Section VI. THEORY OF TUNING UNIT 4

v o l t a g e from the heater bus. Choke L434

(pin 4) through a decoupling net-

s e r v e s as a low-pass filter for powerline

w o r k which consists of rf choke

frequencies and a high rejection filter for

L 4 2 4 and bypass capacitor C456.

i n t e r m e d i a t e frequencies. The 6.3-volt ac

c. If. Amplifier V409 (fig. 32). The am-

is connected through feedthrough capacitor

plified if. signal at the output of V408 is

C 4 9 3 to the junction of decoupling coils

inductively coupled to the control grid of

L429 and L432.

i f . amplifier V409. This stage is essen-

t i a l l y similar to the first three if. ampli-

d. If. Cathode Follower V410 (fig. 32).

f i e r stages (b above), but differs in its

Detailed circuit analysis of the if. cathode

t u n e d plate circuit. The tuned network,

follower, which is identical in tuning units

Z403, in the output of the final if. ampli-

2, 3, and 4, appears in the description of

fier, is single tuned. No set-up trans-

tuning 2 (para 40d). The cathode follower

f o r m a t i o n s required because of the signal

is the final stage in the if. amplifier, which

g a i n produced by the previous amplifier

supplies driving power from a low-

stages. Except for the lower value of

i m p e d a n c e source to the detectors in the

cathode bias resistor R456 and the plate-

d e t e c t o r and electrometer assembly A401.

a n d screen-dropping resistor, R457, the

circuit description is identical with that of

49. Detector and Electrometer Circuit A401

i f . amplifiers V406 through V408. The

(fig. 19)

h e a t e r circuit of V409 uses an additional

A l l tuning units employ an identical

decoupling network which isolates the 6.3-

sealed detector and electrometer as-

v o l t ac line from the if. and rf currents

sembly. E a c h c i r c u i t a c c e p t s d r i v i n g

c i r c u l a t i n g in the tuning unit. This addi-

p o w e r from the if. amplifier, detects the

tional decoupling network consists of filter

FL402, w h i c h c o n s i s t s o f a m e t a l l i c

s i g n a l in separate audio and dc metering

s i g n a l channels, and applies output sig-

c y l i n d e r enclosing rf choke L434. The

nals to the appropriate circuits in the main

choke is encased within the metallic

u n i t . These sealed assemblies are inter-

c y l i n d e r which penetrates the top side of

changeable with each other and perform the

t h e if, amplifier subassembly. The outer

f u n c t i o n s described in detail for detector

shield of the filter is soldered to the

grounded surface of the wall. Feedthrough

a n d electrometer circuit A1 within tuning

unit 1 (para 32 and 33).

c a p a c i t o r C494 accepts the 6.3-volt ac

Section VI. THEORY OF TUNING UNIT 4

50. Block Diagram for Tuning Unit 4

plifier V501 receives the incoming

rf signal which is coupled from the

The block diagram for tuning unit 4 is

main unit to coaxial switch S501. A

s h o w n in figure 33. For complete circuit

path o f c o n t i n u i t y i s c r e a t e d

details, refer to figure 102.

t h r o u g h the coaxial switch to the

a. Rf Tuners. T w o s e p a r a t e r f t u n e r s

d o u b l e - t u n e d cavity in the input

s i d e of V501. The amplified out-

are employed in this tuning unit to house

t h e transmission-line types of rf ampli-

p u t of V501 present in its plate

cavity is coupled to a silicon diode

fiers and oscillators. One tuner covers the

f r e q u e n c y spectrum from 700- to 1,000 -

c r y s t a l mixer. Simultaneously, the

mc; another covers the spectrum from

r f energy circulating in the oscil-

4 0 0 - to 700-mc. The signal path through

lator cavity is coupled to the mixer.

e a c h tuner is similar and is described as

T h e oscillator generates a signal

follows :

t h a t tracks 42-mc higher than the

(1) Tuner, 700-1,000. With

FREQ-

incoming rf signal. The mixer

s t a g e heterodynes the rf incoming

U E N C Y RANGE-MC switch S503

s e t to 700-1,000-mc, first rf am-

s i g n a l with the oscillator signal;