Servo motors are great fun when you want to easily interface a geared motor with a micro controller. However, sometimes, you want a nice little geared motor and don't want to be bothered with control circuitry to drive it. At times like this, it is good to know how to remove the controller from inside the servo motor and convert it to direct drive. Follows are instructions for easily removing the control circuit from a servo motor.
Step 1: Open It Up
Open up the servo by removing the 4 screws fastening it closed.
Once the screws are removed, pry apart the back panel of the motor. You may be tempted to split it in the middle, as this will seem like the natural place to open it up. However, if you split it into two parts as such, it will spill out all the gears and be a mild pain in the neck to put back together.
Step 2: Desolder
Locate the solder terminals for the DC motor. These should be the two largest solder points on the circuit board.
Remove the solder from these with desoldering braid.
Step 3: Pry It Apart
Gently pry the circuit board out of the servo casing once you are sure it is no longer connected to the motor lugs. Be gentle and slowly work it out of the case or it may snap.
It is a good idea to save this circuit board, as it can function later as a small low-power H-bridge circuit for connecting a small motor to a micro controller.
Step 4: Connect New Wires
Connect a red and black wire to the motor.
If there is a marking to indicated one lug is power and one is ground, connect the red one to power (or red to red, as in this case).
Step 5: Case Closed
Close the case back up. Give the shaft of the motor a spin.
It should make the same "wizzing" sound that it made before you did the modification. If it no longer makes the "wizzing" sound, you may need to repair the gear box by realigning the gears.
To control the motor, at this point, now all you need to do is apply power to the motor. To reverse the direction, reverse the wires.
For a similar guide and to learn how to convert servos to be continuous rotation, check out robomaniac's guide here.