A head/arm lock mechanism includes a pawl mounted to the armature of a bistable solenoid, the armature being stable in either of two rotary positions. The armature and pawl are limited to move through an angle less than that defined by the stable positions of the armature, so that the armature is biased by the magnetic circuit of the solenoid to one or the other limit positions. When the head/arm is in position to be locked, the solenoid is energized to lock against a spacer on the head/arm and the magnetic circuit continues to hold the pawl in a lock-engaging position upon de-energization of the solenoid. When the head/arm is to be unlocked, the solenoid is again energized to unlock the pawl from the spacer and the magnetic circuit continues to hold the pawl in an unlock position upon de-energization of the solenoid.