Editing Amstrad CPC 664 Capacitor Replacement Guide (section)

== 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.