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CHAPTER IV

THEORY OF OPERATION

4-1. GENERAL.

Refer to the block diagram figure 4-l and the

schematic diagram figure 5-3 as required for the

following circuit descriptions.

4-2. RF AMPLIFIER.

Signals from the antenna are fed from

(ANT.)

to permeability-tuned transformer

, and are

applied to the grid of V1 through the tuned circuit

formed by

and capacitors Cl through Cl 2. The

amplified signal is coupled from the plate of VI to

the grid of V2 through the tuned circuit formed by

permeability-tuned coil T2 and capacitors Cl8

through C28.

4-3. PREMIXER.

The premixer system includes a transistorized per-

meability-tuned VFO, crystal-controlled oscillator

Q12, premixer tube V6, and the output circuitry

consisting of T3 and T4 and the associated tuning

and coupling capacitors. The PTO is tunable from

4955 kHz to 5455 kHz. The output of the PTO is

applied to the cathode of V6. The output of crystal-

controlled oscillator Q12 is applied to the grid of

V6. The crystal selected to control oscillator Q12

is determined by the settings of the XTALS and

BAND switches. The frequency of the crystal selec-

ted will always be such that the difference frequen-

cy obtained by heterodyning the output of the

PTO with the output of oscillator Q12 will be

5645 kHz higher in frequency than the desired

signal frequency.

4-4. FIRST MIXER.

The output of premixer V6 is coupled to the first

mixer V2 through the permeability-tuned premixer

output coils, T3 and T4, and their associated tuning

and coupling capacitors. Heterodyning of the pre-

mixer output and the RF amplifier output in V2

results in a difference frequency output of 5645

kHz from V2. Gang tuning of the RF coils

and

T2 and the premixer ouptut coils T3 and T4 is

used to maintain a fixed frequency relationship

among these variable circuit elements. The output

from V2 is applied to the input of a four pole

crystal lattice filter with a bandwidth of approxi-

mately 8 kHz.

4-5. FIRST IF SYSTEM.

The output of the crystal filter is applied to the

gate of

Ql,

the 5645 kHz IF amplifier. The ampli-

fied output of this stage is fed first through the

noise blanker, if it is used, and then to grid 1 of V3,

the 2nd mixer. Here the 5645 kHz IF signal is

mixed with the BFO. In the SSB, CW 1.5, CW

.25

modes the output of V3 is the sum of the

5645 kHz IF signal and the BFO or approximately

5695 kHz depending on the setting of the

PASS-

BAND TUNING control. In the AM mode the BFO

is off and V3 functions as a buffer amplifier with

the output at 5645 kHz. The output of V3 is fed

through the MODE switch S2 to the appropriate 8

pole crystal filters. In the AM mode the 5645 kHz

IF signal may be fed through a filter or fed directly

to stage V4 depending on whether an accessory,

filter is installed in the AM filter

osition.

In the

SSB, CW 1.5, CW

.5,

and CW

.25

modes

the

signal

passes through the 8 pole crystal filter with a center

frequency of 5695 kHz. As the frequency of the

BFO

is varied by C86, the PASSBAND TUN-

ING control, the desired sideband or portion of the

5645 kHz IF signal is positioned within the

pass-

band of the 5695 crystal filter. The output of the

crystal filter is then applied to the grid of V4

through the MODE switch, S2. In V4, the 5695

kHz signal is mixed with a 5645 kHz oscillator in

the SSB, CW 1.5, CW

or CW

.25

modes. In the

AM mode the 5645 kHz signal is mixed with a

5595 kHz oscillator. The output of V4 is tuned to

the difference frequency of 50 kHz by the IF trans-

former (T7C). The Q of

T7C

is changed by the

position of the MODE switch. In the CW

.25

position, the high Q tap is connected to

provide additional selectivity.

4-1

1 2 ... 12 13 14 15 16 17 18 19 20 21 22 ... 28 29

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