3. IN A POSITIONER FOR A MOVABLE MEMBER, THE COMBINATION COMPRISING HYDRAULIC FLUID OPERATED ACTUATOR MEANS FOR MOVING SAID MEMBER, SELECTOR VALVE MEANS FOR CONTROLLING FLOW OF HYDRUALIC FLUID WITH RESPECT TO SAID ACTUATOR MEANS AND INCLUDING A PLURALITY OF SLIDABLE VALVE PLATE,S AND PNEUMATIC-TO-HYDRAULIC TRANSDUCER MEANS OPERATIVELY ASSOCIATED WITH SAID SELECTOR VALVE MEANS AND INCLUDING FOR EACH OF SAID VALVE PLATES FIRST, SECOND, THIRD AND FOURTH CHAMBERS, A DIAPHRAGM SEPARATING SAID FIRST AND SECOND CHAMBERS, A NOZZLE CONNECTING SAID SECOND AND THIRD CHAMBERS AND ADAPTED TO BE CLOSED BY SAID DIAPHRAGM WHEN THE PRESSURE OF COMMAND PNEUMATIC FLUID IN SAID FIRST CHAMBER EXCEEDS THE PRESSURE OF HYDRAULIC FLUID IN SAID SECOND CHAMBER, A SLIDABLE DOUBLE ACTING PISTON SEPARATING SAID THIRD AND FOURTH CHAMBERS AND CONNECTED TO THE CORRESPONDING ONE OF SAID VALVE PLATES AND HAVING DIFFERENTIAL END AREAS THE LARGER OF WHICH IS EXPOSED TO SAID THIRD CHAMBER, MEANS ARRANGED TO SUPPLY PRESSURIZED HYDRAULIC FLUID TO SAID THIRD AND FOURTH CHAMBERS, AND MEANS ARRANGED TO CONNECT SAID SECOND CHAMBER TO A HYDRAULIC FLUID RETURN.