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TK-760G/762G
■ Encode
The QT and DQT signals are output from TO of the CPU
(IC502) and summed with the external pin DI line by the
summing amplifier (IC2) and the resulting signal goes to the
D/A converter (IC6) of the TX-RX unit. The DTMF and 2-
TONE signals are output from DTMF of the CPU and goes to
the audio processor (IC508). The signal is summed with a
MIC signal by the audio processor (IC508), and the resulting
signal passes through an analog switch (IC509) and goes to
the TX-RX unit (MO).
The D/A converter (IC6) adjusts the MO level and the bal-
ance between the MO and TO levels. Part of a TO signal is
summed with MO and the resulting signal goes to the MD
pin of the VCO. This signal is applied to a varicap diode in
the VCO for direct FM modulation.
■ D/A Converter
The D/A converter (IC6) is used to adjust TONE and MO
modulation, AF volume, TV voltage, FC reference voltage,
and PC POWER CONTROL voltage level.
Adjustment values are sent from the CPU as serial data.
The D/A converter has a resolution of 256 and the following
relationship is valid:
D/A output = (Vin – VDAref) / 256 x n + VDAref
Vin: Analog input
VDAref: D/A reference voltage
n: Serial data value from the microprocessor (CPU)
Power Supply Circuit
When the POWER switch on the control unit is pressed,
the PSW signal goes low. This signal is inverted by Q26 and
sent to a flip-flop IC (IC14). This IC outputs a control signal
when the PSW goes low. When the power turns on, pin 1
of IC14 outputs a low signal and Q20 turns on. The base of
Q19 goes high, Q19 turns on, SB SW (Q23) turns on and
power (SB) is supplied to the set.
This circuit has an overvoltage protection circuit. If a DC
voltage of 20 V or higher is applied to the power cable, D21
turns on and a voltage is applied to the base of Q21. This
voltage turns Q21 on and turns Q19 and SBSW off.
■ Decode
• QT/DQT/DTMF
The signal (DEO) detected by the TX-RX unit passes
through two low-pass filters of IC513, goes to TOI of the
CPU (IC502) to decode QT, DQT. The DTMF signal is de-
coded by a dedicated IC (IC511) and the resulting signal is
sent to the CPU (IC502) as serial data.
• 2-tone
The detected signal passes through audio processor
(IC508) RX OUT. Then it is filtered through IC504 2-stage
low-pass filtered to enter CPU (IC502) to decode 2-tone sig-
nalling.
CIRCUIT DESCRIPTION
Fig. 14 Encode
Fig. 15 Decode
X1
VCXO
IC6
D/A
A1
VCO
IC1
SUM
AMP
IC509
Analog
SW
IC508
Audio
processor
IC3
PLL
MB
MD
MO
TO
HT
DI
MIC
TX-RX UNIT
TO
DTMF
IC502
CPU
IC502
CPU
IC513(2/2)
LPF
IC513(1/2)
LPF
TOI
IC511
DTMF
DECO.
IC508
Audio
processor
STD
DEO
IC514(2/2)
LPF
IC514(1/2)
LPF
2TN
RXOUT
Q23
SW
Q19
SW
Q20
SW
Q26
INV.
IC14
F. F.
Q21
SW
D21
SW
R135
R134
IGN
SB
+B
PSW
Fig. 16 Power supply circuit