Kenwood TK-290 Service Manual page 20

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 ex-
tremely 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 D601
MAIN
N/C
MIC SW
LIMIT
AMP
MIC
SUB
(CONT)
Q404
MSW
Fig. 6 Microphoen circuit
CIRCUIT DESCRIPTION
MM
Q601
SW
IC607 15 16 18
12
HPF ALC COMP LPF
IC603 IC605 (1/2)
D/A
SUM
I4 O4 AMP
IC603
LSD
D/A
I3
4-3. Drive and Final amplifier
The signal from the T/R switch (D602 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 (IC801) to 5W (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 Q8
From Pre-
DRIVE
DRIVE
T/R SW
AMP
AMP
(D602)
R35
VDD
+B
R37
R39
Q13
REF
VOL SW
(IC3)
Fig. 7 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 8.
OFF
From
LPF
RF
ON
From
LPF
RF
Fig. 8 Internal-External
TONE
19 9 8
6
PRE
HPF IDC
EMP
IC10
VCO
IC604
(1/2)
X1
BUFF
VCXO
O3 AMP
TK-290
In the
universal
IC801 D10 connector
RF TX-RX EXT-INT
LPF
POWER ANT ANT
AMP SW
Q9
ON→EXT
VGG
SW
Q7
SW
IC7 IC7
(1/2) (2/2)
EXT ANT
Switch OFF
↓
INT
Internal
ANT
antenna
EXT ANT
Switch ON
↓
INT
External
ANT
antenna
antenna switching
ANT
SW
19