Rf Tuner Section, Detailed Analysis

u p the voltage of the desired incoming

output circuit. The electrometers supply

s i g n a l . Terminal 6 of the tuner segment

driving voltage to meter cathode followers

a n d the mating wiper contact applies the

V702A and V702B in the main unit.

signal to the grid of rf amplifier V1. Ter-

26. Rf Tuner Section, Detailed Analysis

minal 5 of the tuner segment and the mat-

(fig. 16)

ing wiper contact connects the low-poten-

tial side of T4 secondary winding to chassis

T h e six-position turret tuner (fig. 56)

ground.

c o n t a i n s six separate phenolic segments

c . Rf Amplifier V1. T h e r f a m p l i f i e r ,

o n which are mounted rf transformers,

V1, is a pentode amplifier with tuned cir-

t r i m m e r s , capacitors, and loading re-

cuits in the grid and plate circuits to pro-

sistors for the six different frequency

v i d e selectivity in the stage. These tuned

b a n d s . Each tuner segment has three sets

circuits are contained on the turret tuner,

of coils and trimmers: the input grid cir-

a n d are different for each of the six fre-

cuit of the rf amplifier, the coupling circuit

quency bands. Turning MEGACYCLES band

f r o m the plate of the rf amplifier to the

s w i t c h S1 connects the appropriate tuned

mixer, and the tuned circuit for the oscil-

circuit to the rf amplifier. Capacitor C52C

l a t o r . Since the segments are essentially

i s part of the three-section variable air-

s i m i l a r in design, except for component

dielectric TUNING capacitor which serves

v a l u e s and tuned frequencies, the circuit

to resonate the antenna transformer to the

a n a l y s i s given in the following subpara-

i n c o m i n g signal frequency. Grid bias is

graphs is based on a detailed explanation,

provided by the voltage drop across cathode

w i t h MEGACYCLES band switch set to

r e s i s t o r R10. Capacitor C58 functions as

band 1 (fig. 16). The frequency bands are

the bypass capacitor. Resistor R9 is a

as follows:

s c r e e n - d r o p p i n g resistor, with capacitor

C57 serving as a screen bypass capacitor.

Intermediate

Frequency coverage

frequency

Band

Coils L2, L3, and L4, and capacitors C66,

(mc)

(kc)

C 6 7 , and C68 prevent rf currents from

455

0.15 to 0.36

c i r c u l a t i n g in the heater circuits of all

1,600

0.36 to 0.87

t h r e e stages in the rf tuner section. Re-

455

0.87 to 2.1

1,600

s i s t o r R11, together with capacitors C56

2.1 to 5.2

1,600

5.2 to 12.7

and C61, provide decoupling for the plate

1,600

1 2 . 7 to 30

circuit. Resistor R12 drops the B+ voltage

t o a value required by the plate circuit.

The amplified output of rf amplifier V1 is

a . Rf Transformer Primay Circuit. T h e

rf signal from the main unit is fed through

developed across the primary of interstage

connector P1 (terminal A2) to contact 2 of

transformer T2. Terminals 9 and 10 of the

tuner segment, and their mating wiper con-

the springfinger contacts on the rf chassis.

T h e projecting terminal 2 on the tuner

tacts, create a path of continuity from the

segment couples the incoming signal to the

p l a t e , pin 1, of V1 through the primary

winding of T2, to decoupling resistor Rll

p r i m a r y of antenna transformer T4. Ter-

and the power supply. The output signal is

minal 3 of the tuner segment has a path of

inductively coupled to the signal injection

c o n t i n u i t y to chassis ground through the

w i p e r springfinger contacts. Therefore,

grid, pin 7, of mixer stage V2.

t e r m i n a l 3 on the tuner segment and its

N o t e : Coil L1 and capacitor C59 function as a

1 , 6 0 0 - k c trap. T h i s circuit helps to prevent un-

mating wiper contact connect one side of

wanted 1,600-kc s i g n a l s from appearing in the

the primary winding of T4 to chassis

1,600-kc if. amplifler when this if. amplifier is op-

g r o u n d . This rf transformer presents an

erating. In band 3 (0.87 to 2.1 me), which encom-

i m p e d a n c e of approximately 50 ohms to

p a s s e s 1,600 kc as a frequency within its tuning

r a n g e , the trap circuit is shorted to ground. This

the incoming signal.

circuit arrangement enables an incoming rf signal

b. Rf Transformer Secondary Circuit.

of 1,600 kc to be received and detected in the con-

The secondary of transformer T4, together

ventional manner. In bands 2, 4, 5, and 6, however,

with trimmer capacitors C8 and C9, steps

the 1,600-kc if. amplifier is energized. A method