How to Modify a servo for 360 degrees of Motion
R/C Servos can be modified for 360 degree continuous rotation by following the simple steps below. Servo designs vary so the pictures below might not match that of your servo. Basically, what you are looking to do is locate the feedback potentiometer (5K Pot. shown below in picture) and replace this with static (non-varying) resistors. After this modification, the servo will only be still (no motion) if and only when the controlling stick/slider on the transmitter is centered. Move the stick/slider to the right, the servo rotates to the right, move the stick/slider to the left, the servo rotates to the left, center the stick/slider, the motor stops moving (and holds its position).
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Remove any control horn you have attached to your servo. Open up your servo. In some cases you would remove the four screws on the bottom of the servo and take off the bottom plate. With a small screwdriver, carefully pry out the PC board motor control unit from the plastic case. It will be tight so go slowly.
Once the motor unit is removed from the casing, you will need to de-solder the 5K feedback pot and replace it with a couple of resistors. If there are surface mount parts on the printed circuit board, be careful not to loosen them when de-soldering the 5K pot.
Use two 2.2 K resistors soldered together as a replacement for the 3 terminal potentiometer (2.7 K resistors should also work).
By replacing the pot with the two resistors, it will keep the feedback loop of the servo mechanism at the "centered" value.
Remove the top cover of the servo to expose the gear box. Slide off the gear with the plastic mechanical stop on it and file it off. I used a X-acto knife to shave off the plastic. Replace the gear and make sure everything rotates smoothly.
Reassemble the servo and test it with your R/C gear. It should continuously rotate forward or reverse when you output a pulse width not equal to 1.5ms (anything but center stick).