Kenwood TH-K2AT Service Manual page 8

TH-K2AT/K2E/K2ET
Transmitter System
M icrophone Amplifier Circuit
The audio signal from the microphone passes through a
high-pass filter (IC5) and enters a microphone amplifier (IC16).
The signal passes through a pre-emphasis circuit, limiter
amplifier and splatter filter inside the microphone amplifier.
The splatter filter w ill remove distortion outside the audio band.
M IC IC5
6
HPF
Pre-
em phasis
Fig. 5 M icrophone amplifier circuit
M odulation Circuit
The audio signal amplified by the microphone amplifier
(IC16) passes t hrough a sem i-f ixed volum e (VR5) f or
modulation adjustment, and goes to the VCO modulation
varicap (D6) for variable reactance phase modulation.
Drive and Final Circuit
The output signal from VCO (Q1) passes through an RF
amplifier (Q2, Q5, Q9) and drive amplifier (Q10, Q11), and is
amplified by a pow er amplifier (Q12).
Transmission/ Reception Sw itching Circuit
The signal amplified by the pow er amplifier (Q12) passes
through a transmission/reception sw itching circuit and a low -
pass filter, and is fed to the antenna.
Q11
Q10
Q15
5T
B
8
CIRCUIT DESCRIPTION
IC16
M IC AM P
Lim iter
1
VCO
Splatter
filter
Q12
Q58
APC AM P
1
AOUT
2
A–IN
3
A+IN
4
GND
Q13
B
Fig. 7 APC and Temperature protection circuit
The transmission/reception sw itching circuit consists of D10,
L19, D12 and D13. These diodes are turned ON in transmit
mode and OFF in receive mode to sw itch signals.
M IC IC5 IC16
M IC
HPF
AM P
DRIVE AM P
Q10
DRIVE AM P
Fig. 6 M odulation, Drive, Final and
Transmission/ reception sw itching circuit
APC Circuit
The APC (Automatic Pow er Control) circuit is used to obtain
a stable transmission pow er and controls transmission output
by detecting the drain current of Q11 and Q12.
The transmission output can be changed in three levels:
High, M id, and Low .
The reference voltage is output from the BPF/APC terminal
(pin 6) of the M PU (IC8) and the detection voltage generated by
R70, R71 and R72 are fed to the APC differential amplifier (IC2).
The volt age in proport ion t o t he dif f erence bet w een
reference voltage and detection voltage is output from the
BOUT terminal (pin 7) of IC2 as an APC voltage.
D10
IC2
8
V+
Q14
7
BOUT
6
B–IN
5
B+IN
Q2
Q1 RF AM P
VCO
VR5
RF AM P
M AX DEV
Q11 D10,L19
D12,D13 LPF
ANT
SW
Q12
POWER AM P
ANT
LPF
RX
7
6
Q9
RF AM P
Q5
ANT
Q55
51
THERM
KAPC
BPF/APC
16
FINAL
IC8
M PU