HP7245A plotter mechanical notes

[Tony Duell]

I spent Newtonsday in a non-traditional manner assembling the Y-drive
mechanism of my HP7245A thermal printer/plotter. The machine now sort-of
works, but the motor driver needs setting up (X drive in particular is
uneven) and there may well be other electronic faults. 

I wish I could say it worked first time, but I made 2 silly mistakes. One 
was to get the connector missaligned when I plugged the Y motor harness 
into the backplane (this caused the PSU to shut down when it tried to run 
the Y motor). The other was that after fitting the printhead and trying 
to plot something, nothing appeared on the paper. Yes, I'd got the paper 
in back-to-front (being thermal, the paper is only sensitive on one side).

Anyway, I'll post my notes on the mechanism below. This is not a 
replacement for the HP7245B service manual on hpmusuem.net, it should be 
read along with that manual. You need a couple of special tools to 
asemble the Y drive, assuming you don't have the offical HP ones (I 
wish), I've included insturctions for making them. And I think my way of 
setting the motor coupling (needing 3 homemade spacers) is easier than 
the HP method.

Feel free to archive this somewhere, make it available, etc. All I ask is 
that I am given credit for it.

-tony

PSU Fan Dismantling
-------------------
Dismantle PSU. Disconnect mains input harness from 'line' switch (push-on 
terminals). Remove fan and harness from PSU chassis.

Remove cable clip on fan housing. Undo 2 slot-head screws (under label). 
Feed cable through housing and remove motor/blade assembly. Recover 
thrust pad (inside housing.

Rmmove circlip from end of spindle. Lift off waskers. Remove blade/rotor 
from stator.

Lubricate bearings with light machine oil when reassembling

X Drive Fan Dismantling
-----------------------
Remove fan from X drive chassis (3 screws). Remove adapter from fan (one 
further screw).

Remove 4 crosshead screws on fan motor plate. Lift motor assembly out of 
housing. Remove 2 slothead screws (under label) and take off plate.

Remove circlip from spindle end WITH CARE, it is spring loaded. Take off 
washer, ball race, 'tophat' spacer, spring, second 'tophat' spacer. Lift 
out stator and remove second ball race and spacer from spindle.


X Drive Chassis
---------------

Anticlockwise rotation of motor -> Carriage moves to right

To lock motor, apply 5V to one motor winding (e.g. between red and yellow
wires at motor connnector)

Fit to LH side plate : 
Microswitch harness (2 microswitches)
Paper roll clip
Y drive clip 
Y drive stop (shorter 'tail')
Tensioner assembly

Fit to RH side plate :
Paper roll clip
Y drive clip
Y drive stop (longer 'tail')
Bail spring (loop towards rear of machine)
2 idlers
Motor mounting bracket

Assemble chassis. Fit motor (wires on left side, nuts on front side)
Fit pulley, hole at top. Lock motor (see above). 

Fit cable : 

LH cable (long end). 3 turns anticlockwise from hole, towards back. 

RH cable (short end) : 1+3/4 turns clockwise from hole, towards front.


Y drive special tools 
---------------------

1) 3 spacers, Each 30mm long, made from 1/2" brass rod, 3.5mm hole drilled 
centrally (along axis). Length is reasonably critical. These are used in 
place of the plastic motor mount when using the platten-movement tool and 
checking the motor coupling

2) Platten-movement tool. Brass block, 5/8" square by 3" long. Drill 
centrally 5mm, and tap M6 (1). Drill 3.5mm holes 1+5/16" either side of 
this (approximately 316" from ends of bar). All holes to be aligned along
center of bar, and drilled perpendicualr to the face :
 
Top View:

          Centre Line
               |
+----------------------------+  ---- 
|   o          *          o  |       5/8"
+----------------------------+  ----

|   |          |          |  |
|   | <1+5/16> | <1+5/16> |  |
|   |                     |  |
| <----------3-------------> |

Mill away ends of bar to reduce thickness to aproximately 1/8" (thickness 
of motor flange) for about 1/2"

SIde view:

        +------------------------+         ----------
        |                        |           1/2
        |                        | 
  +-----+                        + ----+   ----------
  |                                    |     1/8
  + -----------------------------------+   ----------
  
  |     |                        |     |
  |     |                        |     |
  |<1/2>| <-------2------------> |<1/2>|

The central hole is fitted with an M6 (1) bolt. Either a stnadard 
commercial one, or make a both from 1/4" or 5/16" brass rod, turned down 
to 6 mm and threaded with a die for a length of 50 mm. Knurl the 'head' 
end to give a suitable grip and part off.

In use, this tool is fitted in place of the stepper motor, using 2 of the 
spacers mentioned above. The M6 screw presses on the end of the platten 
spindle. With the platten fixing screws hand-tight, the M6 screw is used 
to postion the platten to align the LH sprockets.

(1) THis thrad is not critical, although obviously the hole and bolt must 
be the same thread. M6 is 1mm pitch. 0BA, or 1/4-28 UNF would be fine too.

3) A courved straightedge. This is fitted round the platten, in contact 
with the teeth on the LH sprockt. It is then used to line up those teeth 
with the ones on the LH drum sproket.

Turn a brass cylinder 1.27" (32,3mm) diameter by 5/8" thick 
(approximately). Drill a 3.5mm hole along the axis, tap M4. Drill a 2.5mm 
hole radially, 1/4" from one face, tap M3. 

Drill a 4mm hole in a 6" (approx) flat metal square (e.g. 1/8" thick 
aluminium), aporximately 1.5" from the edges. Screw the cylinder there 
with an M4 screww, radial hole closer to the plate.

The actaul straightedge is made from brass strip, 1/2" wide. Drill a 4mm
hole on the centre line close to one end, fix to the cylinder with an M3
screw and washer. Ensure the strip contacts the plate along its full
length. Bend the strip round the cylinder by hand for about 1/2 turn,
keeping it in contact with the plate. Remove from the cylinder, continue
forming by hand so as to fit closely round it. 

Y Drive Assembly
----------------
Assemble the following 6 subassemblies first.

1) Belt cover : Fit the belt spring leaf (2 screws/nuts/washers)

2) Paper clamp : Assemble the tension arms to the side rails, fit the 
springs. Assemble the front roller (presses against drum when machine is 
asembled), screw side rails to this. Fit the white plastic guide roller 
to the tension arms

3) Paper sensor top cover. Assmeble the magnet, metal return plate, 2 
spacers to this plate with 2 screws, washers, and nuts.

4) Paper sensors : Fit the U-shaped brackets to the cross rail. The
easiest way to do this is to fit the spring to the cross rail, using a thin
rod (e.g. a small crosspoint or Torx srewdriver inserted through the hole
on the bottom of this rail to giude it in place. Then fit the bracket to
the rail alongside the sping ( top upringht (tapped hole) inside the
rail), pull out the rodn amd use a small screwdriver to compress the
spring a little and slide the bracket in place. Use the rod to align the
spring and the holes in the rail and U-bracket. Fit the special adjusting
screw from the bottom, screw into the tapped hole in the U-bracket. Fit 
the reflective optoswitches to the sensor PCBs, fit these to the 
U-brackets (PCB away from the bracket), fit the allen-head screws and 
tighten (Sensor pin side against the 'bumps' on the U bracket). Fit the 
cross rail to the left side plate (cable form under the rail), fit the 
fixing screw finger-tight.

5) Platten : Fit an E-circlip into the grouve on the platten spindle (this 
is the one with one end turned down). Fit a plain (not flanged) ball race 
inside the platten at each end. WIth the platten mounting face away from 
you and the flat (prinhead surface) towards the top, slide in the spindle 
from the RHS, fit a samll sprocket on the LH end, fit a clamp and 
tighten. Slide the other small sproket onto the RH end of the spindle, 
fit the coupling/belt sprocket (pin to go through hole in paper 
sprocket), fit clamp. Place assembly on a flat surface and alighn the 
pins on the 2 paper sprockets. Tighten the clamp at the RH side. Fit the 
mouting rail to the platten with 3 allen screws, finger tight.

6) Drum : Fit an E circlip into the grouve nearest the end of the drum 
spindle. Slide a large paper sprocket onto thi from the long end of the 
spinddle (clamp end to be towarfs the long end, fit a clamp. Slide the 
sprocket hard against the E-clrclip, tighten the clamp. Fit the drum 
tube, the metal drum onto the spindle, fit a clamp inside the latter so 
that the screw can be accessed via the hole in the metal drum. Fit the 
other large sprocket, engaging the pin on the metal drum with its hole. 
Alight the teeth on the paper sprokects on a flat surface as before and 
tighten the clamp. Fit an E-circlip into the remaing groove on the drum 
spindle. Slide a flanged ball race onto each end of the drum spridle, 
flanges towards the drum.

Assembling the Y drive : Fit the platten to the left side plate, motor 
coupling/sprocket to the right. Fit and tighten 2 screws. Fit the drum 
into the left side plate, short end of the spindle to the left. Fit the 
right side plate, motor harness loop under the paper sensor cross rail. 
Screw in the the 3 screws that hold the the RH side plate in place. Leave 
the one in the paper sensor rail finger-tight, note that the top screw on 
the platten mouting on this slde is the long one and fitted with a spacer 
under the head (to keep the motor harness away from moving parts.

Fit the hase plate under the paper sesnor, the paper senosr harness and
notor harness fit into the slot on the cross rail. Screw to the cross rail
with 1 screws and to the lugs on the side plates with 4 scress and
hexagonal spacers. Tighten the 2 screws holding the sensor cross rail to
the side plates. 

Fit the platten movement tool to the stepper motor mouting holes with 2 
motor mouting screws and 2 of the spacers made earlier. Slide the platten 
all the way to the right and screw in the the bolt on the tool until it 
just touches the end of the shaft. Fit the curved straightedge tool round 
the platnen, againsts the teeth of the LH paper sprocket. Screw in the 
bolt of the movement tool until the straightedge contacts the teeth of 
the LH drum sprocket. Tighten the 3 allen screws holding the platten to 
the mouting rail (a ball-ended tool is useful here), remvoe the special 
tools.

Fit the toothed belt to the platten motor coupling sproket. Fit the 
sproket to the drum spindle. Fit the tensioner sleeve to the tensioner, 
fit this to the side plate with 1 screw. Turn it to tension the belt, 
tighten the screw. Fit the clamp to the drum sprocket, leave untightened.

Run a length of plotter paper over the sprockets on the drum and platten. 
Routate the latter so that all teeth enter holes on the paper and tighten 
the clamp on the drum sprocket.

Fit the motor coupling to the motor and fit the clamp. Fit the cross 
piece to the platten coupling, fit the motor to the side plate with the 3 
screws and the spacers made earlier. Tighten the clamp on the motor 
coupling. Check everything turns freely. Remove the motor and the 
spacers. Fit the motor to the motor mount (1 screw and nut). The motor 
wires are routed down the side of the motor, through a slot in the mount. 
Fit this assembly to the mechanism, checking that the cross piece is 
correctly engaged and that the cable is routed above the long screw on 
the platten mouning. Fit the 3 motor screws and tighten. Plug the motor 
cable into the motor harness and fit the connectors and cables into the 
grouve on the side plate.

Fit the paper clamp assembly, 1 screw and large washer on one side, 1 
screw and bush on the other. FIt the belt cover (2 screws), check this 
latches the paper clamp

Fit the top cover on the paper sensor (5 screws)