KENWOOD TK373G
Service Manual

TK-373G
CIRCUIT DESCRIPTION
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 preemphasis/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.
MUTE Q35 SW IC14 MIC D23 LIMIT 12 HPF 15 ALC 16 18 19 LPF 9 HPF DTMF 8 PRE EMP IDC 6

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 fig.7) (2) DTMF

COMP

IC17 D/A LPF

VCO QT/DQT/LTR LPF

X1 TCXO

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 to the VCO. TX deviation is adjusted by the CPU. (See fig.7)

TOVCO 22

IC13 CPU

TOTCXO 20 2

DTMF

LPF AF AMP IC15 (2/2)

(RX Audio)

Fig. 7 Microphone amplifier

5. Power supply
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.
ANT Q5 From T/R SW (D5) Pre-DRIVE AMP +B R56 R57 R58 APC (IC13) IC3 (1/2) IC3 (2/2) Q6 DRIVE AMP IC1 RF POWER AMP VDD VGG D3 ANT SW LPF

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
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.

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

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