Heathkit SB-401 Assembly Manual page 151

-
-
-
.05
14/
~
RI49
1M
-
-
-
-
-
-
- -
.0 I - 10
1000
CI36
+
Cl37
ANTI-VOX
INPUT
1M
.ot
V13A
1/2 6J II
ANTI-VOX
AMP.
RI42
1.5M
RI46 C I39
-----
B-BIAS
-
-
.01
1500
IW
0106 RI43
220K
CI38
.01
Figure 6-3
B+
lOOK
MIF
MIR
RI35 C 134
.-A.-
68
RI37
R/36
220K
IW
,
I __.__
RELAY
RLI
B+
-
LSB
0105
FROM VOX
AMPLIFIER
cw
o
USB
V12B
1/3 6DIO
RELAY AMP.
-
-
- 10
RI34
680
+
CI32
Figure 6-2
-
In the LSB, USB, and Tune modes, V12B is
biased through resistor R137 to ground, causing
relay RLl to close. The grounding of resistor
R137 is automatic in the Tune position of the
Mode switch, and is controlled by the button on
the microphone for PTT (LSB, USB) operation.
V13A functions as an Anti-VOX control circuit.
Anti- VOX prevents the transmitter from being
keyed by audio picked up from the receiver
speaker by the microphone. An audio signal
from the station receiver is coupled through the
Anti-VOX input socket and Anti-VOX control
R141 to the grid of V13A. This Signal is ampli-
fied by V13A, coupled through capacitor C13S,
and rectified by diode 0106. The negative volt-
age present at the resistance-capacitance net-
work of resistor R146 and capacitor C139 is
coupled to the input side of diode 0105. This
negative voltage will bias 0105 so that it will
not conduct, and relay RLl will not close.
Additional audio voltage caused by talking into the
microphone will overcome the bias and allow the
VOX
circuit to close the relay.
FROM MIC
PTT
over a considerable range by the VOX Delay
control.
B+
fROM SPEECH
RI33
AMPLIFIER
47K
I
I
V12A
I
1/3 6DIO
FROM
I
I
vox
AMR
SIDETONE
AMPLifiER
CI.JI
t+TO DIODE
7.5
,
0105
PF
CI33
VOX
RI32
\ .005
GAIN
1M
Page 127