Power Supply - Kenwood TK-373G Service Manual

4. Transmitter System
1) Microphone amplifier
The signal from the microphone passes through the limiter
circuit in D23, and through the high-pass filter, the ALC
circuit, the low-pass filter, the high-pass filter, and pre-
emphasis/IDC circuit IC14. When encoding DTMF, the mute
switch (Q35) is turned OFF for muting the microphone input
signal.
The signal passes through the D/A converter (IC17) for the
maximum deviation adjustment, and goes to the VCXO
modulation input.
IC14
15
D23
MIC
12
LIMIT HPF ALC
IC17
D/A
Fig. 7 Microphone amplifier
2) Drive and Final amplifier
The signal from the T/R switch (D5 is on) is amplified by the
pre-drive (Q5) and drive amplifier (Q6) to 50mW.
The output of the drive amplifier is amplified by the RF power
amplifier (IC1) to 4.0W (1W when the power is low). The
RF power amplifier consists of two MOS FET stages. The
output of the RF power amplifier is then passed through the
harmonic filter (LPF) and antenna switch (D3 is on) and
applied to the antenna terminal.
Q5
Q6
From
Pre-DRIVE
DRIVE
T/R SW
AMP AMP
(D5)
+B
R56
R57
R58
APC
(IC13)
Fig. 8 Drive and final amplifier and APC circuit
3) APC circuit
The APC circuit always monitors the current flowing through
the RF power amplifier (IC1) and keeps a constant current.
The voltage drop at R56, R57 and R58 is caused by the
current flowing through the RF power amplifier and this
voltage is applied to the differential amplifier IC3(1/2).
IC3(2/2) compares the output voltage of IC3(1/2) with the
CIRCUIT DESCRIPTION
MUTE
DTMF
Q35
SW
16 9 8
18 19
PRE
COMP LPF HPF IDC
EMP
VCO
X1
LPF
LPF TCXO
QT/DQT/LTR
TOVCO TOTCXO
22 20
IC13
DTMF
2
LPF
CPU
(RX Audio)
IC1
D3
RF
ANT
LPF
POWER AMP
SW
VDD VGG
IC3 IC3
(1/2) (2/2)
reference voltage from IC13, and the output of IC3(2/2)
controls the VGG of the RF power amplifier to make both
voltages the same.
The change of power high/low is carried out by the change
of the reference voltage.
4) Encode signaling
(1) QT/DQT/LTR
QT,DQT,LTR data of the TOTCXO Line is output form pin
20 of the CPU. The signal passes through a low-pass CR
filter and goes to the TCXO(X1).
The QT,DQT,LTR data of the TOVCO Line is output form
pin 22 of the CPU. The signal passes through a low pass
CR filter, mixes with the audio signal, and goes to the VCO
modulation input. TX deviation is adjusted by the CPU. (See
6
fig.7)
(2) DTMF
High-speed data is output from pin 2 of the CPU. The signal
passes through a low-pass CR filter, and provides a TX
and SP out tone, and is then applied to the audio processor
(IC14). The signal is mixed with the audio signal and goes
AF
to the VCO.
AMP
TX deviation is adjusted by the CPU. (See fig.7)
IC15
(2/2)

5. Power supply

There are five 5V power supplies for the microprocessor:
5V,5M,5C,5R, and 5T. 5V for microprocessor is always
output while the power is on. 5M is always output, but turns
off when the power is turned off to prevent malfunction of
the microprocessor.
5C is a common 5V.
5R is 5V for reception and output during reception.
5T is 5V for transmission and output during transmission.
6. Control Circuit
ANT
The control circuit consists of a microprocessor (IC13) and
its peripheral circuits. It controls the TX-RX unit and transfers
data to and from the display unit. IC13 mainly performs the
following:
(1) Switching between transmission and reception by the
PTT signal input.
(2) Reading system, group, frequency, and program data
from the memory circuit.
(3) Sending frequency program data to the PLL.
(4) Controlling squelch on/off by the DC voltage from the
squelch circuit.
(5) Controlling the audio mute circuit by the decode data
input.
(6) Transmitting tone and encode data.
TK-373G
13