Specifications

S7902E. The 16 volts dc is supplied to the
audio amplifier or one of the rf amplifiers
and the test probe rf preamplifier from
S7902E.
3. Audio Amplifier
(fig. 1)
The audio amplifier is a three-stage,
direct-coupled, class A amplifier. Power,
input, and output connections to the audio
amplifier are completed when the selector
switch S7902 is in position 3, 4, 5, 6, 10,
or 11. The output from the audio amplifier
is rectified and fed to the Schmitt trigger
circuit.
a. The input signal to the audio ampli-
fier, developed across resistor. R7401, is
coupled through capacitor C7401 to the
base of transistor Q7401. The amplified
output at the collector of transistor Q7401
is directly coupled to the base of transistor
Q7402. Resistors R7402 and R7403 form a
voltage divider which develops the emitter-
base bias for transistor Q7401. Resistor
R7404 is the output load resistor in the
collector circuit of Q7401.
b. The output at the collector of tran-
sistor Q7401 is amplified by transistor
Q7402. The output of Q7402 is applied
directly to the base of transistor Q7403.
Resistor R7407 is the emitter load re-
sistor for transistor Q7402.
c. The voltage divider, network consist-
ing of resistor R7405, potentiometer
R7406, and capacitor C7402 develops de-
generative alternating-current (at) feed-
back for these directly coupled amplifier
stages. Potentiometer R7406 is a preset
control; its setting determines the overall
gain of the audio amplifier.
d. Resistor R7408 establishes the self-
biasing portion of the emitter-base bias
for transistor Q7403. Capacitor C7403 is
an audio bypass capacitor. Inductor L7401
is the collector load for transistor Q7403.
e. The output of the audio amplifier is
coupled through capacitor C7404 to recti-
fier CR7404. Resistor R7409 is the diode
return resistor, R7410 is the load resistor,
and C7405 is the filter capacitor. The dc
output from the rectifier circuit is fed to
the Schmitt trigger circuit.
4. Amplifier, 11.5-Mc
(fig. 2)
The 11.5-mc amplifier is a two-stage,
intermediate-frequency (if.) amp1ifier.
The overall gain of the two stages is about
40 decibels (db). Power, input, and output
connections to the 11.5-mc amplifier are
made when selector switch S7902 is in
position 7, 9, 13, or 14. The output from
the 11.5-mc amplifier is rectified and fed
to the Schmitt trigger circuit.
a. The input signal from the test probe
rf preamplifier is limited b y resistor
R7612 and coupled to the primary winding
of transformer T7601 through capacitor
C7601. Resistor R7613 matches the im-
pedance of the test probe rf preamplifier
to that of the 11.5-mc amplifier input.
b. First 11.5-mc amplifier Q7601 is
connected in a common-emitter amplifier
stage which provides about 20-db gain.
(1)
(2)
Transformer T7601 and capacitor
C7602 tune the input to Q7601. The
input signal is coupled to the base
of transistor Q7601 through capac-
itor C7603. The output signal from
Q7601 is developed across a tuned
circuit consisting of the primary
winding of transformer T7602 and
capacitor C7608. Resistor R7604
reduces the Q of the tuned circuit
to provide the proper bandwidth.
Resistors R7601 and R7602 form a
voltage divider which provides the
fixed-bias portion of the emitter-
base bias for Q7601. Resistor
R7603 establishes the self-biasing
portion of the emitter-base bias
and stabilizes the emitter current.
Capacitor C7607 is an rf bypass
capacitor. Capacitor C7606 neu-
tralizes the first 11.5-mc ampli-
fier. Diode CR7601 prevents the
emitter-base bias from reversing
when a very large input signal is
applied to the stage. Capacitor
C7604 and inductor L7601 form a
decoupling network in the 16-volt
dc supply line.
c. Second 11.5-mc amplifier Q7602 is
connected in a common-emitter amplifier
stage which provides about 20-db gain.
3