The contrast forming program track of hte control generator is scanned by a photoelectric head moving transversely to a program strip travel in forward direction. The scanning head follows the program track by a servomotor (20) drive, controlled by the scanning head signals. A path sensor (30) generates a signal in dependence on the scanning head position. This signal is supplied to a flip-flop stage (86), having a hysteresis and responding at a present signal value. The flip-flop stage output energises the servomotor in the sense resulting the scanning head into its starting position.
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