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Page Title: If. Input Selector, Detailed Analysis
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d. Oscillator V3. A n A r m s t r o n g - t y p e
R 2 7 and R28 are B+ dropping resistors
oscillator is used to generate a frequency,
c o n n e c t e d to the +150-volt bus; C64 and
which is 455 kc higher than the incoming
C 8 3 are B+ decoupling capacitors. Re-
s i s t o r R16 is a grid resistor, providing
signal when band 1 (or band 3) is switched
a ground return for the oscillator injec-
into the circuit. Contacts 16 and 17 on the
t u n e r segment, and their mating spring-
tion grid (pin 5). The signal injection grid
finger contacts on the rf chassis, connect
( p i n 7) of the mixer contains a resonant
the secondary of T1 to the plate (pin 1) of
circuit that consists of the secondary
the oscillator. Contacts 14 and 15 on the
w i n d i n g of transformer T2 and capacitors
t u n e r segment, and their mating spring-
C4, C5, and C52B. Resistor R2 loads the
finger contacts on the rf chassis, connect
c i r c u i t . Capacitor C52B, the third section
the tuned primary winding of T1 between
of the TUNING capacitor, is used for
the grid (pin 7) of the oscillator and chas-
r e s o n a t i n g the mixer grid circuit to the
f r e q u e n c y of the incoming rf signal. The
sis ground. The oscillator is tuned by
c i r c u i t elements comprising the plate cir-
v a r y i n g capacitor C52A, which is part of
c u i t of the mixer are located in the if.
t h e three-section variable air-capacitor,
input selector subassembly (fig. 17).
driven by the front-panel TUNING control.
T h e oscillator tracks with the frequency
o f the incoming signal, thus maintaining
27. If. Input Selector, Detailed Analysis
a constant difference frequency, which is
t h e intermediate frequency. Transformer
T h e if. input selector subassembly con-
T 1 is the feedback transformer for the
t a i n s two alternate tuned plate circuits
o s c i l l a t o r . Its primary coil is in the grid
for the mixer tube. Relays K1 and K2 select
c i r c u i t . The resonant frequency of T1 is
o n e (455 kc) or the other (1,600 kc) if.
d e t e r m i n e d by TUNING capacitor C52A,
o u t p u t s from the mixer plate. Interstage
trimmer C2, padder Cl, capacitor C3,
transformer T22 or T23 serves as the
and the primary inductance. The secondary
plate load. Circuit action is as follows:
w i n d i n g is in the plate circuit, with Rl,
a. Input Circuit. The primary of trans-
a loading resistor, connected across the
f o r m e r T22 and parallel capacitor C69
w i n d i n g to lower its Q. Resistor R17 is
form a circuit which is resonant to 455 kc.
the grid resistor; C54 is the grid coupling
capacitor.  Resistor R18 and capacitor
R e s i s t o r R22 lowers the Q of the tuned
C 5 3 form a plate decoupling network to
circuit to an optimum value. The primary
p r e v e n t rf from modulating the +105-volt
o f transformer T23 and parallel capacitor
p o w e r supply. Resistor R19 is a voltage-
C 7 1 form a circuit which is resonant to
dropping resistor for the screen and plate
1,600 kc. Resistors R27 and R29 and ca-
supplies. Capacitor C65 is the screen by-
p a c i t o r s C72, C77, and C83 comprise the
p a s s capacitor.
p l a t e decoupling network for mixer V2
e. Mixer V2.  The oscillator output is
(fig. 16). Feedthrough capacitors C75 and
taken from the tuned network in the oscil-
C 7 6 , in the +150-volt power supply line,
l a t o r grid circuit and is coupled through
bypass if. currents to ground. The coil of
c a p a c i t o r C55 to the oscillator injection
relay K1 controls the movable contact that
grid (pin 5) of the mixer tube. At the same
connects either T22 or T23 into the mixer
time, the output of the rf amplifier appears
p l a t e circuit. In the energized position of
at the signal injection grid (pin 7) of this
K1, transformer T23 (tuned to 1,600 kc) is
t u b e . The two signals are mixed within
connected to the plate circuit. Relay K1 is
V2; the plate circuit of V2 (located in the
energized from the +150-volt source when
if. input selector) is tuned to the difference
the band switch is rotated to bands 2, 4,
between them. Resistor R14 is the cathode-
5 , and 6. Feedthrough capacitor C74 is
b i a s resistor; C63 is the cathode bypass
used as an rf bypass. Resistor R21 serves
a s a voltage-dropping resistor to the coil
capacitor. Resistor R15 is the screen-
of the relay; C73 provides a path to ground
dropping resistor, with C62 functioning as
f o r relay contact switching transients.
the screen bypass capacitor. Resistors

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