\section{Overhauling the card reader mechanism}
Most of the faults in the card reader are mechanical. The card reader 
drive roller will certainly need replacement unless this has been done 
recently, and the drive belt may be defective 

The card reader mechanism consists of 2 side plates joined by pillars. 
Plastic card guides are riveted to these side plates and these also hold 
the lampholders and photodetectors for the card sensors. A motor with 
integral gearbox is fixed to the left side plate, this drives the card 
roller spindle via a drive belt on the outside of the reader. The magnetic 
head is fixed in a metal block bolted to the inside of the right hand 
side plate. A pressure roller assembly runs in bushes moulded into the card 
guides and holds the card against the drive roller. A further `centre' card 
guide is carried on the lower front pillar. 

\subsection{Dismantling}
With the card reader mechanism removed from the machine, begin by 
removing the drive belt by simply stretching it over the pulleys. Then 
remove the 2 screws holding the head mounting block in place and remove 
the hard assembly. Put this carefully aside. It is, of course, a good 
idea to clean the head face with propan-2-ol while it is out of the reader.

Remove the 2 lampholders from the card guide on the left side plate by
simply pulling them out of their mountings.  Remove the screw on the right
hand end of the lower front pillar, and the screws from the remaining 4
pillars on the left side. Start to separate the plates. Slide the small
PCBs carrying the photodetectors out of their slots on the left card
guide, taking great care not to break the guide. Separate the plates and
remove the pressure roller assembly. Remove the screw on the lower front
pillar on the left side plate and remove this pillar with the centre card
guide. 

To fully separate the plates it is necessary to disconnect the wires from
the drive motor. Loosen the 2 small screws that retain the rear end cap on
the motor and ease it off Make a note of how the wires are connected and
then remove the terminal screws. Put the right hand side plate with the
wiring harness aside. 

You may want to remove the brushes and clean the commutator at this stage.
The brushes are retained by one further screw each, the plastic cover
that acts as a support for the brushgear is held on by the 2 screws that
retained the rear end cap and one further screw. Removing these parts
should not pose any problems.  Clean the motor parts, the pressure 
roller, etc in the normal way.

\subsection{Replacing the roller}
It is now necessary to remove the old roller hub from the spindle. Clean 
off the remains of the `tyre' and attempt to remove the roller from the 
spindle using the puller tool described earlier. Fit the base plate of 
the puller between the roller hubs and the left side plate with the 
puller bolt pressing on the right hand end of the spindle. Tighten the 
bolt and the roller should be drawn off. Remove the old roller hubs and 
slide the spindle and its earthing bracket out of the left side plate. 
Clean the dismantled parts of the reader.

It is now necessary to make a replacement roller hub. and I am afraid to
say that this requires a small lathe. Start with a length of nominal 7/8''
brass rod about 1'' long and measure its diameter accurately.. Put it in
the 3-jaw chuck and face off the end. Using a centre drill followed by a
twist drill in the tailstock chuck, drill a 3.2mm hole along the axis of 
this rod. Check that tithe roller spindle will slide into this hole.

You now need to machine 2 grooves into the brass to take O-rings so that
the diameter at the base of the groove is 3/4". Calculate the depth from
the diameter you measured earlier. Put the centre of one of the grooves
0.1" from the end of the rod, and the other a further 0.15'' along. I have
found that the brazed-tip round-nose tool available from many model
engineering suppliers\footnote{For example GLR Distributors order code
131160} makes an ideal form tool to cut these grooves. Fit a BS210
O-ring\footnote{That's 1'' outside diameter, 3/4'' inside diameter, 1/8''
thick} into each groove in turn and check that the outside diameter of the
fitted O-ring is 1''. 

Part off a total of 0.35'' of the rod. This will result in a symmetrical 
component with a groove 0.1'' from each end. Drill a radial hole, 2.1mm in 
diameter at the base of one of the grooves and tap it to take an M2.5 set 
screw. As a practical tip at this point, it is easier to use hex socket 
(`Allen') setscrews since they can be held on the end of the hex driver 
(`Allen key') tool and fitted into position.

Temporarily refit the spindle to the left side plate of the reader and fit 
the new hub onto it. Fit the right hand side plate, position the hub and 
mark its position on the spindle. Dismantle the reader again and mill or 
file a flat on the spindle to take the end of the set screw.

In a few cases the roller is firmly bonded to the spindle and the puller 
bends the latter before the roller comes free. In this case it is 
necessary to make a new spindle as well as the roller hub. Remove the 
belt pulley from the end of the old spindle, for example by supporting 
the side plate on a bench vice and driving the spindle out with a 
hammer and pin punch. 

The replacement spindle is made from 1/8'' stainless steel rod. One end 
has to be fitted to the pulley. The end of this rod should be turned down 
in the lathe to be a sliding fit in the bore of the pulley.

A metal pulley can be drilled and tapped radially for an M2.5 setscrews
and a flat milled or filed at the end of the rod for this to locate on. 
If your M2.5 tape is not long enough to tap this hole to the full depth, 
tap it as deep as you can and put a small metal rod under the setscrew to 
press against the flat.

If the pulley is plastic, it is easier to drill and tap the new spindle 
axially for an M2 screw. Fit the spindle onto the end of the new spindle 
(it won't go on too far as the end was turned to fit in the pulley bore) 
and anchor the pulley with an M2 pan head screw and shakeproof washer.

Temporarily reassemble the reader side plates and insert the new spindle in 
its bearing holes. Mark the required length just beyond the right hand 
side plate, remove the spindle, cut it to length and face off the end.

Once the new spindle has been made, fit it to the side plate and the new 
hub as described above, mark the position of the roller and mill or file 
a slot for the setscrew.

\subsection{Reassembly and electrical tests} 
Fit the spindle through the earthing strip and left side plate and fit the
new hub, locking it in place with the setscrew. Fit a pair of BS210
O-rings to the grooves in the new hub. 

Reassemble the motor brush gear carrier and the brushes. Before 
connecting  the wiring or fitting the rear end cap, connect a 24V power 
supply between the brushes. The motor should run, if not, check that the 
brushes are located correctly.

Reconnect the wiring, fit the motor end cap and reassemble the card 
guides, pressure roller and side plates. You will notice that the motor 
wires are connected to a tag strip on the inside of the right hand side 
plate which carries the anti-back-emf protection diode. Connect the 24V 
power supply to these tags, taking care to ensure that the cathode 
(banded end) of the diode is connected to the positive side of the 
supply. Again the motor should run.

Fit the drive belt by stretching it over the pulleys and try again. The 
drive roller should rotate. Feed in a card. If it doesn't pass correctly 
through the reader, check to see what is slipping. Most of the time the 
problem is the drive belt on the left side of the reader.

There are 2 ways to determine the length of this drive belt. A calculator 
enthusiast would measure the diameter of the 2 pulleys and the distance 
between their centres with callipers and enter the data into the Belt 
Length program of the HP67 M.E. Pac 1 (Program ME1-17A) or a similar 
program for the HP41. A practical person would run a length of string 
round the pulleys, mark it, remove it and measure the length between the 
marks. Of course, once the length is known, it is as easy as (dividing 
by) $\pi$ to find the diameter of the O-ring that would make a suitable belt.

Being both a calculator enthusiast and a practical person, I did both. And
they agreed to well within the accuracy of the measurements. This suggested
that a BS231 O-ring\footnote{Outside diameter = 2+7/8'', inside diameter =
2+5/8'', thickness = 1/8''} would be an ideal drive belt for this reader,
and tests showed this was correct. 

With the drive system working, it is worth checking the 4 card sensor 
lamps by measuring the resistance from the lampholder body to the end 
contact. If any are open-circuit, pull them out of the card guide and 
dismantle the lampholder by unscrewing the 5/16'' nut on the front end. 
This will also unscrew the central tube and release the bulb. The latter 
is a T1 midget flange bulb rated at 6V, 50mA, and is still 
available\footnote{For example RS Components 360-7531.}. 

It is worth checking the head windings for continuity at this point by 
measuring the resistance between the centre pin and each of the 4 corner 
pins of the connector `plug' (actually a small PCB with 5 pins soldered
to it). Use an ohmmeter which only supplies a small test current (use it 
on a high resistance range if in doubt), and if possible demagnetise the 
head with a `defluxer' afterwards. I suspect that replacement heads are 
impossible to obtain for these machines, though.

Finally refit the magnetic head and refit the card reader mechanism and 
the {\bf Card Reader Interface} to the machine.

\section{Troubleshooting the card reader electronics}
Most of the (rare)failures that occur on the {\bf Card Reader Interface} 
PCB are in the motor driver circuit. 

\subsection{Motor driver}
With the card reader system connected to the rest of the machine, 
perform any load or store operation. If the card reader motor does not 
start, first check that U5a on the {\bf Card Reader Interface} PCB is 
being set. If not, check the {\bf CROut} signal from the {\bf I/O 
Interface} PCB and the outputs of the I/O register. Then check the 
2-transistor circuit connected to the output of this flip-flop. The most 
common problem is for the PNP transistor that actually feeds the motor to 
fail.

\subsection{Card sensors}
Next check the card sensors. With the motor running (and thus {\bf 
SenseEn} at +12V), check that the appropriate {\bf DI(n)} signal changes 
state as each light beam is interrupted. Of course the most common 
problem here is the failure of the  filament lamp in the card reader 
mechanism, but  you should have already checked these. In any case, a 
quick look at the reader mechanism when the machine is switched on will 
detect this fault.

\subsection{Writing}
The next test it to type in a short test program and attempt to save it 
on a blank card. Using your logic probe, check that U8a's output on the 
{\bf Card Reader Interface} PCB is low throughout writing, and that there is 
activity on the outputs of the 4 flip-flops driving the write drives. If 
you have an oscilloscope, check for activity on the collectors of the 
driver transistors and the head connections. You may have to write the 
program several times to be able to check all the signals.

Faults are normally limited to one channel, and are thus fairly easy to 
find. If the `S' timing channel is malfunctioning, also check the {\bf 
CRSout} signal from the {\bf I/O Interface} PCB.

\subsection{Reading}
Now try to re-load the program you just saved. If it loads without 
errors, there is nothing more to be done, if not, then start by checking 
that U8a on the {\bf Card Reader Interface} PCB is set (to disable 
writing) throughout the read operation. Also check that the outputs of 
U9e, U9f, U7f and U7e are low. 

Next check that {\bf ClRd/}. for example on the reset input of U10b, is 
high when the card is actually passing through the reader. Look for 
pulses on {\bf CRRdClk}, if this signal seems dead, trace the signal from 
the head through the `S' channel read amplifier with an oscilloscope. 
Then look for activity on the outputs of the read flip-flops, U10b, U10a 
and U11b. If any are static, investigate that channel's read amplifier.

It is, of course, possible that the reader is working correctly, but that 
{\bf I/OFlg/} is never being asserted so that the processor ignores the 
incoming data. Check the SR flip-flop U4c and U4d and the data selector U9
on the {\bf I/O Interface} PCB.

