Amstrad PCW 8256 Maintenance Guide

Schneider Joyce, the PCW 8256 as sold in Germany

Routine care of the Amstrad PCW 8256 covers the monitor unit and its power supply, the 3-inch drive and its belt, the dot matrix printer and the keyboard. The Amstrad PCW 8512 uses the same boards and printer, so everything here applies to it; its second drive is covered in the Amstrad PCW 8512 Maintenance Guide. The figures come from Amstrad's service manual and user guide.

Safety

Warning: the monitor unit contains the mains power supply and a CRT. Both can hold a dangerous charge after the plug is pulled.

The rating label on the back of the monitor unit warns of live parts inside. Amstrad's drawing of the monitor board outlines the area at mains potential and marks it "danger mains voltage"; the mains fuse F5001 sits in that area beside the mains connector. The supply's 400 V reservoir capacitor is C5005, 100 µF.

• Unplug the machine and wait before removing the back.
• Discharge the tube before working near it; see CRT Discharge Procedure.
• Measure across C5005 and discharge it through a suitable resistor before touching the supply.
• After any repair, Amstrad's safety tests are a 4 kV flash test between the joined mains live and neutral and every accessible metal point on the outside, and an insulation resistance of at least 4 MΩ between the same points.

To reach the drive, community advice is to remove the back cover of the monitor unit, disconnect the two cables from drive A, lift out the drive in its metal cage and then remove the drive from the cage.

Before the first power-up

A machine brought in from a loft or garage should stand in the room for a couple of days before it is switched on, so that it reaches room temperature and humidity. Check the mains plug: the user guide asks for a 5 A fuse in a 13 A plug, not the 13 A fuse supplied with a new plug. Inside the monitor unit, the mains fuse F5001 is a 2 A time-lag, 250 V fuse (Amstrad part 1400253).

Cleaning

• Clean the screen and the plastic case with an aerosol anti-static foam cleaner. Amstrad says never to use spirit-based cleaners.
• Clear paper dust out of the printer with a blower or the nozzle of a vacuum cleaner. Keep the printer out of damp, dusty rooms and direct sunlight.
• Amstrad's drive fault notes start with "always ensure the head is clean". See Floppy Drive Repair for head cleaning.

Disc drive

CPU board; the drive cables run from here to the drive behind the screen

The cabinet parts list gives drive A as a Compact Floppy Disc Drive EME-155 (part 190005). Amstrad told its agents not to repair drive mechanisms or their control boards during the warranty period, and to return them for exchange. The service manual still gives the mechanical replacement procedures and the alignment checks, marked "for information only".

Drive belt

The belt is the usual failure. After years in storage it perishes into a sticky black residue; the drive motor runs but the disc does not turn, and the machine stays on a blank screen with the system disc in drive A. The PCW's drive A is the same as the drives used in the CPC range, so belts and instructions for the CPC drive apply. CPCWiki gives the CPC replacement belt as 3 mm wide, 0.6 mm thick and 230 mm in outer circumference, often sold as "115 mm".

1. Remove drive A as described above.
2. Turn the drive over on a desk or bench. The small write-protect pin can fall out when the drive is upside down, and without it every disc reads as write-protected.
3. Remove the old belt. Clean every trace of the old rubber off the motor pulley and the flywheel; residue left on the pulley stops the new belt working properly.
4. Fit the new belt and turn the flywheel by hand to check it runs true.
5. Check the index period (below) before refitting the drive.

Spindle speed and index period

Amstrad's spindle motor procedure is: remove the transistor fitted to the motor, unplug CN5 from the control board, remove the drive belt, undo the two screws holding the motor, refit in reverse, and adjust VR201 so that the index period is 200 ±2 ms. The same page gives the head assembly and stepper motor replacements; after either, the azimuth burst and track 00 positioning have to be checked.

Alignment checks

Amstrad's alignment checks use a double-beam oscilloscope and the EME-CF2 test disc (EME-CF2DD for the double-sided drive B of the PCW 8512). The test points on the drive board are TP1 and TP2 (read signal at the filter output), TP3 and TP11 (signal ground), TP5 (track 00 sensor output) and TP9 (index).

Drive A alignment figures (service manual pp. 10–14)

Check	Track (drive A)	Figure
Index period	–	200 ±2 ms, set with VR201
Index timing, from index to the first data pulse	39	500 ±500 µs to pass; adjust the LED board to 500 ±100 µs, then check 500 ±200 µs at track 00
Head output	39	140 mV p-p or more; clean the head before testing, replace it if still low
Positioning (off-track)	19	below 26 µm; adjust to below 15 µm
Track 00 and track 39 difference	00 and 39	within 18 µs

Index timing is corrected by loosening the two screws on the LED board and moving it; positioning by loosening the bolt of the rotation stopper on the screw shaft.

Fitting a Gotek

The power cable to the Amstrad drive is not a standard PC floppy power lead. Community advice is to modify it or fit a converter before connecting a Gotek floppy emulator, because the wrong wiring destroys the Gotek.

Power supply and monitor

Monitor board, with the mains area outlined (service manual p. 21)

The monitor board (MM0010B in the parts list) carries the switch-mode supply, built round the STK7308 hybrid (IC5001) and switching transformer T5001, with an AN78N24 at IC5004. It also has the deflection circuits, the flyback transformer FB4001 and the brightness control VR2001.

Amstrad's alignment chart for the monitor needs a digital voltmeter, an oscilloscope, a frequency counter, a test pattern generator and the RP3 diagnostic or a pattern disc:

Monitor and supply alignment (service manual p. 17)

Step	Adjustment	Measure at	Method
1	12 V	Pin 2 of CD102, with CD101 disconnected	Turn the brightness control VR2001 to maximum; set VR5002 for 12 V within 2%
2	5 V	Pin 3 of CD102, with CD101 disconnected	VR2001 at maximum; set VR5001 for 5 V within 2%
3	Vertical size	Screen	VR4002 for full size
4	Vertical linearity	Screen	VR4003 for equal spacing above and below the centre of the test circle
5	Horizontal hold	Pin 1 of CP4001, free running with no input	VR4005 for 15.625 kHz
6	Size and linearity with RP3 or pattern disc	Screen, CD101 reconnected	VR4002 for 15 mm top and bottom borders; VR4003 for 16 mm left and right borders
7	Cut-off	Screen	Brightness to minimum; VR4004 for a faint screen; then brightness up to 100% white and VR2001 set at the centre of the screen

The user-accessible controls are the brightness thumbwheel on the monitor and the vertical hold, which the cabinet parts list gives as a separate pot (171024). For the supply's electrolytic capacitors see the Amstrad PCW 8256 Capacitor Replacement Guide.

Printer

The printer has no supply or controller of its own: the monitor unit drives it through the 34-way ribbon cable and powers it through a separate lead from the Power Out socket. The user guide has the machine unplugged from the mains before the printer or keyboard is connected.

Changing the ribbon

The ribbon is a cassette, Amstrad Printer Ribbon PR-1.

1. Lift the dust cover and release it from its hinges.
2. Hold the old cassette by the fin on top and lift it out.
3. Hold the new cassette with the fin upwards and the ribbon away from you, and turn the feed knob to the left of the fin in the direction of the arrow to take up the slack.
4. Slot the cassette in so that the ribbon passes between the print head and the ribbon guide, without twisting it. Do not touch the head straight after printing; it can be very hot.
5. Press both sides of the cassette down, check the ribbon lies in front of the head, and take up any slack with the feed knob.

Ribbons for the PCW printers are now hard to find.

Print head lever

The head adjustment lever on the inner right-hand side of the printer has five positions. Use the lower settings for thin paper and the higher ones for several thicknesses; position 2 suits a single sheet of ordinary paper. A wrong setting gives light or smeared print.

Parts

Amstrad supplied the printer mechanism as an exchange assembly (171127) and lists its parts individually, among them the print head assembly (171186), timing belt (171204), paper feed motor (171166), head drive motor (171167), home sensor (171180) and paper sensor (171184). The printer's driver board (MS0030B) carries an NE556 and two HA13408 driver ICs (IC706, IC708). Wikipedia notes that the chassis of the dot matrix printer is entirely plastic.

Keyboard

The keyboard plugs into a 4-pin DIN socket on the right of the monitor unit. New keyboards are no longer made, so a faulty one is replaced from another PCW; the PCW 8256 and 8512 keyboards are interchangeable even though one is marked 256K and the other 512K. The keyboard has three option links. LK1 puts it into a self-test mode in which it sends test patterns continuously and the Shift Lock LED stays lit, so check that LK1 is open on a keyboard that behaves oddly.

Related pages

• Amstrad PCW 8256 · Amstrad PCW 8256 Troubleshooting Guide · Amstrad PCW 8256 Capacitor Replacement Guide
• Amstrad PCW 8512 Maintenance Guide · Amstrad PCW 9256 Maintenance Guide
• Floppy Drive Repair · CRT Discharge Procedure · Recommended Tools