TM 55-4920-414-13&P
3-2. Pilot Assist/Nulling Test Box Theory of Operations
a. 28 VDC Power Circuit. The purpose of this circuit is to
supply voltage to, or remove voltage from, three valves (trim
turn on, SAS shutoff and boost shut off). This circuit also sup
plies excitation voltage to three pressure switches (SAS, yaw
boost and collective boost).
b. 115 VAC Power Circuit. This circuit has only one purpose
in this test set, that is to supply input voltage for power sup-
plies PSI (+ 15 VDC) and PS2 ( +5 VDC output).
c. ± VDC Power Circuit This circuit has three purposes.
First, it supplies circuit card power to the 70700-20687-041 cir-
cuit card assembly. Second, it supplies ± 15 VDC to the meter
as a reference voltage. The third purpose for these voltages is
as excitation voltages for the stick input and trim piston posi-
tion transducers located inside the pitch trim assembly (when
connected).
d. +5 VDC Power Circuit. This circuit has only one function
in this test set, that is to supply power for the meter’s inter-
nal circuitry and digital display.
e. Circuit Card Assembly. This card has two purposes. First,
it supplies valve current (milliamps) for one of the following ser-
vo valves; pitch SAS, yaw SAS, roll SAS or pitch trim (when
connected). Secondly, this card produces a voltage (millivolts)
for the meter. The number of millivolts sent to the meter is
equal to the number of milliamps sent to the servo valves. This
is accomplished by two parallel circuits. Both utilizing the out-
put of RI (current control). The circuit utilizing amplfier AR1
produces millivolts for the meter. The circuit for the valves,
contains amplifer AR2 and an array of power transistors Q1
thru Q4. When RI is varied, both circuits respond simultane-
ously to produce the above mentioned outputs.
f. Meter. The meter is a 3 1/2 digit millivolt meter, The full
scale deflection of this meter is from – 199.99 mV to + 199.9
mV. A small set screw is located on the front of the meter
(recessed). This screw is used to adjust for a full scale reading
(+ or - 199.99) when a calibrated voltage source is connected
to the meter input. This screw has no effect on measurements
near zero, such as our application of – 10.5 mV to +10.5 mV.
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