Amstrad 3" drives have two basic forms. One form has a single head and formats CF2 discs to 180K each side. The other has two heads and formats both sides of the disc at once to 720K. Initially it was said that a different disc, the CF2DD, would be needed for this drive, but in practice it was found that the CF2 worked very well and CF2DD discs are seldom seen, although the 720K format is still known as the CF2DD format.
The single-headed drive is used as the A: drive on the PCW8256 and PCW8512 and also on other Amstrad machines such as the CPC6128 and the Spectrum +3. The double-headed drive is used as the A: drive on a PCW9512 and as the B: drive of a PCW8512 and PCW9512. The A: and B: drives are not immediately interchangeable as they are "hard wired" on the printed circuit boards of the drives. This can be reversed by using a special cable with twisted strands which will convert an A: to a B: or vice versa.
Both single and double-headed drives come in several varieties, with different printed circuit boards and other parts, but they all work in a similar fashion. Each has two connections to the computer; power and data cables. The power connection has four pins. The outer pins carry 5 and 12 volts respectively and the two inner pins are both earthed. Of the 26 pins of the data cable connector, one row of 13 pins is earthed, the other carries instructions to and from the computer. When a disc is inserted into the drive the head(s) contacts the disc. The disc is rotated by a 12 volt motor and drive belt and the head(s) is moved across the disc by a 12 volt stepper motor. The printed circuit board and it's electronic components operate on 5 volts.
Information is stored on a disc in circular tracks, the positions of which are determined when the disc is formatted. The 180K format uses 40 tracks each side and the 720K format uses 80 tracks each side. The position of these tracks is precise so that any disc can be read by any drive of the correct type which is correctly adjusted.
Near the centre of the disc there is a small hole. As the disc revolves at a precise speed the position of the hole at the beginning of each revolution is detected by a light cell so that any part of any track can be accurately accessed by the head(s) for reading or writing information.
Tracks, in common with many things in computing are numbered from 0, and track 0 is the outermost track. When a disc is put into a drive, the head(s) move first to track 0 which carries information which identifies which type of disc it is. If it's a start-up disc the head(s) will then move towards the centre of the disc, reading in the information to boot the computer. If it is a program disc the drive will identify the disc then wait for a command from the person using the computer. If the drive doesn't recognise the disc (it may be an incorrect format or corrupted) then it will not be able to read it and will let you know. In the case of a bad start-up disc it will make some noises caused by the head(s) trying to make sense of the disc, then when it doesn't succeed the machine will emit some beeps. When the machine is up and running, a disc which the drive doesn't recognise will cause various error messages on the screen.
Drive belt fault
Drives usually fail because the rubber belt which turns the disc becomes perished
or stretched. Unless the disc is turned at the correct constant speed the drive
cannot read it. A drive which has a deteriorating drive belt may exhibit symptoms
for days or weeks before the belt finally fails; throwing up error messages
or having to be run for some time before it will work properly. A slack belt
will warm itself up whilst running which will increase the friction on the pulleys
but will soon fail. A drive will sometimes refuse to work after the machine
has not been used for a time. In this case the drive belt has become hard, stretched
and set in shape so that the small diameter pulley on the drive motor cannot
drive it. The solution to a drive belt problem is simple; replace the belt with
a new one of the correct specification.
Head drift fault
Another fault which is less common is when the drive appears to go out of adjustment,
and cannot find track 0 of a standard disc. The symptoms are that the head goes
to track 0, cannot identify it, so goes towards the centre of the disk. It then
returns to the outside of the disc where it again fails to find track 0 and
this cycle is repeated several times with the head travelling back and forth
across the disc until it gives up and an error message appears on the screen.
With this fault, the drive is often able to format a disc, but the disc cannot
be read by other drives as track 0 is in the wrong place. On rare occasions
it is possible to readjust the drive but I have never succeeded with a double-headed
drive yet.
Adjusting to track 0
The head is driven by a shaft with a spiral groove and the shaft is turned by
a stepper motor. The stepper motor can be positioned at every 36 degrees, in
other words 10 increments for one rotation. With both the 40 and 80 track format
discs the head moves 10mm so that for the 40 track disc the motor shaft does
4 revolutions for the full travel. With the 80 track drive the motor rotates
8 times. Comparing the two it is easy to see the difference in the pitches of
the spiral grooves on the respective shafts; the 40 track drive has a coarser
spiral and thus moves the head further for each revolution of the motor. The
approximate head position for track 0 is determined by a proximity switch which
is crudely adjustable. It is secured by a screw in a slot. The proximity switch
can be moved in and out relative to the centre of the disc and then secured
by a single screw. After adjustment by the manufacturer it is stuck in position
with varnish. It is advisable to leave this alone. Fine adjustment is by loosening
two screws and rotating the stepper motor. Remember that a 36 degree rotation
alters the head position one track. A set-up disk and an oscilloscope are needed
for optimum results; I have neither!
I don't hold out much hope for saving a drive this way. I don't know what causes
this problem; it could be a faulty head, a printed circuit board fault, even
incorrect rotation speed. As far as I am concerned it is a fatal fault, but
if you know more about it I would be grateful to hear from you. I'm still experimenting.
Power fault
Some drives exhibit a partial or total power failure fault. The disc fails to
rotate and/or the lights do not illuminate. This is usually due to a power surge,
maybe caused by the drive having been connected or disconnected with the power
on the computer. In some drives the 5 volt and 12 volt supplies pass through
an "IC protector". This acts similarly to a fuse and blows if excessive voltage
or current is applied. These can be obtained from some electronics suppliers
or taken from scrap drives. In emergency they can be replaced by a wire link,
although a second mishap may then blow up other components. The "IC protector"
is labelled Q3, Q4 or something like this, is black or dark in colour and is
about 6mm x 3mm, flat with two legs. They are definitely used on PC boards Z80264
and Z80265 and may be on others; look out for them if you have a power fault.
Replacing rotation and stepper motors.
Occasionally one or other of these develop a fault and need to be replaced.
There is no problem here for the rotation motor provided a direct replacement
is available, but the stepper motor can be difficult. The screws holding the
motor are usually very tight and one is close to the small terminal board which
is easily damaged. Once the screws are removed, the motor pulls off the screw
shaft, but replacing the motor involves adjustment of the heads as detailed
above.
Data cable pin connections
The 26-way data cable is connected by plug and socket to the drive. The pins
on the drive are numbered from 1 to 26 and connections are as follows:-
All odd number pins are grounded.
| Pin No. | Function | Description |
|---|---|---|
| (2) | Index | Detects start of track |
| (4) | Drive Select 0 | Selects drive A: |
| (6) | Drive select 1 | Selects drive B: |
| (8) | Motor on | Runs drive motor |
| (10) | Direction | Head movement in or out |
| (12) | Step | Runs stepper motor |
| (14) | Write data | Write to disc |
| (16) | Write gate | Enable write to disc |
| (18) | Track 0 | Track 0 detected |
| (20) | Write protect | Indicates disc state |
| (22) | Read data | Read from disc |
| (24) | Side 1 select | Identifies side currently used |
| (26) | Ready signal | Indicates drive is ready |
At any time when the drive is working, there will be a signal,
either high or low voltage, on each pin. The high voltage would be between 4
and 5 volts, the low voltage from zero to 1 volt. On pin 10, for example, a
low voltage would turn the stepper motor in one direction, a high voltage would
turn it in the other.
From the table it can be seen how an A: and a B: drive can be connected by the
same cable. A signal on pin 4 will only affect A: and a signal on pin 6 will
only affect B: Thus an A: drive can be used as a B: drive by making a special
cable with wire strands 4 and 6 reversed. If necessary the wires could be reversed
by a switch. For example, a 3.5" B: drive could be fitted to a PCW9512 and an
"ABBA" switch fitted so that the machine can be booted from either.
The table also shows how a standard PCW9512 A: drive can be used as a B: drive
by using a short cable with the wires to pins 4 and 6 reversed.
The ready signal on pin 26 is derived from the indexing circuit and denotes
that the drive is rotating and ready for work. Without a suitable ready signal
the drive will not function. Unfortunately, in general, 3.5" drives have dispensed
with the ready signal which raises a problem when fitting a 3.5" drive in place
of a 3" drive and an "ersatz" signal has to be provided.
Ron King
Email address............ron@king27.freeserve.co.uk