Heathkit SB-401 - Page 52

HBEATHEKIT:
over a considerable range by the VOX Delay
control,
In the LSB, USB, and Tune modes, V12B is
biased through resistor R137 to ground, causing
relay RL1 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 C138,
and rectified by diode D106, 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 D105, This
negative voltage will bias D105 so that it will
not conduct, and relay RL1 will not close,
Additional audio voltage caused by talking into the
microphone will overcome the bias andallowthe
VOX circuit to close the relay,
FROM MIC
PTT
Figure 6-3
ANTI-VOX
INPUT
CI36 +) C/37
Page 127
FROM SPEECH
AMPLIFIER
V12A
|
\ 143 6DIO FROM
vox AMR SIDETONE
AMPLIFIER
TO DIODE
DI
C133 as
vox 005
GAIN
t+le132, $134
T 680
Figure 6-2
Bt
RELAY
RLI
V12B
1/3 6DIO
RELAY AMP. ©
FROM VOX (5)
AMPLIFIER ool
DIO5 ’ \
---1
\ 3f RIZ6
‘e G) 220K
RI37 iw
R147 ig RIBS _\C/34
1500
100K eH iis
B-BIAS
DI06
RIGS R146 _\C/39
‘470K IM -
RI48
@) VOX
- DELAY
Cl4i R/ag
R1la4
1M
1000 ] .