Hitec HS-422 Servo Modification

Caution, wear eye protection for this step. Use a wood backer as illustrated in Figure 3 and Figure 4. A 1/2 wide, square, Exacto blade (#18) works well. First make a short (vertical) cut, then make the long (horizontal) cut.

Figure 3.

Figure 4.

A file or fine tooth saw may also be used, as shown in Figure 5 and Figure 6. Use whatever method you are most comfortable with to remove the plastic stop from the gear.

Figure 5.

Figure 6.

Step 3, Next we need to bring the potentiometer outside of the housing. This allows the precise adjustment of the servos stop value. I use a pair of needle-nose pliers to remove a small section of the case to allow the wires to exit. Just give the pliers a twist and the plastic will come out easily. Figure 7 illustrates the procedure.

Figure 7.

Figure 8.

Step 4, A nice feature of the Hitec servo modification is the ability to reverse the wires to the motor. The motor is the large round object with the brown and orange wires going to it as shown in Figure 8. When done to one of the servos the relationship of left and right servos are the same. For example an increase in pulse length on both sides will result in forward motion. Reverse the motor leads on one of the servos and mark it as the "left" servo.

Figure 9.

Step 5, Gently push the control board back in place. Replace the nylon gears into position starting with the middle one. When all of the gears are in place install the servo case top and bottom. They should fit together squarely with little effort. Finally replace the four long servo case screws. Your servos should look like Figure 9, and are now ready for installation into the robot body.

Step 6A, To use the servo with a microcontroller, program it to generate a 1.5mS pulse every 10mS. A program for the First Step (BS1) can be found here. A program for the Next Step (BS2) can be found here. When you run the program the servo should immediately begin rotating. Adjust the potentiometer until the servo stops rotating. Now altering the pulse value will result in rotation of the servos output shaft. For the right side servos, shortening the on time below 1.5mS results in CCW rotation. The farther from 1.5mS you go the faster the rotation. Lengthening the time more than 1.5mS results in CW rotation. The farther from 1.5mS you go the faster the rotation. The legal range is 1.0mS to 2.0mS.

Step 6B, To use the servo with an RC system, connect it to a receiver as usual. When you power it up the servo should immediately begin rotating. Adjust the stick and trimmers on the RC transmitter for center of throw. Adjust the potentiometer until the servo stops rotating. Now moving the stick will result in changing the speed and direction of the servos output shaft.

Figure 10.

Step 7, Attaching a 3" wheel to a servo is not that difficult. Remove the servo horn screw, but leave the round servo horn in place. Attach a small section of foam double sided tape to the servo horn then push the wheel onto the tape, aligning the center hole to the servos. Install a 1" #4 self tapping screw into the wheel / servo combo, and drive till tight. Other wheels will require different methods.