Kenwood TK-390 Service Manual page 19

TK-390
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 be-
comes 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 mi-
crophone is supplied to the microphone amplifier of the TX-
RX unit.
The signal from microphone passes through the limiter
circuit in D601, and through the high-pass filter, the ALC cir-
cuit, the low-pass filter, the high-pass filter, and pre-empha-
sis/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 sum-
ming 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.
4-2. 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 in-
puts 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
extermely 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.
EXT.
MIC
IC3
Q403
MAIN
N/C
MIC SW
AMP
MIC
SUB
(CONT)
Q404
MSW
18
CIRCUIT DESCRIPTION
IC607 15 16
D601
12
HPF ALC COMP
LIMIT
IC603 IC605 (1/2)
D/A
SUM
I4 O4 AMP
LSD
Fig. 7 Microphoen circuit
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) 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 univer-
sal connector) and applied to the antenna terminal.
Q6
From Pre-
T/R SW DRIVE
AMP
(D7)
R35
+B
R37
R39
Q13
REF
VOL SW
(IC3)
Fig. 8 Drive and final amplifier and APC circuits
4-4. Internal-External (INT-EXT) antenna switching
The INT-EXT antenna switch housed inside the universal
connector only switches to the EXT ANT side when an an-
tenna speaker-microphone has been installed.
This INT-EXT antenna switch works mechanically and
switches based on the operation shown in Figure 9.
From
LPF
From
LPF
MM
TONE
Q601
SW
18 19 9 8
6
PRE
LPF HPF IDC
EMP
IC10
VCO
IC604
IC603 (1/2)
X1
D/A
BUFF
VCXO
I3 O3 AMP
Q8 IC501 D10
RF TX-RX
DRIVE
POWER ANT LPF
AMP
AMP SW
Q9
VDD VGG
SW
IC7 IC7
(1/2) (2/2)
EXT ANT
Switch OFF
↓
OFF
INT
Internal
ANT
antenna
RF
EXT ANT
Switch ON
↓
ON
INT
External
ANT
antenna
RF
Fig. 9 Internal-External
antenna switching
In the ANT
universal
connector
EXT-INT
ANT
SW
ON→EXT
Q7
SW