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resistor. Coil L411, which is wound on re-
h e a t e r voltage from feedthrough
sistor R403, functions as a parasitic sup-
C411 to feedthrough capacitor
p r e s s o r . Plate voltage is filtered by re-
C417. Rf choke L413 and bypass ca-
s i s t e r s R404, R407, and R408, together
p a c i t o r C418 prevent rf currents
from circulating in the heater sup-
w i t h bypass capacitor C416. Feedthrough
p l y circuit.
c a p a c i t o r s C421 and C422 are best de-
(2) F e e d t h r o u g h capacitor C417 pene-
scribed by reference to the physical view
t r a t e s the upright center partition
of the rf tuner (fig. 69).
o n which tuning capacitors C402,
a. +105- Volt Dc Power Path. The dc path
C404, and C413 are mounted. The
for applying +105-volts to the plate of os-
inner conductor of the feedthrough
c i l l a t o r V402 consists of the following
capacitor connects the heater volt-
c i r c u i t elements:
age to oscillator V402 heater (pin
(1) The +105-volt bus is connected to
6 ) . The second heater (pin 1) is
the rear of the rf tuner (A, fig. 69)
connected to the rf chassis ground
b y a wire lead which connects to
to complete the heater circuit path.
t h e inner conductor of the feed-
The
c. Oscillator Power Signal Path.
t h r o u g h bypass capacitor, C422.
f r e q u e n c y generated across oscillator coil
(2) A n i n s u l a t e d l e a d c o n n e c t s t h e
L 4 0 9 (B, fig. 69) in V402 plate-to-grid
+105-volt bus from C422 to a sec-
circuit is inductively coupled to the crystal
o n d feedthrough capacitor, C421.
diode pickup coil L405. This coil is phys-
T h i s capacitor penetrates the up-
ically located so that it is also within the
right center partition which serves
m a g n e t i c field surrounding the rf ampli-
as a mounting surface for the three
fier plate circuit inductor loop, L404.
v a r i a b l e tuning capacitors, C402,
C404, and C413.
(3) D e c o u p l i n g resistor R408 (B, fig.
45. Crystal Mixer Circuit, Detailed
69) connects the dc voltage to the
Analysis
high-potential terminal of rf bypass
capacitor C416. The dc voltage
The incoming rf signal and the oscillator
continues through the series- con-
output signal are mixed in this stage. The
n e c t e d decoupling resistors, R407
s i g n a l from rf amplifier plate inductor
and R404, and rf choke L411, to the
L404 is inductively coupled to mixer pickup
l / 4 - i n c h wide loop of brass strap-
l o o p L405. The signal from oscillator-
ping mounted to the frame of C413.
tuned circuit L409, physically located
This strapping functions as a high
about 1 inch away from mixer pickup loop
Q inductor for the oscillator at the
L405 (B, fig. 69), is also within the mag-
ultra-high frequencies employed
netic field of L405 and inductively couples
in tuning unit 3.
t h e oscillator signal to the crystal mixer
b. 6.3 Volt Ac Path.  T h e a c p a t h f o r
c i r c u i t . The nonlinear characteristics of
applying 6.3-volt ac voltage to the heater
a crystal diode, CR401, are untilized for
o f the oscillator tube consists of feed-
t h e heterodyning process because of the
t h r o u g h capacitor C417, rf choke L413,
f a v o r a b l e signal-to-noise ratio obtained
a n d bypass capacitor C418. This decou-
at the high frequencies encountered in the
p l i n g circuit is physically located within
200-400-mc frequency range of tuning
the rf tuner (B, fig. 69) as follows:
u n i t 3. The diode produces a number of
f r e q u e n c i e s at its output circuit. Two of
(1) A parallel path is provided inside
the rf tuner for heater voltage dis-
these are the sum and difference fre-
t r i b u t i o n . One path is through rf
quencies produced by the mixing process.
choke L408 for heater voltage (para
S i n c e pickup coil L405 and trimmer ca-
4 3 b ( 2 ) ) to rf amplifier V401. A
p a c i t o r C408 are resonated to 42-mc, the
difference frequency is passed through
second path is provided by the
output coils L4-15-L416 and coupling
I n s u l a t e d lead which connects the
63


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