A pick-up arm guiding mechanism for an automatic record player having a command disc with pin-guiding concentric grooves for a sensor pin which is connected to the pick-up arm, each groove corresponding to a specific record size, the pin being directed into the desired groove by a mechanical catching means. During the rotation of the command disc in a changing cycle the pin engages a diversion groove which directs the pin toward the disc center, and the pin is forced out of the groove when the inward movement is blocked by a stop disposed such that the pin engages with the selected pin-guiding groove.