File: servomod.txt
Date: 17 April 1993


  Modifying a Futaba FP-S148 Hobby Servo Motor for Use as a Robot Motor


In the following instructions, "front" means the part of the motor case
that encloses the motor's output shaft (and has the Futaba label on it);
"back" means the opposite side of the motor case.


You will need:

Jeweler's screwdrivers (Phillips)
Small solder iron
Solder sucker
Needlenose pliers
Diagonal cutters
Two 2.7K ohm, 1/4-watt resistors (you could probably use 2.2K ohm
	resistors in a pinch; those are Radio Shack 271-1325)
Exacto knife


1.  If your motor already has some form of mechanical coupler device
screwed onto the end of the output shaft, remove it.

2.  Remove the four screws from the back of the case.

3.  Remove the front and back covers.

4.  Remove nylon center (top) gear and nylon gears on output shaft and
motor shaft.

5.  Using an Exacto knife, carefully trim and remove the nylon spur on
the surface of the large output gear.  This spur normally limits the
servo's movement to an arc of about 270 degrees.  Make sure you remove
the spur completely.  You must not leave any chunks of nylon that might
prevent the output gear from rotating freely.

6.  Pry off the bronze sintered bushing from the plastic hub around the
potentiometer (pot) shaft.

7.  Remove the two small screws on either side of the motor shaft.

8.  Firmly press on the pot's shaft to push it back through the servo's
case.  This should push the pot and the printed circuit board (PCB) out
the back of the case.

WARNING:  DO NOT pry on the PCB at all!  DO NOT push on the motor's
spindle!

9.  Remove the pot from the PCB by carefully heating its connections,
then removing the excess solder with a solder sucker.  Work carefully
and do not damage the PCB's traces.

10.  Install two 2.7K resistors, wired in series, in place of the pot. 
The two resistors will appear to the servo's circuit as a 5K pot rotated
to its center position.  Refer to the following (crude) text schematic:

             +------ 2.7K -------+------- 2.7K -------+
             |                   |                    |
             |                   |                    |
             |                   |                    |
             O                   O                    O

where the Os represent the solder pads that previously held the pot's
leads.  Make sure you install the junction of the two resistors in the
center pad on the PCB.  Trim the leads so the resistors will fit inside
the case when you later reassemble the motor.  Make sure you don't
accidently short any traces on the underside of the PCB when you solder
the resistors in place.

11.  Carefully reassemble the servo motor.  Simply reverse the above
steps for disassembling the motor.