\section{Overhauling the printer mechanism}
As a mechanical device, the internal printer of the HP9810 and HP9820 
machines benefits from being dismantled and cleaned. The platen roller may 
also have decayed with time and turned into a black, sticky, goo and this 
clearly needs to be cleaned off and the roller replaced.

\subsection{Dismantling the printer mechanism}
Being by removing the 3 screws holding the {\bf Printer Interface} PCB to 
the right-hand side of the printer chassis. Hold the {\bf Printer Driver} 
PCB in place on the underside of the printer and ease the {\bf Printer 
Interface} PCB off, separating the edge connector from the card edge of 
the {\bf Printer Driver} PCB. Inspect the {\bf Printer Interface} PCB 
for burnt r damaged components and set it aside.

At the front of the printer, there is a metal beam running across the 
chassis in front of the printhead. In the middle of this beam is a 
hexagonal domed housing that contains the printhead pressure spring. 
Unscrew this with a 5/16'' nutdriver -- carefully so the spring doesn't 
escape -- and remove it and the spring. Undo the 2 screws that hold the 
beam to the bearing plates (noting the position of the `P' clip for the 
microswitch wires) and pull the beam out. If you want, remove the 
microswitch screws to completely free the beam from the printer.

The printhead is carried on a pair of arms with adjusting cams at the 
lower ends. Remove the self-locking nuts from the fixing screws and 
remove the springs, cams and screws from the printer. At the rear edge of 
the {\bf Printer Driver} PCB, unplug the edge connector that carries the 
motor and switch wiring and then remove the PCB and printhead from the 
chassis.

To separate the printhead from the {\bf Printer Driver} PCB, remove the 
setscrews in the clamp. Slide the printhead, the flexible PCB tail and the 
clamp bar out of the block. Handle the printhead with great care, it has a 
brittle ceramic substrate and the only source of replacements is another 
HP9810 or HP9820 printer.

Back on the chassis, remove the circlip on the platen (upper) 
spindle at the front left side and slide off the gear. Loosen the 4 
motor mounting screws and slide the motor forwards to relieve the belt 
tension. Remove the sprocket from the loading belt (lower) shaft at the 
front left, it is supposed to turn freely on this shaft, and remove the 
drive belt. On the right hand side, remove the circlips and gears from 
both shafts, the larger gear fits on the loading belt (lower) shaft.

On the underside of the printer, you will see the 2 loading belts that 
feed the paper towards the platen. Remove the circlip from one end of the 
rear belt shaft and slide this out, recovering the plastic rollers. Moving 
towards the front of the chassis, remove the (smaller) circlip from one 
end of the next shaft and slide that out too, again recovering the 
rollers. This shaft also carries the paper-out contacts, which can be 
left hanging on the wires.

There are actually 2 types of paper-out contact. One type has one wire 
connected to this contact assembly and the other soldered to a metal 
plate on the front section of the printer chassis. The other has both 
wires connected to separate contacts on the assembly carried on this 
spindle, they are connected together by coming into contact with an 
isolated metal plate on the front section of the chassis when the paper 
runs out. This makes little difference to the dismantling and repair 
procedures though.

The platen and loading belt shafts are carried in bearings in plates
screwed to the sides of the chassis. Remove this assembly, note the
positions of the plastic spacer strips under them.  There is a thin idler
roller carried in holes in these palates that may fall out at this point,
make sure you don't loose it. 

Now, on the right side of this assembly is the red manual paper freed 
leaver. Turn this through 180$^\circ$ to free it from the left bearing 
plate (this will take some force if the platen has not decayed) and then 
separate the various parts.

On the main printer chassis again, remove the 2 rear through-bolts.  Push
the rear chassis section downwards and unclip the paper guide wire.  Remove
the 2 front through-bolts, the top one carries the threaded spacer for the
{\bf Printer Interface} PCB, and separate the chassis parts. Remove the 4 
screws loosed earlier and separate the motor from the left side plate.

Clean all the parts of the printer. If the platen has decayed, it will 
take a lot of elbow-grease to remove the goo from the various chassis 
parts,m the {\bf Printer Driver} PCB and the printhead. But clean the 
latter with care, it is very easy to damage

\subsection{Dismantling the motor}
Once the motor has been removed from the printer it can be dismantled to 
inspect and lubricate the bearings or repair winding faults.

Check the windings for continuity between the appropriate pins of the edge 
connector that plugs into the {\bf Printer Driver} PCB. I have never had 
to do it, but it should be possible to re-wind the motor once it is 
dismantled.

Remove the rear end cover by unscrewing the 3 screws on it. These screws 
tap into pillars which are fitted with plastic sleeves, remove these 
sleeves next. Loosen (or remove) the 2 setscrews in the motor sprocket 
and slide this off the spindle followed by the plastic bush under it. 
Remove the circlip and the shim washers under it and slide the rotor, with 
more shim washers on the shaft, out of the rear end for the motor. Make 
sure that no shims have been left adhering to the bearing faces, or they 
are likely to be lost. Treat the rotor with care, it is fairly brittle 
and spares are difficult to obtain.

To dismantle the stator, use a suitable bar (such as the shaft of a large
screwdriver) between the pillars that take the rear end cap screws to
rotate the locking plate and free it from the motor casing. Remove the
plate and the large spring washer under it, then lift out the stator pole
pieces and the coils, keeping them in order. Ensure you refit the coils
the same way up, if one (and only one) is turned over, the motor will run
in reverse when it is reassembled. This causes no damage but you will have
to take everything apart again to turn a coil over. The metal band around
each coil slides off followed by plastic covers. At this stage it is
possible to re-wind the motor. 

Reassemble the motor is the reverse order to dismantling it and lubricate 
the needle-roller bearings with high melting point grease.

Next, reassemble the main chassis. If the platen is in good condition, 
reassemble the platen, bearing plates and belts too. But do not refit the 
PCBs or print head just yet. If the platen has decayed, the next section 
will tell you how to repair it.

\subsection{Repairing the platen roller}
To repair the platen the best material\footnote{Suggested by David Smith 
and Katie Wasserman on the Museum of HP Calculators Forum} appears to be 
3M 8242-8 Cold Shrink(TM) kit\footnote{Available from RS Components, 
251-7566 or Farnell 126-6864}. This kit contains a length of the Cold 
Shrink tubing on a plastic support, 2 sleeves, and a packet of silicone 
grease. Ignore the grease, you do not want the platen to slip once assembled.

Push the 2 sleeves onto the ends of the original platen spindle (after 
cleaning the latter, of course), so that they meet near the middle of the 
spindle. It will take considerable force to do this. Make sure they go on 
evenly and touch, but don't force them on so far that there is a bulge at 
the joint between them

Slide the Cold Shrink over this and start to pull out the helically-wound
support. When about half the support is removed, cut round the Cold Shrink
itself and reposition it over the layer already fitted. Remove the
remainder of the support. 

Thus the spindle is covered with the sleeves and 2 layers of Cold Shrink 
tubing. The next job it to cut it to length. Rest the roller on the 
partially-open jaws of a bench vice and use a sharp knife to trim off the 
ends. Make the platen too long at both ends at this stage. 

Clamp the spindle in the 3-jaw chuck of a small lathe and support the
other end on a tailstock centre (HP kindly provides centring holes at each
end of the spindle). Hold the knife against the toolpost and position the
carriage so that the knife will cut that end of the platen at the correct
place. Then while {\em turning the lathe by hand} (Don't even thin of
running the lathe under power unless you want to make a trip to hospital
-- or worse -- when the knife ends up in you), cut the sleeving to length.
Reverse the platen in the lathe and cut the other end. 

You should now have a repaired platen of the correct size. Reassemble the 
platen, belts and bearings and refit this assembly to the front of the 
printer chassis. Reassemble the gearing.

\subsection{Checking and refitting the print head}
the printhead has 88 connections in a row along the bottom edge which 
connect to corresponding pads on the flexible PCB that's integral with the 
{\bf Printer Driver} PCB. These pads, starting at the left side of the 
printhead follow the pattern of 10 individual resistor connections 
followed by the common for those 10 resistors, followed by another 10 
individual connections, their common and so on for a total of 8 groups.

Begin by testing the printhead. Check the resistance from each individual 
connection to the corresponding common one. A new printhead would have 
resistors measuring between 120$\Omega$ and 260$\Omega$. The resistance 
may increase with wear, but it should still be less than 300$\Omega$. If 
any resistors are totally open-circuit, then I am afraid the printhead 
needs replacement. The only source is another machine.

If the printhead is good, fit the clamp bar to the clamp block and 
position the head and flexiprint so that the contacts are aligned. 
Carefully tighten the setscrews. Do not overtighten them, there is a very 
real risk of cracking (and ruining) the printhead. 

Now check that the printhead is connected properly by measuring the 
resistance between the collector (or metal case) of the transistor Q4 on 
the {\bf Printer Driver} PCB and the cathodes (banded end) of each of the 
diodes in the row alongside it. Then check from the collector of Q3 to the 
cathodes of the next row of diodes, the collector of Q2 to the next row 
and finally the collector of Q1 to the cathodes of the front row of 
diodes. In each case you should see the resistance of a printhead 
resistor. 

If some or all are open-circuit, release the clamp, reposition the head 
and flexiprint and try again. I am sure there was a jig at the factory 
for this, but it's not that difficult to do by hand.

When this is correct, refit the remaining mechanical parts and the PCBs to
the printer chassis. Then fit the printer into the machine (it is not
necessary to fit the fixing screws, just plug the {\bf Printer Interface}
PCB into the backplane) ready for electronic troubleshooting. 

\section{Printer electronic troubleshooting}
With the printer fitted and the machine turned on. press the manual paper 
feed lever at the right hand side of the printer. The printer motor 
should turn. 

If it doesn't, begin by checking {\bf MotorClk}, for example at the 
output of U96 on the {\bf Printer Interface} PCB. If it is not pulsing, 
check that the manual feed clock astable is running (there should be a 
square wave at the collector of Q21) and that {\bf Feed/} at the 
collector of Q24 is being asserted. Then work through the few logic gates 
involved. 

When {\bf MotorClk} is behaving correctly, check that {\bf MotorEn/} at 
the output of U5b is being asserted (low). Next, on the {\bf Printer 
Driver} PCB check that the motor state counter is running. If so, then 
check the motor driver stages.

Once the printer will feed paper correctly, insert a roll of the correct 
thermal paper and try to print something. There are a number of 
possibilities: 
\begin{description}
\item[The printer prints correctly] : Not a lot more to be said, all you 
have to do is screw the printer down and it's done.
\item[The paper feeds but nothing appears on it] : First check the paper
is in the right way round with the thermally-sensitive surface against the
printhead. The method I use for this is to hold the end of the paper over a
hot soldering iron. It will soon be apparent which side changes colour.
Then try adjusting the printhead height using the adjustment cams so that
the row of heater elements is against the paper. It may be necessary to
remove the head and reposition it in the clamp. Check that {\bf PrtHdEn/}
is being asserted and that U4 on the {\bf Printer Driver} PCB is enabling
the printhead selection transistors,. It is possible, albeit unlikely that
all the drivers have failed at once, it may be worth checking this if not
other fault can be found
\item[It prints, but some dots are missing] : If only a few odd 
dot-columns are blank, then it is likely the fault is either a defective 
printhead or the appropriate diodes on the {\bf Printer Driver} PCB. If 
an entire electrical row or column of the printhead matrix is missing, 
then investigate the driver for that line. 
\item[Nothing happens at all] : First check that the printer doesn't 
think it's out of paper. The collector of Q23 on the {\bf Printer 
Interface} PCB should be high.  Then check if {\bf PrtEn} is being 
asserted. If not, check the logic around the SR flip--flop U4b and U12a 
on the {\bf I/O Interface} PCB. If {\bf PrtEn} is being asserted, check 
that it is asserting {\bf PrtHdEn}via U7a on the {\bf Printer Interface} 
PCB, and check the motor signals as above.
\end{description}

Once the printer is correctly working, refit the 4 screws that anchor it 
to the main chassis.
