Audio Detector VI

tected signals to the threshold of audibility.

unit. This receptacle is the termi-

T h i s circuit arrangement achieves a high

nating point for the wiring harness.

degree of discrimination between the mon-

When a tuning unit is inserted into

itored signal and other interfering signals

t h e compartment provided in the

t h a t may be present. In the METERED

m a i n unit, this 20-pin connector

S L I D E B A C K position of function switch

mates the 20-pin receptacle located

S702, the output of the negative 150-volt

in the interior of the main unit tun-

p o w e r supply is applied across SLIDE-

compartment.

BACK control R92 (for tuning unit 1), volt-

a g e divider resistor R709 (mounted on

33. Audio Detector VI

function switch S702), and chassis wound.

(fig. 19)

Note: For tuning unit 2, the SLIDEBACK control

Audio detector V1 is also mounted within

is R273; for tuning unit 3, the control is R473; for

t h e detector and electrometer assembly.

tuning unit 4, the control is R587.

T h e amplified if. signal, coupled through

(1) D e p e n d i n g on the position of the

t h e if. input selector, is detected by this

variable arm of this control, a

s t a g e , filtered, and fed to the main unit

n e g a t i v e voltage within the range

for audio monitoring.

from zero to approximately -35

a. Input Circuit. Input connector J1 re-

volts with respect to ground is

c e i v e s the if. signal from the if. output

p r e s e n t at terminal 13 of connec-

selector. Capacitor C6 couples this signal

tors P9 and J2.

to one plate (pin 1) of the dual diode audio

(2) T h i s negative voltage is coupled

d e t e c t o r , One diode of the audio detector

to two L-section filters, which

is not used, and its elements (pins 6 and 7)

c o n s i s t of resistors R3 and R4,

a r e connected to chassis ground.

a n d capacitors C4 and C5, con-

b. Output Circuit, without S l i d e b a c k

nected in series with the L-section

A c t i o n . Positive alternations of the in-

f i l t e r s (R1-C1 and R2-C3) which

c o m i n g signal are detected at the plate.

are tied to the plate output of the

Residual if. signal frequencies are filtered

a u d i o detector.

f r o m the detected audio signal by two

(3) T h e n e g a t i v e v o l t a g e f r o m t h e

L-section pads, which consist of resistors

SLIDEBACK control has a path of

R1 and R2, and capacitors C2 and C3. The

continuity through R4, R3, R2, and

f i l t e r e d output signal is coupled through

R 1 to bias the plate of audio de-

c a p a c i t o r C1 to terminal 14 of multipin

t e c t o r V1. Under this condition,

receptacle J2 and its mating plug P9 (when

only those incoming if. signals with

tuning unit 1 is in use) for application to

an amplitude that can override the

t h e audio amplifier in the main unit. Ca-

negative bias potential are de-

p a c i t o r s C19 and C21 bypass all rf cur-

tected. In slideback operation of the

rents in the filament circuit to ground.

t e s t set, the SLIDEBACK control

c. Output Circuit, with Slideback Action.

is rotated to that setting that per-

The SLIDEBACK control, which is mounted

mits the operator to hear the least

on the tuning unit front panel, provides a

a u d i b l e signal. This signal Is at

m e a n s of reducing the amplitude of de-

the threshold of audible sensitiviy.

Section IV. THEORY OF TUNING UNIT 2

34. Block Diagram of Tuning Unit 2

sembly which houses the two different

s e c t i o n s for covering the 20- to 220-mc

(fig. 20)

f r e q u e n c y spectrum in two bands. These

T h e block diagram for tuning unit 2 is

a r e as f o l l o w s :

s h o w n in figure 20, For complete circuit

details, refer to the overall schematic

(1) 2 7 - m c section. With band selector

diagram (fig. 100).

S201 set to the 20-70 mc position,

a. Rf Tuner. The rf tuner is a subas-

first rf amplifier V201 receives