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Page Title: If. Amplifiers K207 and V208
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b. If. Amplifiers K207 and V208 (fig. 25).
p l i f i e r V208. Capacitor C275 pre-
A l t h o u g h all four if. amplifiers are es-
vents the bias voltage from short-
sentially the same in function and descrip-
ing to ground. Grid resistor R252
tion, the method of biasing V207 and V208
i s connected to a fixed source of
differs from that employed in the circuits
n e g a t i v e bias potential (R244 and
for V209 and V210 (fig. 26). A second dif-
R 2 4 5 ) through feedthrough capac-
ference exists because double-tuned trans-
itor C271. Resistors R244 and R245
formers are connected in the plate circuits
form a voltage-divider network
of V207 and V208; single-tuned impedance
f o r the -15-volt bias line and are
n e t w o r k s are sufficient to maintain the
physcially 10 c a t e d on terminal
desired selectivity in third and fourth
b o a r d TB201 (fig. 64). Except for
the difference in biasing, cor-
amplifiers, V209 and V210 (fig. 26).
r e s p o n d i n g parts in the second if.
(1) T h e s e c o n d a r y o f i f . i n t e r s t a g e
amplifier state function identically
transformer T205 is tuned to 10.7
with those in the first if. amplifier
rnc and feeds the signal to the con-
trol grid of V207 through capacitor
C266. Cathode bias is supplies by
c. If. Amplifiers V209 and V210 (fig. 26).
The output of second if. amplifier V208 is
the voltage drop across R249, which
is bypassed by C268. Grid resistor
c o u p l e d to the grid of third if. amplifier
V 2 0 9 . This stage operates with cathode
R 2 4 8 is connected through feed-
bias developed across resistor R259, which
through capacitor C267 to the var-
i a b l e arm of GAIN control R241.
i s bypassed by capacitor C282. Resistor
T h e feedthrough capacitor func-
R 2 6 1 , bypassed by C283, decreases the
screen grid and plate voltage. Tuned coil
tions as an rf bypass and couples
t h e bias voltage from the panel-
L 2 2 7 completes the dc path to the plate
mounted GAIN control to the grid of
(pin 5) of amplifier V209. Resistor R263
V207. The amplification of the sig-
a n d capacitor C286 provide decoupling.
nal through V207 is dependent on
A d d i t i o n a l decoupling in the +150-volt
the bias voltage applied to the grid
supply line is provided by low-pass filter
(pin 1) through GAIN control R241.
F L 2 0 1 . This filter permits the 150-volt
Resistor R251, bypassed by C273,
B + output from the power supply to be
decreases the screen grid and
c o u p l e d through feedthrough capacitors
p l a t e voltage. Resistor R254 and
C297 and C296 to the 150-volt bus in the
capacitor C269 provide decoupling
i f . amplifier subassembly; simultaneously,
f o r the plate and screen circuits.
t h e filter coil presents a high-impedance
Bypass capacitors C299 and C302,
i n p u t to the 10.7-mc frequencies present
together with chokes L231 and
i n the plate and screen circuits. Bypass
L232, prevent rf currents from
capacitors C303, C304, and C305, together
c i r c u l a t i n g in the heater circuit,
w i t h chokes L233 and L234, prevent rf
The output of V207 is applied to the
c u r r e n t s from circulating in the heater
circuit. A second low-pass filter network,
primary of T206. The primary
winding and the parallel capacitor,
F L 2 0 2 , provides additional protection in
C272, resonate together at the
a s s u r i n g that the heater power source is
i n t e r m e d i a t e frequency. The out-
f r e e of residual rf and if. currents. The
output of V209 is applied to tuned imped-
put of T206, developed across the
ance network Z201. This network consists
secondary winding and parallel
capacitor C274, is coupled to sec-
o f tuned coil L227, which resonates with
o n d if. amplifier V208. Resistor
c a p a c i t o r C284 to the desired center fre-
R 2 5 3 lowers the Q of the trans-
quency of 10.7 mc. Capacitor C285 is in-
c l u d e d within the shielded network, and
f o r m e r tuned circuit.
(2) The signal from tuned transformer
c o u p l e s the output to fourth if. amplifier
T206 is coupled through capacitor
V 2 1 0 . Resistor R262, also housed within
C275 to the grid of second if. am-
t h e shield, is the grid return for V210.

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