Amstrad CPC 6128 Troubleshooting Guide

A CPC 6128 with the keyboard removed: main board at left, disc drive at right (photo: Jesus Abizanda, CC BY 2.0)

The 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. The drive procedures are in the Amstrad CPC 6128 Maintenance Guide.

Power first

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

Amstrad's basic hardware analysis

CPC6128 basic hardware analysis (service manual p. 4)

Symptom	Check	Amstrad's suspects
Red LED does not light	Monitor mains fuse; 5 V DC present on the cable from the monitor	Mains fuse; cable or plug assembly from the monitor. With 5 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.

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 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. Later boards combine several of these into one ASIC; see the 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 CPC 664 Troubleshooting Guide. The chart is marked for information only, because Amstrad's agents returned faulty mechanisms for exchange. Its key figures:

Disc flow chart thresholds (pp. 8–10)

Check	Figure	If it fails
Index period	200 ±5 ms	Motor rotation chart: adjust VR201 to 200 ±1 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 mV or more	Check head touch and head output (1 mV or more); replace the head or the drive
Read output, track 00	220 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 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."

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 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.

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.
• 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.
• 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 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