C1 which, in turn, is dependent upon the amplitude of the
action across unbypassed cathode resistor R29 of V4
audio signal.
The amplitude of the audio signal
assures stability of operation.
determines the amount of frequency deviation in the rf
c. The audio output signal at the plate of V4 is fed to
output signal.
The proportionate dc current flowing
DEVIATION control R15 (fig. 16) through capacitors C2B
through meter M1 is representative of the frequency
and C2A, resistor R16, and MODULATION switch S2
deviation.
section 1 rear (fig. 44). Screen voltage for V3 and V4 is
c. Potentiometer R6
(fig. 16) in
the cathode of V1 is
applied through resistors R20 and R30. Capacitors C5
a deviation meter adjustment. It adjusts the operating
and C8B are screen bypass capacitors.
bias of V1. Resistors R3 and R4 in series with M1 cancel
the effect of diode contact potential which, otherwise,
50. Deviation Indicator Circuit V1 and V2
would produce a current through M1 with a consequent
a. The deviation indicator circuit (fig. 16) consists of
error in deviation indication, particularly at low deviation
V1, a type 5654 pentode, operating as a voltage amplifier;
levels. Screen voltage for V1 is obtained from the po-
tube V2, a type 5654 connected as a diode, operating as
sitive 200-volt source and is applied through resistor R81.
a rectifier; and a 200-microampere meter (M1) calibrated
to indicate deviation in kc.
Figure 16. Deviation indicator circuit.
Capacitor C3A is the screen bypass capacitor.
is fed through the MODULATION switch to DEVIATION
d. Meter M1 has a calibrated scale from which the
control potentiometer R15, which also serves as the grid
deviation in kc can be read directly for any of the three rf
resistor for V1. The voltage developed across R15 also is
bands when DEV MULT switch S1 is in the X1 position.
fed to DEV MULT switch S1 through isolating resistor R7
In the X5 and X10 positions of S1, the meter reading must
(fig. 44). After amplification by V1, the positive half -
be multiplied by 5 and 10, respectively, to determine the
cycles of the audio signal cause C1 to charge through V2.
amount of deviation. When DEV MULT switch S1 is in
During the negative plate voltage swings, C1, unable to
the X10 position (fig. 44), the audio voltage across R15 is
discharge through V2, discharges through R2 and M1.
fed through R7 and the rf filter to the input of the
The amount that the needle of M1 is deflected is
modulator, by way of temperature compensation network
dependent upon the amplitude of the discharge current of
32