KENWOOD TK390
Service Manual

TK-390
CIRCUIT DESCRIPTION
4. Transmitter System
4-1. Microphone amplifier
The signal from IC3 (control unit) goes through the mute switch (Q403). When the SP-MIC is not attached, the microphone switching terminal (MSW) on the universal connector becomes High, and mute switch (Q403) is turned on. When the SP-MIC is attached, MSW is connected to GND at inside of SP-MIC. For this reason, Q403 is turned off, the internal microphone is muted, and only the input of the external microphone is supplied to the microphone amplifier of the TXRX unit. The signal from microphone passes through the limiter circuit in D601, and through the high-pass filter, the ALC circuit, the low-pass filter, the high-pass filter, and pre-emphasis/IDC circuit in IC607. When encoding DTMF, mute switch (Q601) is turned off for muting the microphone input signal. The signal passes through the D/A converter (IC603) for the maximum deviation adjustment, and enters the summing amplifier consisting of IC605 (1/2), and is mixed with the low speed data from the CPU (IC406). The output signal from the summing amplifier goes to the VCO modulation input. The other output signal from the summing amplifier passes through the D/A converter (IC603) again for the BAL adjustment, and the buffer amplifier (IC604 1/2), and goes to the VCXO modulation input.

4-3. Drive and Final amplifier
The signal from the T/R switch (D7 is on) is amplified by the pre-drive (Q6) and drive amplifier (Q8) to 20mW. The output of the drive amplifier is amplified by the RF power amplifier (IC501 or IC801) to 4W (1W when the power is low). The RF power amplifier consists of two stages MOS FET transistor. The output of the RF power amplifier is then passed through the Transmit-Receive (TX-RX) antenna switching (D10 is on) and the harmonic filter (LPF) and the Internal-External (INT-EXT) antennal switching (in the universal connector) and applied to the antenna terminal.
In the ANT universal connector EXT-INT ANT SW ON�EXT

From T/R SW (D7) +B

Q6 PreDRIVE AMP R35 R37 R39

IC501 or IC801 D10 RF TX-RX DRIVE POWER ANT AMP AMP SW Q9 VDD VGG SW

Q8

LPF

Q7 SW IC7 (1/2) IC7 (2/2)

Q13 REF VOL (IC3) SW

Fig. 7 Drive and final amplifier and APC circuits

4-4. Internal-External (INT-EXT) antenna switching 4-2. Noise cancelling microphone circuit
The two signals from INT MIC (Main & Sub) are input to the positive (+) input (Sub) and to the negative (�) input (Main) of the IC3. If the same signal is input to both Main and Sub, the Main signal is canceled at the output of IC3 (pin 7). In other words, noise from nearby sources not directly connected to the transceiver enters the Main and Sub inputs at the same signal and is therefore canceled out. When a signal is only input to Main and there is no signal at Sub, the Main signal is output as is, from IC3 (pin 7). In other words, only the voice audio of the operator in extremely close proximity to the Main MIC is input to Main so that the signal is output as is from IC3 (pin 7). Also, when the �N/C� switch is set to �L�, transistor Q14 turns off so Sub microphone turns off and operation is the same as above.
MM Q601 SW IC3 IC607 MAIN MIC SUB (CONT) Q404 IC603 D/A I4 O4 IC605 (1/2) SUM AMP LSD IC603 D/A I3 O3 IC10 IC604 (1/2) BUFF AMP VCO X1 VCXO N/C AMP MIC Q403 SW D601 LIMIT 12 HPF ALC COMP LPF HPF PRE EMP IDC 6 15 16 18 19 9 8

The INT-EXT antenna switch housed inside the universal connector only switches to the EXT ANT side when an antenna speaker-microphone has been installed. This INT-EXT antenna switch works mechanically and switches based on the operation shown in Figure 8.
EXT ANT

From LPF RF

OFF

Switch OFF � INT Internal ANT antenna

EXT ANT Switch ON � INT External ANT antenna

From LPF RF

ON

TONE

EXT. MIC

Fig. 8 Internal-External antenna switching

MSW

Fig. 6 Microphoen circuit 19