The invention is a motorized gearbox assembly. The motorized gearbox assembly includes a motor that drives an output shaft. The output shaft actuates a window covering by applying torque to a tilt rod of the window covering. The output shaft extends substantially an entire length of the motorized gearbox assembly. It also has a through-channel that extends substantially an entire length of the output shaft, so that the tilt rod can pass entirely through the motorized gearbox assembly. Because of this, the motorized gearbox assembly can be used for retrofitting blinds. In some embodiments, the motorized gearbox assembly has a diametrically polarized magnet driven by the output shaft, and a position encoder that measures the output shaft's position and number of rotations.