Kenwood TH-D74E Service Manual page 8

BAR Antenna circuit
The bar antenna can be selected for HF reception.
The reception signal from the bar antenna passes through the tuning circuit and enters the RF amplifier (Q190). Then the signal is
received on the same path in the HF receiving circuit.
INVERTER
Q187
SWANT_S /SWANT_S
MWANT_S /MWANT_S
ANT
DUPLEXER
LPF LPF LPF
2.1.3.4 FM Tuner receiver circuit
FM Tuner receiver circuit
The reception signal from the antenna passes through the filter circuit and enters the RF amplifier (Q195). The signal amplified by the
RF amplifier is further amplified by the second RF amplifier (Q675) after the signal passes through the filter circuit, and enters the FM
Tuner IC (IC674). The RF signal is demodulated in the FM tuner IC (IC674) and converted into an audio signal.
ANT
DUPLEXER
LPF LPF
2.1.4 Transmitter System
2.1.4.1 Transmitter circuit
The VCO (B-band) VHF transmission signal that is sent from the RF amplifier (Q361) passes through the band switches (D35 and
D32). The pre-drive amplifier (IC1) and drive amplifier (Q6) amplify the signal. The signal passes through the band switches (D22 and
D26), and the final amplifier (Q2) amplifies the signal to the final output level. Then the signal passes through the antenna switches
(D13, D14, and D45), triplexer, and LPF, and is fed to the antenna.
The VCO (B-band) UHF transmission signal that is sent from the RF amplifier (Q361) passes through the band switches (D36 and
D33). The pre-drive amplifier (IC1) and drive amplifier (Q6) amplify the signal. The signal passes through the band switch (D31), and
the final amplifier (Q2) amplifies the signal to the final output level. Then the signal passes through the antenna switches (D42, D43,
D18, and D19), triplexer, and LPF, and is fed to the antenna.
2.1.4.2 APC circuit
The automatic power control (APC) circuit ensures stable TX power and is in control of TX power by detecting voltage generation from
the drain current of the final amplifier (Q2) in use.
The operational amplifier circuit (consisting of IC3 and Q12) amplifies the voltage on R55 and R57 generated by the drain current of
the final amplifier, and the generated voltage (CURR) is input into the main MPU (IC702) through the A/D converter (IC725).
The main MPU (IC702) compares the transmission CURR voltage and adjustment CURR voltage and changes the APC voltage
according to the comparison result. The main MPU (IC702) inputs the APC voltage into the gates of the drive amplifier (Q6) and
the final amplifier (Q2) through the D/A converter (IC671) and operational amplifier (IC287), thus changing the TX power level.
In the power adjustment menu, power adjustments are made by changing the APC voltage (APC digit) value. The APC loop function is
OFF in the power adjustment menu. When an appropriate APC voltage level is determined, the APC voltage and CURR voltage will
be stored as adjustment values. The adjustment values are used for APC performance while the transceiver is in user mode.
1-8 (No.RA044<Rev.001>)
BAR ANT
TUNING
D198
D194/195/202
RF SW
SWANT_S OBPFB/APC SWANT_S
BAND B 1
MWANT_S MWANT_S
BHFS
st
1 MIXER
IC283
BAND B 0.1~76M
RF AMP
D204 D197 Q190
LPF
RF SW RF SW
/SWANT_S
OAGCB
BHFS
/MWANT_S
BHFS
Fig.4 HF receiver circuit
RF AMP
D201
Q195
LPF LPF
RF SW
BFMS
Fig.5 FM Tuner receiver circuit
IC284
st
IF 58.05M
MCF
nd
BAND B 2 IF 450k
XF282
nd
PGA0 2 MIXER AGC1/2
+/-7.5kHz
57.6M
RF AMP
Q191 D199
RF SW
OAGCB BHFS
FM TUNER 76~108M
RF AMP
D672 D673/674 Q675
RF SW
/EANTS BPFR
rd
BAND B 3 IF 10.8k
rd
3 MIXER IFBUF(LPF)
ATT IC285 IC286
(To CODEC IC )
IFOUT
460.8k
IFBUF
IFBPF DISCRI
(To CODEC IC)
LIMITER
NOISE RECTIFIER
NOISE AMP
IC674
FM TUNER
nd
LNA(AGC) 2 MIXER ADC
FM TUNER Dout
(To MAIN MPU)
IFB
SMB
AFB
SQB