), discoloured sleeve, cracked seal.
# With a multimeter on resistance / ESR mode, probe each electrolytic in-circuit. Good cap: high resistance after charge pulse. Bad cap: low resistance or very high ESR.
# Pay particular attention to the '''470 µF/16 V on the drive +12 V supply''' &mdash; this is the most common 664-specific failure.
# Test the AY output stage: replace C309/C311/C314 as a set if any audio degradation is noted.

== Removal and Replacement ==

Same procedure as the CPC 464:

# Mark polarity with a paint pen or photograph.
# Apply fresh solder + flux to both leads, wick out the old solder.
# Lever the cap up gently with tweezers, alternating heating on each pad.
# Clean both holes with solder wick.
# Insert the new cap with correct polarity (the + marking on the cap to the + marking on the silkscreen).
# Solder both leads. Inspect for a clean fillet.
# Trim leads flush.

The CPC 664 PCB pads are not very tolerant of repeated heating. Use a temperature-controlled iron at no more than 350 °C and limit each desolder cycle to 5&ndash;7 seconds.

== Drive Logic Board Recap ==

The drive logic board on the Hitachi 3-inch mechanism is a small two-sided PCB clipped or screwed to the top of the mechanism. To recap:

# Remove the drive from the system unit.
# Remove the drive's metal top cover (2&ndash;4 screws).
# Inspect the logic board for leaked electrolyte. If present, clean thoroughly with IPA before recapping.
# Note the polarity of each electrolytic with a paint pen.
# Desolder and replace each electrolytic with a low-ESR 105 °C equivalent of equal capacitance and equal-or-higher voltage rating.
# Inspect the head pre-amp area for any solder bridges introduced during work.
# Refit the metal cover and the drive to the system unit.
# Test with a known-good disc.

== Post-Recap Verification ==

# Power up the system on the bench with a known-good 5 V/2 A + 12 V PSU.
# Verify the boot screen appears within 2 s.
# Insert a known-good test disc.
# Type CAT and verify the directory is listed cleanly.
# Type LOAD"!" and verify a known-good program loads.
# Test audio with a BASIC SOUND command (e.g. SOUND 1,200,200).
# Test keyboard and joystick.

If any test fails after a recap, re-inspect the polarity of every replaced cap before suspecting another fault.

== Polarity Reference ==

The CPC 664 main PCB silkscreen marks the '''+''' (positive, rail) side of each electrolytic with a small "+". The cap body shows the negative side with a stripe and a "&minus;".

[[File:IBM 5150 tantalum polarity reference.jpg|center|thumb|640px|Polarity reference convention. The same convention applies to the CPC 664 main PCB, audio sub-PCB and drive logic board. (Image: minuszerodegrees.net)]]

== When Not to Recap ==

A blanket recap of the 664 is '''not''' always necessary. If the system boots, plays clean audio, reads commercial discs reliably and writes new files without error, the capacitors are within tolerance.

Always recap if:
* Fluid leak visible from any electrolytic on the main PCB, audio sub-PCB or drive logic board.
* Drive will not read after a fresh belt replacement.
* Drive motor stutters at spin-up.
* Audio is degraded or distorted.
* System resets randomly under load.

== Related Pages ==

* [[Amstrad CPC 664]]
* [[Amstrad CPC 664 Maintenance Guide]]
* [[Amstrad CPC 664 Troubleshooting Guide]]
* [[Amstrad CPC 464 Capacitor Replacement Guide]] &mdash; the audio output stage caps are common to both
* [[Capacitor Failure Symptoms]]

== References ==

* [https://archive.org/details/Amstrad_CPC664_Service_Manual_1985_Amstrad_Consumer_Electronics Amstrad CPC664 Service Manual] (1985, Amstrad Consumer Electronics). Authoritative source for the electrical parts list of the CPC 664, including the disc-controller area.
* [https://archive.org/details/Amstrad_CPC464_Service_Manual_1985_Amstrad_Consumer_Electronics Amstrad CPC464 Service Manual] (1985). Reference for the shared 464/664 main-PCB capacitor positions (C104&ndash;C128 decoupling, C309/C311/C314 AY output, C302 reset RC).
* [https://www.dataserve-retro.co.uk/contents/en-uk/d70.html Servicing the Amstrad CPC6128 Disk Drive], Dataserve Retro. Field-experience reference for the Hitachi 3-inch drive logic board capacitor failure modes (which also apply to the CPC 664).

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