\section{General Dismantling}

To gain access to the internals of the HP9866A for repair, begin by 
removing the {\bf covers}. The top cover is held on by 2 screws  on the 
rear of the machine, after removing these the cover lifts off leaving the 
paper access door attached to the mechanism.

The 5 plug-in PCBs are not exposed. These have colour-coded  ejector 
handles which match the card guides and also give the last digit of the 
PCB part number using the standard resistor colour code. To the left of 
the print mechanism are : 
\begin{description}
\item[Data Path] : Brown handles
\item[Control] : Red handles
\item[Logic PSU] : Orange handles
\end{description}
While on the right side of the print mechanism are : 
\begin{description}
\item[Printhead PSU] : Yellow handles
\item[Motor Driver] : Green handles -- The motor is connected to an edge 
connector on the top of this PCB
\end{description}

Each set of PCBs is retained by a hold-down bracket retained by 2 screws. 
Remove these nest; it is easier to remove the countersunk screw holding 
the bracket to the side pane before the pan head screw holding it to the 
mechanism. The 2 hold-downs are different, but they are colour-coded to  
match the PCBs they fit on, so there should be no confusion when the 
time comes to reassemble the printer. After removing the hold-downs, 
unplug and remove the 5 PCBs. AS mentioned earlier the motor wiring is 
connected to the {\bf Motor Driver} PCB, unplug this edge connector 
before removing the PCB. The {\bf Data Path} PCB contains the MOS 
character generator ROM and MOS shift registers and should therefore be 
stored in an anti-static bag.

Next turn the printer upside-down and remove the base cover which is 
retained by 6 obvious countersunk screws. Loosen the 2 screws at the 
front of the foot rails that go onto the plastic tabs of the front cover. 
Turn the printer the right way up again and loosen the 2 screws at 
 the front of the top flanges of the side plates. Pull the front cover 
forwards, then loosen the 3 screws that hold the switch bracket to it. 
Slide out the switch bracket and completely remove the front cover.

The {\bf Printhead} is removed next. Remove the 2 pan head screws and 
large washers from the front edge of the mechanism side plates that hold 
the head against the platen roller. Then remove the 2 countersunk screws 
fixing it to the chassis plate. Swing the printhead forward and down and 
unplug the 4 ribbon cables from the connectors on the {\bf Printhead 
Driver} PCB. Put the printhead carefully aside.

With the printhead removed, the {\bf Printhead Driver} PCB comes out 
next. Unplug the 20V power input edge connector from the right hand side 
of this PCB and then remove the 7 screws holding the PCB to the chassis. 
Slide the PCB to the right to unplug it for the edge of the {\bf Logic 
Backplane} and lift the PCB out from the underside of the machine.

Before removing the {\bf Mechanism}, the card guides for the plug-in  
PCBs must be removed. Although all 20 card guides are identical, the 
mounting methods for the 4 groups are not. Thy are removed in the 
following manner :
\begin{description}
\item[Front Left] : Unscrew the 2 screws on the plate, remove the plate 
and 3 card guides. Remove the 2 screws holding the fixing bar in place 
(one to the side plate, one of the mechanism) and remove this bar.
\item[Front Right] : Unscrew the 2 screws on the plate, note that the left 
hand screw is the longer one (it goes through the mounting bar into a 
tapped bush on the motor mount). Remove the plate and 2 card guides. 
Remove  the mounting bar by unscrewing the countersunk screw holding it to 
the side plate.
\item[Rear] : Each set of rear guides is retained by a bracket fixed by 2 
screws. Remove the screws, bracket and guides.
\end{description}

With the printer upside-down, remove the 2 screws holding the paper-out 
microswitch bracket to the mechanism chassis. Slide the microswitch out 
and leave it hanging on the wires. Remove the 2 screws on the rear that 
tap into the right-hand mechanism sideplate followed by the 8 screws on 
the underside of the chassis. With the printer standing on its rear panel, 
lift the complete mechanism out. 

The {\bf Logic Backplane} is removed by first removing the foot rails. 
These are retained by the screws that held the front cover in place and 
3 further screws. Under each rail is a  m,tale chassis rail. The one of 
the left side is now free and can easily be removed, that on the right 
side is retained by a further (small) countersunk screw on the front. Then 
unplug the chassis wiring and input cable edge connectors from the {\bf 
Logic backplane}, undo its 6 fixing screws, and lift it out.

\section{Removing the Rear Panel}

To completely separate the rear panel from the printer chassis is a 
fairly major job. All the above parts must be removed, and then continue 
as follows : 

Remove the cable clips from the switch bracket to free the wiring and 
unscrew the 2 screws holding the mains on/off switch to the bracket. 
Unclip all the grey plastic cable clips under the chassis and feed the 
mains switch cable through the chassis to the underside. Next desolder 
all the transformer secondary winding leads. On the underside of the 
chassis, there are 2 orange and 2 blue wires on the logic backplane edge 
connector and 2 yellow and a brown wire on the {\bf Motor Driver} PCB 
connector. Above the chassis there are 2 grey wires to the input of the 
printhead supply bridge rectifier and 2 green wires to the input of the 
+5V supply bridge rectifier. When reassembly, note that the polarity of 
identically-coloured wires is unimportant. Feed the transformer wires 
through the chassis to the top side.

Next remove the plastic bumper strips from the sides of the rear panel . 
They are retained by 3 screws each. Often these strips have decayed with 
time and become brittle, it is then probably best not to refit them but 
to simply refit the screws into their tapped holes. With the bumpers 
removed,, remove the 4 countersunk screws that retain the rear panel to
the side panels and remove the rear panel, freeing the switch cable 
grommet from the chassis plate..

This gives complete access to all parts mounted on the rear panel. 
However, it is often only necessary to work on the cooling fan or the 
connector panel. In this case it is not necessary to desolder the 
transformer secondary wires. Instead, first remove all the parts described 
in the previous section (up to and including the {\bf Logic Backplane}. 
Remove the bumper strips and the 4 countersunk screws under them. Then 
stand the printer on the rear panel and lift the side panels / chassis up 
to separate them. Free the switch grommet and turn the chassis assembly 
about a vertical axis to allow access to the fan and connector panel.

The connector panel is retained by 4 nuts on studs on the bezel and can 
easily be removed. The fan can be overhauled in the same was as the fans 
in the HP9800 calculators; this procedure was described in that series of 
articles.

\section{Dismantling the Printhead}

The printhead pressure springs (2 per printhead hybrid circuit) can be 
removed by unscrewing their housings with a 5/16'' nutdriver. The hybrid 
circuits can then be removed by loosening the 2 setscrews that clamp each 
one to its heatsink, but note that they must be carefully aligned when 
refitting them -- the procedure is in the service manual. Hewlett-Packard 
specifically warn against dismantling the printhead carrier/heatsink 
unit, my experience is that it can be dismantled and reassembled but that 
aligning it is very difficult and it is best not to dismantle this unless 
necessary

\section{Dismantling the Mechanism}

With the printer mechanism removed for the printer, dismantle it in the 
following way: First remove the coupling bolt that links the motor shaft 
to the platen roller using a 5/16" nutdriver. If the mechanism is not 
seized, it is  easy to rotate the roller to allow access to the bolt 
head. If the mechanism is seized, remove the 4 motor fixing screws and 
rotate the motor body to get the bolt to a suitable position for removal.

Next, if the motor screws have not been removed, remove them. Then pull
the motor off the side of the mechanism. Undo the 3 screws thus exposed
and remove the motor mount.  When the motor is refitted, fit the coupling
bolt loosely, then screw the motor to the motor mount and finally tighten
the coupling bolt. 

Continue dismantling by removing the screws from the ends of the paper 
feed roller shafts on the left side of the unit. These retain the belt 
sprockets which should then be drawn off together with the toothed 
coupling belt. Unscrew the pivot nuts for the paper feed bearing plates 
(using a 1/4'' nutdriver) and remove their screws from inside the printer 
mechanism frame. Take off the bearing plates.

On the left side, remove the 6 screws (4 pan head, 2 countersunk). Remove 
the left hand side plate, the thin spacer plate on the inside of it and 
the 3 rollers. The platen roller is the plain one with a bearing in the 
left hand side plate, the front feed roller is divided into 3 sections 
while the rear feed roller has 4 sections. When refittng the feed rollers, 
the send with the flats and the axial tapped hole goes to the left side 
of the machine.

If necessary, remove the similar 6 screws on the right hand side plate 
and separate the mechanism chassis parts. 

The various ball races can be removed from the spindles or chassis 
parts using conventional methods (a 3-legged puller or suitable drifts in 
a bench vice) if it is necessary to replace them. 

I  have  never hand to rebuild the rollers in an HP9866A so I can't 
describe how to do it. Suffice it to say for the moment that it does not 
look to be straightforward, owing to the fact that the feed rollers are 
divided into sections. 

The {\bf Motor} can be dismantled for repair (after removing it form the 
mechanism) by removing the 4 slot-headed through-bolts from the front 
face. The remove the front cover. Often the rotor comes away with this 
cover, separate them by tapping the ed of the spindle on a wooden bench 
top. Recover the crinkle washer and the shims from the rear bearing 
housing and then remove the rear cover of the motor by carefully tapping 
it free. 

\section{Initial reassembly}

Reassemble the printer mechanism by reversing the dismantling procedure. 
Refit the rear panel and reconnect the transformer leads if they were 
desoldered. Refit the logic backplane and reconnect the 2 edge connectors 
to it. It is best to leave the mechanism out of the chassis until the 
initial electrical tests have been performed.
