Amstrad CPC 6128 Capacitor Replacement Guide

The original CPC6128 main board (Amstrad service manual pp. 16–18). Most of the electrolytics are 300-series parts in the audio and cassette circuits at the left-hand end.

The original Amstrad CPC 6128 main board has twelve aluminium electrolytic capacitors in four values. The values, circuit references and Amstrad part numbers below are from the electrical parts list on page 21 of Amstrad's CPC6128 service manual, checked against the cassette control circuit diagram on page 36. The list covers the first board design only; the later boards are drawn in Amstrad's amendment manual and are covered under Later boards below.

Safety

The CPC 6128 takes 5 V and 12 V from the CTM644 or GT65 monitor and carries no mains voltage. The monitors contain the mains supply and a CRT and hold a dangerous charge after switch-off; their capacitors are listed in the Amstrad CTM-644 Maintenance Guide. Read CRT Discharge Procedure before opening a monitor.

Electrolytic capacitors

CPC6128 electrolytic capacitors (parts list, p. 21)

Value	Circuit reference	Amstrad part no.
100 µF / 16 V	C315	20028
100 µF / 10 V	C301, C304	20028
47 µF / 10 V	C136, C303, C306	1400244
22 µF / 10 V	C308	20025
1 µF / 50 V	C309, C311, C314, C317, C318 (printed; see below)	20062

Two points on the 1 µF line need care:

• The parts list puts C318 in both the electrolytic line and the polycarbonate list (0.047 µF). The cassette control circuit diagram shows C318 as 0.047 µF and C316 as a 1 µF 50 V electrolytic, so the electrolytic line should read C316, as it does in the CPC 664 list.
• The circuit diagram marks C314 "50V1 (NP)", a non-polarised 1 µF part. Replace it with a bipolar capacitor, not a standard polarised one.

C136 is the only electrolytic with a 100-series reference; the rest are in the 300-series audio and cassette circuits around IC301 (LA4140) and IC302 (LA6358S).

Later boards

The amendment manual's chassis schematic for the MC0057A board shows C136, 47 µF 10 V, across the 5 V supply, with a 0.1 µF capacitor (C135) in the reset circuit. That schematic covers the digital circuits only and does not redraw the audio and cassette section.

The later cost-down board, PCB001 MC0100 with the Amstrad 40226 ASIC at IC101 (the amendment's "CPC6128/III" parts list), is renumbered throughout. That parts list gives no capacitors; the schematics show nine electrolytics.

CPC6128 MC0100 board: electrolytic capacitors (amendment pp. 21, 24)

Value	Circuit reference	Circuit
100 µF / 16 V	C308, C311, C316	LA4140 output (C308) and supply (C311); op-amp supply (C316)
47 µF / 10 V	C305, C312	LA4140 (IC302)
22 µF / 10 V	C309, C315	LA4140 feedback (C309); op-amp bias (C315)
4.7 µF / 50 V	C101	Reset timing (with D101, R117 and R118)
1 µF / 50 V	C306	LA4140 input

On this board IC301 is the LA6358S and IC302 the LA4140, the reverse of the original board, so the 300-series numbers do not match the original list.

Other capacitors

CPC6128 film and ceramic capacitors (parts list, p. 21)

Type	Value	Circuit reference
Polycarbonate, 50 V	0.1 µF	C319, C320
Polycarbonate, 50 V	0.068 µF	C302
Polycarbonate, 50 V	0.047 µF	C318
Polycarbonate, 50 V	0.01 µF	C305
Polycarbonate, 50 V	0.001 µF	C312
Ceramic	0.1 µF	C101–C135, C201–C213
Ceramic	470 pF	C306 (printed)
Ceramic	270 pF	C313
Ceramic	220 pF	C139–C141, C310
Ceramic	15 pF	C137, C138

The ceramic line "470 pF C306" clashes with C306 in the electrolytic list; the circuit diagram shows a 470 pF capacitor at C307, which suggests the ceramic line is the misprint. These parts are not part of a routine recap.

The disc drive's board

A note under the parts list says no part numbers are given for anything on PCB30001, the board on the drive mechanism; a faulty board went back to Amstrad with the whole mechanism. The disc control circuit diagram on pages 35–36 is the schematic of that board, labelled FDC201 EME-150A, and shows its capacitor values. Read the values from the diagram, or from the parts on your own drive, since 6128s were fitted with more than one mechanism: the original parts list names an EME-155 drive and the amendment's MC0100 list an EME-156V.

Replacement procedure

1. Unplug both leads from the monitor and open the case as described in the Amstrad CPC 6128 Maintenance Guide.
2. Photograph the board and note each electrolytic's polarity before removing it.
3. Remove one part at a time with a desoldering iron or braid, and clean the pads.
4. Fit parts of the same capacitance and the same or a higher voltage rating, matching the polarity marking; fit a bipolar part at C314.
5. Reassemble and check the 5 V and 12 V at the computer's sockets, then sound, a disc load and an external tape load.

Related pages

• Amstrad CPC 6128 · Amstrad CPC 6128 Maintenance Guide · Amstrad CPC 6128 Troubleshooting Guide
• Amstrad CPC 664 Capacitor Replacement Guide · Amstrad 6128 Plus Capacitor Replacement Guide
• Amstrad CTM-644 Maintenance Guide – the monitor's capacitors
• Capacitor Failure Symptoms · Recommended Tools