Section Il. DETAILED CIRCUIT FUNCTIONING

11-6625-467-34

Section Il. DETAILED CIRCUIT FUNCTIONING

produces a constant-amplitude square wave at the

2-21. Introduction

input of emitter follower Q2. Emitter follower Q2

The test set is composed of several functional cir-

current amplifies the square wave and applies it to

cuits listed in a through p below. The referenced

the base of Q3. The amplifier circuit of Q3 amplifies

p a r a g r a p h presents the function for that circuit

the square wave signal and applies it to the base of

only. Many of the circuits are not located entirely on

tuned amplifier Q4. The high-Q LC tank circuit in

o n e circuit card; therefore, following the signal

the collector circuit of Q4 is tuned to 150 Hz. The

paths on the overall schematic diagram is difficult.

tuned amplifier produces a sine wave output at the

A simplified schematic diagram of the associated

collector of Q4. Capacitor C7 couples the sine wave

circuit accompanies each circuit description. The

signal to a voltage divider consisting of R19 and 150

simplified diagrams show only the details of the cir-

CPS CAL control R3. The 150 CPS CAL control

cuit being considered and leave out wires and com-

adjusts the sine wave voltage level input to the

ponents concerned with other circuits. If the circuit

metering circuit. The output of the tuned amplifier

components are located on more than one circuit

is frequency dependent as shown in B, figure 2-17.

card, or on a circuit card and the chassis, the simpli-

The tank circuit in the collector of Q4 has a high-Q,

fied diagrams show them connected in a logical

causing a sharp voltage dropoff when the frequency

manner. Notes and reference designations indicate

deviates from 150 Hz. Before the counter circuit is

the physical location of the components.

used, the circuit must be calibrated by connection of

a. 150 Hz oscillator/counter (para 2-22).

the circuit in the 150-Hz oscillator mode of operation

b. 1 kHz oscillator (para 2-23).

(b below) and adjustment of the 150 CPS CAL con-

c. 10 kHz oscillator (para 2-24).

trol for a predetermined meter reading. (Note that

d. 500 kHz fm oscillator (para 2-25).

the circuit may be calibrated in the oscillator mode

e. 3.975 and 5.925 MHz oscillators (para 2-26).

because the output of the clipper circuit is the same

f . 30 and 68 MHz oscillators (para 2-27).

regardless of the mode of operation of the circuit. )

g. Distortion detector (para 2-28).

This meter reading represents the indication for an

h. Audio amplifier/detector (para 2-29).

input frequency of exactly 150 Hz.

i. RF amplifier/detector (para 2-30).

(2) After the circuit is calibrated, the oscillator

j. 500 kHz filter (para 2-31).

feedback loop is disconnected and the unknown fre-

k. 500 kHz switch (para 2-32).

quency applied to the counter input. If the unknown

1. 500 kHz discriminator (para 2-33).

frequency is exactly 150 Hz (the same as the oscil-

m. 30 MHz switch (para 2-34).

lator frequency), the output voltage to the meter is

n. Frequency selection circuit (para 2-35).

the same as that produced by the internal oscillator;

o. Power supply circuits (para 2-36 through

however, if the frequency is either above or below

2-43).

the 150-Hz center frequency, the tuned amplifier

p. CIRCUIT SELECTOR switches (para 2-44).

voltage is proportionally lower and causes a pro-

portionally lower meter reading.

2-22. 150 Hz Oscillator/Counter

b. 150-Hz Oscillator Mode. In the 150-Hz oscil-

(fig. 2-17)

lator mode, a feedback loop (shown by dashed lines)

is connected from the timed amplifier output to the

The 150 Hz oscillator/counter operates in either of

circuit input through resistor R27. The circuits of

two modes, the 150 Hz oscillator mode or the 150 Hz

transistors Q1 through Q4 operate the same as for

counter mode. In the 150 Hz oscillator mode, the

the counter mode of operation. The feedback volt-

circuit generates a 150 Hz sine wave. In the 150 Hz

age at the base of Q4 is in phase with the existing

counter mode, the circuit operates as a frequency

voltage and sustains circuit oscillation. The circuit

measuring device. The two operational modes are

oscillates at the natural resonant frequency of the

discussed in a and b below.

tank circuit composed of L3 and the parallel capaci-

a. Counter Mode.

tors in the collector circuit of Q4. The reason for this

(1) In the counter mode of operation, the input

p r i m a r i l y is that the maximum regenerative (in

to the circuit is an externally generated 150-Hz

phase) feedback occurs at this frequency. The cir-

sine wave signal. The circuit of transistor Q1 is an

cuit output is taken at the emitter of Q1 and applied

emitter follower which presents a high impedance

to a second emitter follower Q5 through 150 CPS

to the external circuit and a low impedance to the

AMPL control R10. A voltage divider consisting of

clipper circuit. The clipper circuit, consisting of

R25 and R26 at the emitter of Q5 provides high and

diodes CR1 and CR2 and associated resistors, clips

low level circuit outputs. The high-level output (150

the sine wave output of the emitter follower and

2-57