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Rx (receive) signal is louder, and then it processes the signals so that the louder signal is given
precedence.
The Voice Detector provides a DC input to the Attenuator Control corresponding to the Tx signal.
The Comparator receives a Tx and Rx signal, and supplies DC input to the Attenuator Control
corresponding to the Rx signal.
The Attenuator Control provides a control signal to the Tx and Rx Attenuator to switch the
appropriate signals ON and OFF.
The Attenuator Control also detects the level of the volume control to automatically adjust the
volume for changing ambient conditions.
1) Control Signal Path
Control signal for transmission and reception are input to IC3 via the following path:
(Transmission Control Signal Path)
MIC(CN2)
IC3 Pin 9
(Reception Control Signal Path)
Telephone Line
Q3
2) Transmission/Reception Switching
The comparison result between Tx and Rx output is a DC level at IC3 Pin 26.
Tx level is high .................... Pin 21-Pin 26=6mV
Rx level is high .................... Pin 21-Pin 26=150mV
The comparator output is connected to the attenuator control inside IC3.
3) Voice Detector
The output of the mic amp (Pin 10 of IC3) is supplied to Pin 14 of IC3 as a control signal for the
voice detector.
4) Attenuator Control
The attenuator control detects the setting of the volume control through Pin 25 of IC3 and
automatically adjusts the volume for changing ambient conditions.
5) Transmission Signal Path
The input signal from the microphone is sent through the circuit via the following path:
MIC(CN2)
C64
Pin 4
C41
R41
6) Reception Signal Path
Signals received from the telephone line are output to the speaker via the following path:
Telephone Line
D1
R42
C46
IC3 Pin 29
C63
CN3 Speaker
7) Busy Tone Detector Circuit
The busy tone detection for automatic redealing is executed as follows.
Telephone Line
D1
Pin 64
IC3 Pin 10
IC3 Pin 3
Q4
IC3 Pin 7
R206
C59
IC3 Pin 9
Q3
C4
T1
Q1
C3
T1
IC3 Pin 28
Q1
C3
T1
IC3 Pin 4
IC3 Pin 10
C3
Q1
D1
C4
Q3
R10
R57
C62
C61
C91
R91
IC6
14
IC3 Pin 5
R45
C48
IC3 Pin 3
JK1
Telephone Line
C8
R12
Q4
IC3 Pin 20
IC3 Pin 16
D3
R99
Q20
IC3
C42
IC1
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