TM 11-5826-225-25
S302 is set to either ANT or LOOP, +15
and R305, forms a bridged-T attenuator which
volts dc is attenuated by variable resistor
presents a constant 600-ohm output impedance
R301-C and applied as bias voltage to transis-
to the aircraft interphone system. This im-
tors Q2, Q4, Q7, and Q8. The network com-
pedance value matches the aircraft voice range
p r i s i n g thermistor RT1 and resistor R118
f i l t e r network. impedance. When function
stabilizes bias voltages by temperature com-
switch S302 is set to either ANT or LOOP,
pensation. In the LOOP position of switch
the variable resistors are shorted out by con-
S302, resistor R303 decreases RF gain to com-
tacts 4 on the front and rear wafers of S302-A.
pensate for the increase in gain caused by
In LOOP operating mode, switch wafer S1-L,
switching out certain signal attenuating net-
connects capacitor Cl 11 in parallel with termi-
works in the receiver RF stages during loop
nals 3 and 4 of output transformer T20. This
operation, thus, RF gain is practically the
reduces the audio output level so that sharper
s a m e in either antenna or loop operating
aural nulls may be obtained during manual
modes for the same RF gain control setting.
direction finding. It should be noted that turn-
In adf mode, variable resistor R301-C is by-
ing the GAIN control clockwise rotates R301
passed through switch S302 applying + 15-volt
counterclockwise.
bias \vithout attenuation. It should be noted
b. Manual RF Gain Control Circuit (fig. 1-
that turning the GAIN control clockwise, ro-
tates R301 counterclockwise.
mode, only RF gain can be varied manually.
This is accomplished by varying the bias on
1-31. Receiver Bfo Switching
two IF amplifier stages and on the push-pull
RF amplifier. An increase in positive bias
The control unit BFOOFF switch controls
voltage decreases gain. When function switch
Figure
1-18. Bfo control circuit, schematic diagram.
Change 5 1-29
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