DRAKE R-4A Spezifikationen PDF herunterladen (Seite 25)

Audio Output

Audio signal from the AF GAIN potentiometer is applied to the grid of the

6EH5

audio output tube V-7 through two stages of AF amplifiers consisting

of Q7 and Q8 and associated circuits.

The audio output from this tube is then applied to the speaker or phones

through audio output transformer T16.

The ANTI-VOX jack is connected

directly to the plate of V7 through

C122.

AVC System

The 2N3394 AVC amplifier(Q-4) is biased past cut-off to provide AVC delay.

When sufficient R. F. voltage from V5 is applied to its base, collector current

flows during part of the cycle.

This causes amplified negative voltage to

appear across its load resistor R83 thus charging

Cl08

when the SSB/CW-AM

switch is in the fast AVC position and both

Cl08

and

Cl09

when the switch

is in the slow AVC position.

The capacitor or capacitors then discharge through R83 to provide a time

constant of about

.75

seconds on slow AVC and about

.025

seconds on

fa st AVC

When the switch is in the AVC off position the load resistor

R83 is shorted, thus preventing AVC voltage from being developed.

AVC voltage is applied to the grid of

Vl,

V4, V5, and Noise Blanker IF

amplifier V9.

Rotating the RF Gain control counter clockwise applies increasingly

more negative bias to the AVC controlled tubes thus limiting their gain.

Adjustment of the receiver sensitivity control

(R87)

varies the minimum

bias on the AVC controlled tubes under no signal conditions with RF Gain

control at maximum.

When the mute line is opened from ground, the bias voltage rises to

-28 volts, thus cutting off the AVC controlled tubes.

S-Meter

The S-Meter operates in a bridge circuit with the plates of V-4 and V-5

on one leg and

and V3 on the other leg,

When AVC voltage is applied to the grids of V-4 and V-5, the tubes

draw less current, thus unbalancing the bridge and causing the S-Meter

to read up scale,

Adjustment of S-Meter zero pot (R42) allows the bridge balance to be

varied for zeroing the meter.

Zero adjustment holds under varying line

voltage conditions due to current characteristics of V-2 and V-3.

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