Amstrad CPC 6128 Maintenance Guide

The original CPC6128 main board (Amstrad service manual pp. 16–18). The 300-series audio parts are at the left-hand end and the disc interface at the right.

Maintenance on the Amstrad CPC 6128 centres on its 3-inch disc drive, its two supply leads from the monitor, and the keyboard. The drive pages of Amstrad's 6128 manual are the same as those in the CPC 664 manual. Part numbers, procedures and figures here are from the CPC6128 service manual unless another source is given. Fault diagnosis is in the Amstrad CPC 6128 Troubleshooting Guide and capacitor values in the Amstrad CPC 6128 Capacitor Replacement Guide.

Power and safety

The CPC 6128 has no mains connection. Amstrad's specification lists a 5 mm socket for the 5 V supply, "supplied exclusively from monitor", and a 5 mm plug and lead to the 12 V disc power socket on the monitor. The monitors are the CTM644 colour and GT65 green screen; see the Amstrad CTM-644 Maintenance Guide.

CPC 6128 power connections (parts lists, pp. 3 and 21)

Ref	Part	Amstrad part no.	Notes
J301	DC jack HEC0470-01-630	170024	5 V in from the monitor
CD302	DC cord	170882	12 V lead to the monitor; the keyboard parts list gives "Cord DC 14550401" as 170822

Amstrad's cassette control circuit diagram labels the 12 V lead's plug "inner GND, outer +12V (to monitor TV)", and the 5 V socket "+5V (from monitor TV)". The 12 V plug has its centre at ground. Check the polarity of any replacement supply against this before connecting it.

• The keyboard unit carries no mains voltage.
• The CTM644 and GT65 contain the mains supply and a CRT, and hold a dangerous charge after switch-off. See CRT Discharge Procedure.
• Amstrad's safety test for the monitors in this manual is a 3 kV flash test between the mains lead (live and neutral joined) and all accessible metal, and an insulation test requiring at least 4 MΩ.

Identifying your board

The CPC 6128 was built on three main board designs, from the original board to the 1988 cost-down board with a single 100-pin ASIC; the table is on the CPC 6128 page. Amstrad's amendment manual has drawings of two later boards. The parts list in the original manual covers the first board only.

Opening the unit

1. Switch off and unplug both leads from the monitor, the 5 V feed and the 12 V lead.
2. Disconnect any joystick, printer, second drive or expansion device.
3. Lay the keyboard unit face down on a soft cloth and remove the screws from the underside.
4. Separate the cabinet halves carefully and photograph the cable routing.
5. Unplug the drive cable (CD201) and the keyboard, noting the orientation of each.

The keyboard parts list gives the cabinet top assembly (170855), the drive frame (170856), the cabinet bottom (170857) and the keyboard assembly ESU-244 (170858).

The disc drive

The parts list gives the drive as a Compact Floppy Disc Drive EME-155 (FDD201, part 190005). CPCWiki records that early 6128s used the earlier EME-150A. Amstrad's specification sets the software for a 12 ms step rate and 30 ms settling time.

The drive pages are marked for information only: service agents were not to repair or adjust the mechanism or its board, PCB 30001, during warranty, and a faulty mechanism went back to Amstrad for exchange. The parts list gives no part numbers for PCB 30001, but the disc control circuit diagram on pp. 35–36 is that board's schematic, with component values and test voltages.

Drive belt

A drive whose motor runs but whose disc does not turn usually has a perished belt. Amstrad gives no belt size. CPCWiki gives the replacement as 3 mm wide, 0.6 mm thick and 230 mm in outer circumference, and describes the removal procedure for the CPC 6128; it warns that the small write-protect pin can fall out when the drive is turned over. Amstrad's spindle motor procedure removes the belt as its third step (below).

Spindle speed

Amstrad's flow chart sets the index period with VR201 to 200 ±1 ms; the spindle motor replacement procedure gives 200 ±2 ms. The first box of the chart accepts 200 ±5 ms as working. 200 ms is one revolution at 300 rpm.

Mechanical replacements

Amstrad's procedures on p. 37:

Head assembly

1. Remove two screws from the front panel and take the panel off.
2. Remove four screws from the control PCB.
3. Unplug the stepper motor and the LED PCB, disconnect the transistor from the spindle motor and the index sensor from the front of the PCB.
4. Raise the PCB from the side opposite the LED, unplug the head, and remove the control PCB.
5. Remove the four screws holding the loading unit to the chassis from the flywheel side and take out the loading unit.
6. Remove the spring and the rod support screws and slide the head gently off its rod.
7. Refit in reverse order, then check the azimuth burst and track 00 positioning.

Spindle motor

1. Remove the transistor fitted to the motor and unplug CN5 from the control PCB.
2. Remove the drive belt.
3. Undo the two motor screws and replace the motor.
4. Adjust VR201 for an index period of 200 ±2 ms.

Stepper motor

1. Remove the control PCB as for the head.
2. Remove the two screws of the stepper motor bracket and take out the motor.
3. After replacement, check and adjust index and positioning.

Alignment checks

The alignment checks need a double-beam oscilloscope and Amstrad's EME-CF2 test disc: radial adjustment on track 19, index burst on track 39 and azimuth on track 39. The test points are TP1 and TP2 (read signal at the filter outlet), TP3 and TP11 (signal ground), TP5 (track 00 sensor) and TP9 (index). Amstrad's note says that if the wave patterns do not appear, the mechanism is faulty, and to check the waveforms thoroughly before replacing anything.

Head cleaning

The software errors page ends "Always ensure the head is clean." Clean the head with isopropyl alcohol on a foam swab, and let it dry before inserting a disc.

Keyboard

The keyboard is a single assembly, ESU-244 (part 170858). Amstrad's fault chart for keys that do not work sends you to inspect the keyboard PCB for cracks, liquid spillage and distorted mounting, and then to check the connector lead and its termination.

• Remove loose dirt with a soft brush and low-pressure air.
• Clean the keycaps with a damp cloth; keep liquid out of the keyboard.

External cassette and sound

The 6128 keeps a 5-pin DIN socket (J302) for an external cassette recorder (Amstrad's CL1 lead), with the LA4140 (IC301) and LA6358S (IC302), the motor relay RY301 and the 300-series parts that hold most of the board's electrolytic capacitors. The speaker is SP301 and the volume control VR301 (20k).

Cleaning

• Brush and low-pressure air for the main board, the disc controller area and the keyboard.
• Clean the sockets and the printer and expansion edge connectors with a little contact cleaner on a swab.

Tools

• Cross-head screwdrivers and an anti-static strap
• Digital multimeter, for the 5 V and 12 V leads
• Double-beam oscilloscope and the EME-CF2 test disc for Amstrad's alignment checks
• A screwdriver small enough for VR201
• Isopropyl alcohol and foam swabs
• Replacement drive belt
• See Recommended Tools and Floppy Drive Repair.

Related pages

• Amstrad CPC 6128 · Amstrad CPC 6128 Troubleshooting Guide · Amstrad CPC 6128 Capacitor Replacement Guide
• Amstrad CPC 6128 Service Manual · Amstrad CTM-644 Maintenance Guide
• Amstrad CPC 664 Maintenance Guide – the same drive pages
• Amstrad 6128 Plus Maintenance Guide
• Floppy Drive Repair · CRT Discharge Procedure · Capacitor Failure Symptoms