When the pilot selected the landing gear down for landing it failed to extend, and the circuit breaker (CB) popped. He reset it, but each time the gear was selected down it popped again. He decided to return to Broken Hill because of the emergency services availability. On arrival he attempted to extend the gear manually, but he could not turn the handle. He was now committed to a wheels up landing. An inspection revealed that the "up" limit microswitch attachment bracket had broken, preventing the lever arm making contact with the microswitch. This allowed the gearbox motor to continue running when the landing gear was selected up after take-off. The motor then ran the gearbox pinion and segment gears to the end of their travel, forcing and jamming their teeth together. When the gear was selected down the motor was unable to move the pinion gear and stalled, causing the CB to pop. The jammed pinion and segment gears also prevented the manual extension handle being turned.