Editing Amstrad CPC 6128 Troubleshooting Guide

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[[File:Amstrad CPC 6128 with keyboard removed.jpg|thumb|right|300px|A CPC 6128 with the keyboard removed: main board at left, disc drive at right (photo: Jesus Abizanda, CC BY 2.0)]]
The [[Amstrad 6128|Amstrad CPC 6128]] has no numbered self-test codes. Amstrad's service manual gives a basic hardware analysis chart for the computer and a four-page diagnostic flow chart for the disc drive, with measured thresholds, and refers full CPU diagnostics to its RP2 Test Pack.<ref name="sm6128">Amstrad Consumer Electronics plc, ''CPC6128 Micro Computer / CTM644 Colour Monitor / GT65 Green Monitor Service Manual''. Described at [[Amstrad CPC 6128 Service Manual]]. Basic hardware analysis p. 4; software errors p. 7; diagnostic flow chart pp. 8–11; alignment checks pp. 12–15; electrical parts list p. 21; cassette control circuit diagram p. 36; technical specification on the back page.</ref> The drive procedures are in the [[Amstrad CPC 6128 Maintenance Guide]].

== Power first ==

The CPC 6128 takes 5&nbsp;V and the drive's 12&nbsp;V from the CTM644 or GT65 monitor on two separate leads.<ref name="sm6128" /> A computer that starts normally with a dead drive may simply have its 12&nbsp;V lead unplugged. Amstrad's circuit diagram marks the 12&nbsp;V plug as inner ground and outer +12&nbsp;V; wire any substitute supply to match.<ref name="sm6128" /> For monitor supply faults see the [[Amstrad CTM-644 Maintenance Guide]].

== Amstrad's basic hardware analysis ==

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''CPC6128 basic hardware analysis (service manual p. 4)'''
! Symptom !! Check !! Amstrad's suspects
|-
| Red LED does not light || Monitor mains fuse; 5&nbsp;V DC present on the cable from the monitor || Mains fuse; cable or plug assembly from the monitor. With 5&nbsp;V present: internal connectors and soldered joints (faulty LED, broken PCB track, faulty connector), then the switch on the keyboard unit
|-
| LED lights, no start-up message || RGB video connector inserted; brightness turned up; any picture at all? || With a picture: video connector, cable assembly, ULA or PCB. No picture: try another monitor. Works on another monitor: the monitor's cable termination, internal connectors or brightness control. Still nothing: ULA, Z80A, 6845, crystal or RAM
|-
| No reaction to the keyboard || Keyboard connector || Keyboard connector
|-
| Some keys do not work || Keyboard PCB || Cracks, liquid spillage or distorted mounting; connector lead and termination
|-
| Program disc does not load || Try another disc || If another disc loads, replace the disc; if not, go to the disc flow chart. If the unit is still faulty, seek specialist advice
|-
| No sound || – || Audio amplifier or speaker, volume control, audio connector plug or socket, AY-3-8912 sound IC
|}

The chart's margin note reads "Remember to turn brightness up!". It prints the sound chip as "AY-3-8192"; the parts list gives AY-3-8912 at IC102.<ref name="sm6128" />

On the original board the suspects are the gate array ("ULA") IC116 (HSG3130/3170, part 40010), the Z80A IC111 (Z8400APS), the 6845 IC108 (HD6845SP), the 16&nbsp;MHz crystal X101 and the sixteen MSM3764-20RS DRAMs IC119–IC134. The audio amplifier is IC301 (LA4140), the speaker SP301 and the volume control VR301.<ref name="sm6128" /> Later boards combine several of these into one ASIC; see the [[Amstrad 6128#Main board versions|board versions]].

== Disc faults ==

The disc flow chart and alignment pages in the 6128 manual are the same as in the CPC 664 manual, and the step-by-step tables are in the [[Amstrad CPC 664 Troubleshooting Guide#Disc faults: Amstrad's diagnostic flow chart|CPC 664 Troubleshooting Guide]]. The chart is marked for information only, because Amstrad's agents returned faulty mechanisms for exchange.<ref name="sm6128" /> Its key figures:

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''Disc flow chart thresholds (pp. 8–10)'''
! Check !! Figure !! If it fails
|-
| Index period || 200&nbsp;±5&nbsp;ms || Motor rotation chart: adjust VR201 to 200&nbsp;±1&nbsp;ms; if it cannot be adjusted, try a known-good motor, then replace the drive
|-
| Read output, track 39, "2F" pattern (EME-CF2 disc) || 140&nbsp;mV or more || Check head touch and head output (1&nbsp;mV or more); replace the head or the drive
|-
| Read output, track 00 || 220&nbsp;mV or less || As above
|-
| Read error on one track only || – || Replace the disc
|-
| Seek error || Track 00 sensor output normal? || If normal, replace the drive; if not, replace the track 00 sensor
|}

A motor that runs while the disc does not turn usually has a perished belt; see the [[Amstrad CPC 6128 Maintenance Guide#Drive belt|drive belt]] section.

=== Soft, write and seek errors ===

Amstrad's software-errors page explains that soft errors come from bursts of external noise or small off-tracking and timing shifts and can often be recovered by re-reading; that a write error which persists after more than ten attempts and moves with the disc means a bad disc, while one that stays with the drive means a defective drive; and that seek errors come from the stepper motor or its drive circuit, or incorrect carriage torque, and are retried after recalibrating to track 00. It also notes that the drive is not ready until two revolutions after a disc is inserted, and ends "Always ensure the head is clean."<ref name="sm6128" />

=== AMSDOS messages ===

AMSDOS on the 6128 reports disc problems with the same messages as the CPC 664 ("Drive A: disc missing", "read fail", "write protected" and so on) followed by "Retry, Ignore or Cancel?". The table is in the [[Amstrad CPC 664 Troubleshooting Guide#AMSDOS error messages|CPC 664 Troubleshooting Guide]]. CP/M loads only from a SYSTEM-format disc, which reserves two tracks for it; DATA-format discs have no reserved tracks.<ref name="sm6128" />

== Other faults ==

* Things that are not faults: Amstrad fitted 6845 CRTCs from several makers, and later boards replaced the 6845 with an ASIC. CPCWiki groups these into types with small behavioural differences, so some demos that rely on undocumented CRTC behaviour run differently on different 6128s.<ref name="cpccrtc">CPCWiki, [https://www.cpcwiki.eu/index.php/CRTC "CRTC"]. Retrieved 2026-10-01. The CRTC parts Amstrad used and their type numbers.</ref>
* Cassette: the 6128 has no built-in deck; an external recorder plugs into the 5-pin DIN socket J302 through Amstrad's CL1 lead, with the motor switched by relay RY301.<ref name="sm6128" />
* Printer and expansion: as on the CPC 464; see the [[Amstrad CPC 464 Troubleshooting Guide]].
* Capacitors: see the [[Amstrad CPC 6128 Capacitor Replacement Guide]].

== Related pages ==

* [[Amstrad 6128|Amstrad CPC 6128]] · [[Amstrad CPC 6128 Maintenance Guide]] · [[Amstrad CPC 6128 Capacitor Replacement Guide]]
* [[Amstrad CPC 6128 Service Manual]] · [[Amstrad CTM-644 Maintenance Guide]]
* [[Amstrad CPC 664 Troubleshooting Guide]] · [[Amstrad 6128 Plus Troubleshooting Guide]]
* [[Floppy Drive Repair]] · [[Capacitor Failure Symptoms]] · [[CRT Discharge Procedure]]

== References ==
<references />

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