Kenwood TH-28A Service Manual page 4

TH-28A/E
CIRCUIT DESCRIPTION
* S-merer circuit
The S-meter signal is output from pin 13 of IC1 as a
direct current corresponding to the input signal, con-
verted to a voltage by R63, then input to pin 3 of the
microprocesser. The DC voltage is digitized to control
the LCD S-meter display.
SM —
Microprocessor
ay -
oO oO
Fig. 4 S-meter circuit
Transmitter System
¢ Microphone amplifier
The signal from the microphone is passed through a
6dB/oct pre-emphasis circuit consisting of C79 and
R91, 92 to amplifier 1C7 (1/2), then limited. Distortion
components exceeding the audio band of the resulting
signal are then eliminated by a splatter filter consisting
of 1C7 (2/2).
* Modulator circuit
The output from the microphone amplifier is passed
through variable resistor VR8 for modulation adjust-
ment to varicap diode D3 of the VCO, controlling the
VCO frequency and so producing a frequency-modu-
lated RF output.
« Drive and final circuits
The modulated RF signal from the VCO is amplified
to about -5dBm by a buffer amplifer. The signal is then
amplified to about 15dBm by the drive. The amplified
signal is input through pin diode D208 for transmission
output adjustment to power module IC202. The power
the signal to about 5W for output.
° Transmmission/ reception selector.circuit
The. transmission output is passed through the
transmission/reception selector circuit and low-pass
filter to the antenna. .
The transmission/reception selector circuit, which
consists of D209 and D210, is turned on during trans-
mission and off during reception to switch the signal.
Fig. 5
Microphone amplifier