Meter Cathode Follower V702 and Metering Circuit, Stage Analysis

a n d B of the bridge-type amplifier are

headset. Depending on the position of the

derived from meter detector V2 and high-

a r m of the VOLUME control, all or part

impedance electrometer stages V3 and V4

of the audio voltage is applied to the con-

( p a r a 32) which are housed within the

trol grid, pin 2, of V701A. Cathode bias is

sealed assembly of each tuning unit (fig. 1).

d e v e l o p e d across resistor R702, which is

In the static condition of the bridge ampli-

b y p a s s e d by capacitor C702. The signal

fier, R715 is adjusted SO that the potential

voltage is developed across plate load

at C and D are equal, and no current flows

r e s i s t o r R703, which is connected to the

t h r o u g h the metering circuit (0 MICRO-

+150-volt output of the power supply. Ca-

VOLTS). When signals (negative voltages)

pacitor C701 serves to bypass any if.

a r e applied to the grids of electrometer

c o m p o n e n t which may still be present in

t u b e s V3 and V4 (fig. 19), the negative

t h e input signal. Capacitor C703 couples

going outputs taken from the cathodes are

the amplified output from the plate, pin 1,

applied to the grids of V702A and V702B

of V701A to the input of the second audio

r e s p e c t i v e l y . Negative going voltages on

amplifier.

the grids of V702A and V702B cause the

b . Second Audio Amplifier V701B. T h e

d c plate resistance of each triode to in-

a m p l i f i e d audio voltage coupled through

c r e a s e . Meter detector tube V2 and elec-

c a p a c i t o r C703 and parasitic suppressor

t r o m e t e r tubes V3 and V4 are arranged

R 7 0 5 is developed across grid resistor

in a signal voltage-divider circuit so that

R 7 0 4 . Cathode bias is provided by the

V702A receives one-third more signal

v o l t a g e drop across unbypassed cathode

v o l t a g e than V702B. Therefore, the dc

resistor R707. Plate voltage for V701B is

p l a t e resistance of V702A increases more

supplied from the +150-volt supply through

than that of V702B. This will cause point

the primary winding, terminals 1 and 2, of

C to become negative with respect to point

o u t p u t transformer T701, The amplified

D and current will flow through the meter-

a u d i o voltage from the plate, pin 6, of

i n g circuit.

V 7 0 1 B is coupled to transformer T701.

b. Cathode Follower V702 (A, fig. 8).

A u d i o f r e q u e n c i e s within the range from

The two sections of cathode follower V702

150- to 5,000-cps, flat within 8-db of the

are connected in a nearly symmetrical

1,000-cps amplitude, are transformer-

c i r c u i t . In the cathode circuit, one re-

c o u p l e d by the secondary winding, ter-

sistor (R712) for V702A is fixed; the cor-

m i n a l s 3 and 4, to the input of rf filter

responding resistor (R715) for V702 B is

F L 7 0 1 . The filter circuit, which consists

variable. In this manner, the voltages

of feedthrough capacitor C705 and rf coil

across the bridge circuit can be equalized

L 7 1 9 , prevents rf signals which may be

b y adjusting ZERO ADJ control R715 in

p i c k e d up by the headsets or the headset

t h e absence of an incoming signal. Fixed

cords from entering the test set. Two

resistors R712 and R713 serve as the

PHONE jacks, which are integral parts of

c a t h o d e load resistor, connected to the

f i l t e r FL701, permit connecting one or

-150-volt output of the power supply, for

two headsets to the final output signal from

V702A. Resistors R714 and R715 function

audio amplifier V701B.

identically in the corresponding circuit for

V702B.

17. Meter Cathode Follower V702 and

(1) Grid circuits. Resistor R719 is the

Metering Circuit, Stage Analysis

grid return for V702A; an identical

(fig. 8)

v a l u e resistor, R718, is the grid

a. General. Cathode follower V702 is a

r e t u r n for V702B. Since both the

dual triode which is connected as a bridge-

grid return resistors and the

type amplifier (B, fig. 8) to provide drive

c a t h o d e resistors are connected to

for the panel-mounted meter and for either

t h e -150-volt output of the power

a recorder or the remote meter, when con-

s u p p l y , the bias voltage for each

n e c t e d to RECORDER EXT METER jack

s e c t i o n of V702 is determined by

J704. The dc input voltages to sections A

t h e current flowing through their