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MODIFYING A STANDARD SERVO FOR CONTINUOUS ROTATION 263

much of the lubricant used for the internal gears of the servo. If you think lubrication has been
lost, you can always add more just prior to reassembly. Clear (or white) gear grease is available
at any hobby store that sells R/C parts. Apply the grease sparingly. Do not use a spray- on
lubricant such as WD- 40.

G Before performing the modification, test the servo for proper operation, just to be sure it’s not
bad to begin with. Though rare, some servos fail to work right out of the box. Once it is
modified, you cannot return the servo for in- warranty replacement.

1. Using a Phillips screwdriver, remove the horn wheel, if one is attached to the servo.
2. Untighten the four casing screws from the bottom of the servo. If they don’t get in the
way in the following steps, you can keep the screws in place in the bottom of the case.
Unscrew just enough to remove the top of the servo case. Note that on a few servos,
notably the S03 series from GWS, the case screws are removed from the top, and
these should be removed all the way and set aside while you work.
3. Observe the orientation of all the gears. Remove the center gear, being careful not to
unseat its metal shaft. On most servos the center gear cannot be easily removed with-
out also lifting up the output gear at the same time. Place the center gear aside.
4. Remove the output gear.
5. Using nippy cutters, X- Acto blade, or razor saw, remove the nub on the top side of the
output gear. I prefer the cutters, but exercise caution! The harder the plastic, the more
likely the nub will break off at high speed. Wear eye protection. Always nip first on the
long side, to prevent possible breakage of the gear, and cut off only a small portion at
a time. When using an X- Acto blade or razor saw, the obvious precautions against cut-
ting your fingers off should be observed. Work slowly.
6. Odds are, no matter what cutting technique you use, a small portion of the nub will
remain. This can be filed down with a small flat file.
7. Use the small- bladed screwdriver to remove the metal retaining ring from the underside
of the output gear. This ring retains the potentiometer shaft clip and also serves as a
bearing surface.
8. Use the small- bladed screwdriver to remove the retaining clip.
9. Replace the metal retaining ring back into the output gear.
10. Align the potentiometer shaft so that it’s centered. If needed, rotate it back and forth
to find the center.
11. As an optional step, you may want to connect the servo to your control circuit. Apply
1.5 ms (1500 s) pulses. If the motor turns (even slowly), rotate the potentiometer
shaft until all rotation is “nulled out.”
12. Once set to its center, you can leave the shaft as is or apply a very small dab of cyano-
acrylate glue (Super Glue) to keep the shaft in place. Do not apply too much glue, or
the potentiometer may be damaged.
13. Reassemble by placing the output gear on its seat over the potentiometer. Replace the
middle gear, and observe that all gears properly mesh. Add more grease at this point,
if needed. Finally, replace the top case and the four case screws. Don’t overtighten the
screws.

Test the servo for proper operation by connecting it to your control circuit. A series of
1.5 ms pulses should stop the servo. A signal of 1.0 ms pulses should rotate the servo in one
direction; 2.0 ms pulses should rotate the servo in the other direction.

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