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757 MAINTENANCE MANUAL
 c)  The air inlet assembly has two tube connections, two diaphragms, and a spring. The diaphragms, which are attached to the actuator rod, divide the air inlet assembly into three chambers. One chamber, which has the spring, bleeds to the condensor inlet tube. The middle chamber bleeds to ambient. The third chamber bleeds to the condensor outlet tube.
 (b)
The low pressure differential servo assembly

 1)  The low-pressure-differential-servo assembly is almost the same as the high-differential-pressure-servo assembly. The only difference is that the middle chamber of the air inlet assembly is not installed. Also, the air inlet assembly bleeds to the turbine muff and to the pack outlet.

 (c)
Electro-magnetic-control valve

 1)  The electro-magnetic-control valve has a torque motor, a flapper, a supply nozzle port, a control nozzle port, and a outlet port. The supply nozzle port bleeds to the vent connection. The control nozzle port bleeds to ambient. The outlet port bleeds to the pneumatic actuator. When the torque motor is off, the flapper pushes against the supply nozzle port. When the torque motor is energized, it moves the flapper against the control nozzle port. Thus, the supply nozzle port will open and the control nozzle port will close.

 (d)
Reference Pressure Regulator

 1)  The reference pressure regulator has a regulator diaphragm, a poppet valve, and an adjustment. The regulator diaphragm divides the reference pressure regulator into two chambers. One chamber, with the spring and the adjustment, bleeds to ambient. The other chamber bleeds to the vent connection and the poppet valve. The poppet valve bleeds to the inlet-pressure-sensing probe.

 (e)
Pneumatic Actuator

 1)  The pneumatic actuator is the mechanism that controls the butterfly plate in the valve flow section. The pneumatic actuator has an actuator housing assembly and an actuator linkage assembly.
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757
MAINTENANCE MANUAL

 2)  The actuator housing includes an actuator diaphram that divides the pneumatic actuator into two chambers. The open chamber bleeds to the electro-magnetic-control valve and the two differntial-pressure-servo assemblies. The other chamber, with the spring, bleeds to ambient. The closed stop adjustment controls how far the spring can push the diaphragm.
 3)  The actuator linkage assembly has an actuator arm and a connecting link. They connect the diaphragm to the butterfly shaft. Movement of the actuator diaphragm causes the butterfly shaft to turn.
 (f)  
The Valve Flow Section

 1)  The valve flow section is a 2-inch diameter straight tube. It has the inlet pressure sensing probe, the butterfly plate, the butterfly shaft, and the open stop adjustment. The butterfly plate has a ring that seals against the valve body when the butterfly plate is closed. The butterfly plate is bolted to the butterfly shaft. The butterfly shaft turns on two annular ball bearings in the valve body. The open stop adjustment controls how far the butterfly plate can turn.

 (g)  
The Visual Position Indicator

 1)  The visual position indicator is a red groove on the end of the butterfly shaft. As the butterfly shaft turns, the groove moves to the words OPEN or CLOSED, on the outside of the pneumatic actuator housing assembly.
 (4)  
Function

 (a)  
The position of the low-limit-control valve is controlled by the pneumatic actuator. When the pressure of air in the open chamber of the pneumatic actuator is sufficient, the low-limit-control valve opens. The pressure of air comes from the inlet-pressure-sensing probe. The amount of air that flows from the inlet-pressure-sensing probe to the open chamber controls the position of the low-limit-control valve.

 (b)  
The reference pressure regulator keeps a constant air pressure at the vent connection. When the poppet valve is open, air from the inlet-pressure-sensing probe flows into chamber A or to the vent connection. As the inlet pressure increases, the pressure in chamber A increases until the regulator poppet valve closes. The air in chamber A slowly flows through the restrictor in the vent connection, and the regulator poppet valve opens again.
　
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