what i did was i used the circuit out of two other unused servos with three wires and soldered them onto the 4 wire servos. i used the original potentiometers and motors from 4 wire servos for compatibility reasons. taking note of wire placement on the potentiometers and the extra wire (black) on the 3 wire servos that was soldered onto the outside of the servo motor. it was easy. the whole thing took about 30 min. main thing is, use the original gears, pots and motors and just replace the circuit and wiring. leave the wires long on the donor circuit. spectrum 3 wire circuit to losi 4 wire servo. works great.
