Circuit Description; Transmitter Circuit; Microphone Amplifier Section (X57 B/3); Modulation Section (X57 A/3) - Kenwood TK-790 Service Manual

TK-790/ ( B

1. Transmitter Circuit

1-1. Microphone amplifier section (X57 B/3)

The audio input from the microphone is attenuated by
VR501 and passes through the active high-pass filter (pre-
emphasis circuit) in IC505, the compressor circuit in IC504,
the IDC (limiter circuit) in IC505, the summing amplifier cir-
cuit in IC510, the active low-pass filter in IC510, the sum-
ming amplifier circuit in IC513, and the D/A converter in
IC512, and is output from the CN502 to the CN202. Q504 is
used as a microphone mute switch.
to TX VCO
MO
IC512
DAC PC
IC513 IC510 IC510 IC505
SUM SUM
LPF
AMP AMP
CONTROL UNIT X57 B/3

1-2. Modulation section (X57 A/3)

The signal from the MO terminal of the CN202 goes to
D305 in the VCO, and the VCO is directly modulated. The
transmit signal output from the VCO passes through switch
D306, is amplified by Q201 and Q204, and is output from
the CN203 to the CN1.
TX-RX UNIT X57 A/3
Q201
AMP
SW
TX-VCO
MO
1-3. Final amplifier section (X57 C/3) : TK-790/(B)
The signal from the DO terminal of the CN1 is applied to
drive the power module and the output is fed through trans-
mit/receive switching diode ; D3,D4, low-pass filter, and CM
coupler to the antenna connector.
CM coupler is a line for detecting traveling and reflected
waves. Traveling waves are detected by D5 and fed through
the APC control to differential amplifiers Q5, which compare
the signal level with the reference voltage of PC tuned.
The output is amplified by DC amplifier Q1 to control APC
DC amplifier Q2. Q1 controls the power supply voltage for
the power module, keeping the transmitter output constant.
To protect the transmitter power amplifier stages, there is
protection circuit, against abnormal antenna loading.
If an abnormal antenna load is connected the reflected
wave level increases. Reflected waves are detected by D6
and the output level is fed to the differential amplifier, lead-
ing to the transmitter output power being reduced in the
way already described.
26
) /H ( B )

CIRCUIT DESCRIPTION

IC504 IC505 Q504
VR501
MUTE
IDC COMP HPF
SW
Fig. 1
Q204
DO
AMP
APC
D306
8T
Fig. 2
FINAL UNIT (X57 C/3)
POWER
DO
MODULE
DB
Q1
APC
DRIVE
Q3
DC
AMP
MIC
1-4. Final amplifier section (X45) : TK-790H(B)
The signal from the DO terminal of the CN1 is applied to
drive the power module, and the output is divided into two
signals which are amplified by Q6 and Q7. The signals are
mixed and the resulting signal is fed through CM coupler,
transmit/receive switching relay ; K1, and the low-pass filter
to the antenna connector.
CM coupler is a line for detecting traveling and reflected
waves. Traveling waves are detected by D1, D3 and fed
through the APC control to differential amplifiers Q5, which
compare the signal level with the reference voltage of PC
tuned.
The output is amplified by DC amplifier Q4 to control APC
DC amplifier Q3. Q4 controls the power supply voltage for
the power module, keeping the transmitter output constant.
To protect the transmitter power amplifier stages, there are
two protection circuits which one is against abnormal an-
tenna loading and the other is against overheating.
If an abnormal antenna load is connected, the reflected
wave level increases. Reflected waves are detected by D2,
D3 and the output level is fed to the differential amplifier Q5,
leading to the transmitter output power being reduced in the
way already described. If an abnormal high temperature is
detected by the thermistor TH1, DC SW Q1 is made to re-
duce the reference voltage of the PC tuned. This also leads
to the transmitter output power being reduced. Antenna
switching is done by a relay K1 with 8R.
FINAL UNIT (X45)
IC1 Q6
POWER
DO AMP
MODULE
B
DB
Q7
Q3
APC
AMP
DRIVE
Q4
DC
+B
AMP
Q5
Differential
AMP
D3,4
ANT
CM
LPF
coupler
SW
B
D6
D5
FWD REF
8T
DET DET
VR1
+B
Q5
Differential
AMP
Q6
DC
AMP
Fig. 3
K1
CM
ANT
coupler
SW
D2 Q9 Q8
D1
FWD REF DC
DET DET SW
Q1
D3
VR1
Rectified
DC
Diode SW
Q2
DC
AMP
FIg. 4
LPF
PC
LPF
DC
8R
SW
8T
TH1
PC