Vox Amplifier - Heathkit SB-401 Assembly Manual

Figure 6-1
TO VOX
GAIN
CONTROL
TO RELAY
AMPLIFIER
CATHODE
-
\ MIF
\ MIR
\.
-------------
\
\
M3F
aTUNE
USB CW
a
LSB
B+
B+
-
-
-
-
- -
-
-
-
1M
............
----
MIC
LEVEL
MIC
BlK-RED
RI
TO
BALANCED
MODULATOR
- --
.2
RIOT
1500
ClOT
_-",I
,
.0 I
BLK-VEl
-
I
r----
I I
,
,
.......
_
I
~,__.-,-., I
.01-== '
I
RI06
220K
-
~
- - ---
r------~ ~
t--++---_
I
-==-
RI02
-
15K
PHONE
PATCH
V1B
1126EA8
CATHODE
FOLLOWER
BlK-GRN
7
I
I
I
I
CI05
V1A
112 6EA8
AUDIO
PREAMPLIFIER
- -~-
VOX delay is determined by the time constant
of the resistance-capacitance combination of
resistor R149, capacitor Cl41, and VOX Delay
control R148. The delay time is made variable
RELAY AND ANTI-VOX AMPLIFIERS
(Figure 6-3)
The positive DC voltage from diode 0105 is
coupled directly to the grid of relay ampli-
fier V12B.
For VOX operation, Vl2B has its bias level set
by a voltage divider network, consisting of re-
sistors R136 and R135. This fixed value is at
such a level that a small amount of positive
voltage applied to the grid of V12B will cause
current to flow through Vl2B and close relay
RLI. The closing of RLl causes the circuit
changes required to place the Transmitter on
the air.
An audio signal applied through the Microphone
input is coupled by capacitor CIOl to the grid of
Audio preamplifier tube VIA. Capacitor C101
limits the low frequency response of the input
signal, while capacitor C102 limits the high fre-
quency response. Any RF signals that may be
picked up by the microphone are bypassed to
ground by capacitor CI02.
The amplified signal from VIA is coupled through
capacitor Cl05 and Mic Level control Rl to the
cathode follower tube, VI B. VIB is used to
match the low impedance input of the balanced
modulator circuit.
The push-to-talk (PTT) and voice operated (VOX)
transmitter circuits are disabled by the Mode
switch in the CW and Tune positions.
VOX AMPLIFIER (Figure 6-2)
When the Mode switch is in either the LSB or
USB positions, the amplified audio signal from
VIA is also applied through VOX GAIN control
R13l and resistor R132 to the VOX amplifier
tube, Vl2A. Any distortion that may occur by
grid rectification in V12A is limited by re-
sistor R132.
In the CW mode of operation, a signal (approxi-
mately 1000 Hz) from sidetone amplifier V12C
is coupled through capacitor C131to VOXampli-
fier VI2A. Depending on the mode of operation ,
either the audio signal from VIA or the sidetone
signal from V12C is amplified by Vl2A and then
is coupled by C133 to diode 0105. Diode D105
rectifies this signal to produce a positive DC
voltage.
AUDIO PREAMPLIFIER AND CATHODE
FOLLOWER (Figure 6-1)
Page 126