Section V. THIRD RECEIVER MIXER AND THIRD OSCILLATOR SECTION

Section V. THIRD RECEIVER MIXER AND THIRD OSCILLATOR SECTION

4 (fig. 10). When switch S401 is in this

29. Filters Z401 Through Z403, 3.0 to 3.9 Mc

p o s i t i o n , and when relay K401 is deen-

(fig. 10)

ergized, 3.4-mc crystal Y405 is connected

a. The 3.0- to 3.9-mc if. from second

to the grid circuit of third oscillator

receiver mixer V303 (para 27) is applied

V401B.

through jack J401 to tuned circuit Z401,

which is the plate load for second receiver

31. Third Oscillator V401B

mixer V303 (fig. 8). Capacitor C403 cou-

(fig. 10)

ples the signal developed across Z401 to

Z 4 0 2 , and C407 couples the signal from

a. Third oscillator V401B operates at

Z402 to Z403.

the frequency of the crystal switched into

b. Filter networks Z401, Z402, and Z403

the grid circuit. The voltage divider action

are bandpass filters that are tuned by the

of capacitors C412 and C413 controls the

10-position, 0.1-mc shaft of the frequency

f e e d b a c k in the oscillator circuit. The

selector. The shaft positions the powdered

v a l u e s of these capacitors are such that

iron cores in the tuning coils of L401, L403,

oscillation is maintained at the fundamen-

and L405. Trimmer coils L402, L404, and

tal frequency of the crystal. Resistor R402

L406 adjust the LC ratio for proper track-

i s o l a t e s jack J404 from the control grid

ing. Resistor R401 and capacitor C406 de-

a n d prevents bias measurements at jack

couple rf from the +125-volt supply which

J404 from loading the oscillator grid cir-

provides plate and screen voltage to V303

cuit. Resistor R404 develops cathode bias

through Z401. Capacitor C411 couples the

to protect V401B in case oscillation stops.

3.0- to 3.9-mc signal to the cathode (pin 2)

Rf choke L407 isolates bias resistor R404

of third receiver mixer V401A.

from the crystal circuit.

b. Oscillator V401B receives plate volt-

30. Crystal Selection

age through plate voltage dropping resistor

(fig. 10)

R407. Capacitor C416, resistor R407, and

filter FL404 decouple rf from the + 125-volt

a. Two signals are applied to V401A.

supply. Capacitor C418 provides rf ground

One signal is the 3.0- to 3.9-mc output rf

a t the junction of resistors R408, R409,

f i l t e r networks Z401 through Z403; the

and R410. Resistor R408 is a voltage drop-

other signal is the voltage generated by

ping resistor for V401A. Coupling capac-

selected crystal Y401 through Y410. The

i t o r C417 couples the signal from the

crystal frequency is always 500-kc higher

c a t h o d e of V401B to the control grid of

or lower than the 3.0- to 3.9-mc if. Third

third receiver mixer V401A.

o s c i l l a t o r V401B is crystal-controlled by

one of 10 crystals. On receive, relay K401

32. Third Receiver Mixer V401A

is deenergized and relay contacts 1 and 2

(fig. 10)

o f K401 connect crystal selector switch

S401 to the control grid of third oscillator

The 3.0- to 3.9-mc signal from Z403 and

V401B.

the third oscillator output mix in the third

receiver mixer V401A to produce the 500-

b. Switch S401 is driven by the 10-posi-

kc difference frequency. The 500-kc signal

tion, 0.1-mc shaft of the frequency selector

is coupled to filter FL901 through blocking

and selects crystals so that there is a 500-

k c difference between the oscillator fre-

capacitor C410. The third receiver mixer

quency and the 3.0- to 3.9-mc if. For ex-

receives plate voltage from voltage divider

ample, assume that the rt unit is tuned to

resistors R409 and R410 through resistor

receive 286.9 mc. The if. is then 3.9 mc.

R 4 0 8 . Resistor R406 is the grid-return

Selector switches S401 and S402 are posi-

circuit to ground. Resistor R405 provides

tioned by the 0.1-mc tuning shaft so that

cathode bias and the coupling impedance

the rotor of S401 is in contact with terminal

f o r the 3.0- to 3.9-mc second if. signal