Kenwood TK-863G Service Manual page 22

TK-863G
Encode
The LTR, 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 (A/2). The DTMF
signal is output from DTMF of the CPU and goes to the au-
dio processor (IC508). The signal is summed with a MIC
signal by the audio processor (IC508), and the resulting sig-
nal passes through an analog switch (IC509) and goes to the
TX-RX unit (A/2) (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.
TO
MIC
IC502
CPU
IC508
DTMF
Audio Analog
processor
Fig. 14 Encode
Decode
• QT/DQT/LTR
The signal (DEO) detected by the TX-RX unit (B/2) passes
through two low-pass filters of IC513, goes to TOI of the
CPU (IC502) to decode QT, DQT and LTR.
TOI
IC502
CPU
Fig. 15 Decode
22
CIRCUIT DESCRIPTION
TX-RX UNIT (A/2)
IC1
TO MB
X1
SUM
VCXO
AMP
DI
MD
IC6
IC509
D/A
MO
SW
IC513(2/2) IC513(1/2)
LPF
LPF
DEO
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 TX-RX unit (B/2) 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 con-
trol signal when the PSW goes low. When the power turns
IC3
on, pin 1 of IC14 outputs a low signal and Q20 turns on. The
PLL
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
A1
VCO
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
MO
voltage turns Q21 on and turns Q19 and SBSW off.
IGN
Q23
SB
SW
Q19
SW
Fig. 16 Power supply circuit
Q20 IC14 Q26
SW F.F. INV.
Q21 D21
SW SW
+B
PSW