Instruction Manual
D101728X012
3710 and 3720 Positioners
September 2017
33
6. Inspect the zero adjustment pivot (key 32). Be sure it rotates freely. If necessary, replace the zero adjustment pivot.
Also, if necessary, replace the feedback arm assembly (key 21) and cam roller (key 23), following the instructions
given in this manual.
7. Install the new span adjuster assembly by sliding the span adjustment shaft (threaded rod) through the zero
adjustment pivot. Install the zero adjustment locknut on the span adjustment shaft.
8. Install the feedback arm assembly with the new span adjuster assembly by returning the feedback arm assembly to
the feedback pivot. Turn the zero adjustment locknut and zero adjustment knob as needed to fit the range spring to
the summing beam. Install the washer and retaining ring that hold the feedback arm assembly in position.
9. Install the cam and cam locknut. Be sure the cam roller on the feedback arm assembly is properly positioned in
contact with the cam.
10. Perform the calibration procedures in this manual.
Principle of Operation
Figure 23. Fisher 3710 Positioner Schematic
A5944‐1
INPUT SIGNAL PRESSURE
INPUT MODULE
SUMMING BEAM
PIVOT
FEEDBACK SPRING
SPOOL VALVE
SUPPLY
CAM
PIVOT
FEEDBACK LEVER
PIVOT
ZERO ADJ
OUTPUT B
OUTPUT A
PISTON ACTUATOR
SPAN ADJ
The 3710 pneumatic positioner accepts a pneumatic input signal and the 3720 electro‐pneumatic positioner accepts a
milliampere (mA), direct current (DC), input signal from a control device. The 3720 positioner uses a 3722 converter
to provide a pneumatic input to the pneumatic portion (3710) of the electro‐pneumatic positioner.
The pneumatic portion of the positioner is a force‐balance instrument that provides a control valve position
proportional to a pneumatic input signal. The balance of opposing forces in the positioner occurs at the summing
beam.
One force applied to the summing beam is developed from the input signal pressure on the diaphragm. The other
force is from the range spring and is proportional to the position of the feedback lever. The feedback lever position is