Kenwood TH-K20A, Service Manual

Page: 5 / 40

TH-K20A/K20E
5
3. Transmitter System
3-1. Microphone Amplifi er Circuit
The signal from microphone amplifi ed by IC301 and goes
through mute switch (IC300).
IC304 is composed of high-pass fi lter, low-pass fi lter and
pre-emphasis/IDC circuit.
The output signal from the DAC IC (IC300) goes to the
VCO modulation input.
Fig. 3 Microphone amplifi er circuit
3-2. Driver and Final Amplifi er Circuit
The signal from the T/R switch (D8 is on) is amplifi ed by
the pre-drive amplifi er (Q101) to 30mW.
The output of the pre-drive amplifi er is amplifi ed by the
drive amplifi er (Q102) and the RF fi nal amplifi er (Q103) to
5.5W (1W when the power is low).
The drive amplifi er and the RF fi nal amplifi er consist of
two MOS FET stages.
The output of the RF final amplifier is then passed
through the harmonic fi lter (LPF) and antenna switch (D101
and D102) and is applied to the antenna terminal.
CIRCUIT DESCRIPTION
X1
TCXO
VCO
LSDO
IC300 MIC
DAC
AMP/LIM
IC304
MIC
IC301
MOD
AMP
MIC
Q3,D4
ANT
FINAL AMP
ANT
SW
From
T/R SW
(D8)
D101,102
+B
POWC
Q102
DRIVE AMP
Q101
VDD
VG
Pre-DRIVE
Q103
R125
R127
R129
CURDET
Fig. 4 Drive and fi nal amplifi er circuit
2-3. IF Amplifi er Circuit
The fi rst IF signal is passed through a four-pole mono-
lithic crystal fi lter (XF200) to remove the adjacent channel
signal.
The fi ltered fi rst IF signal is amplifi ed by the fi rst IF ampli-
fi er (Q202) and then applied to the lF system IC (IC200).
The IF system IC provides a second mixer, second local
oscillator, limiting amplifier, quadrature detector and RSSI
(Received Signal Strength Indicator).The second mixer
mixes the fi rst IF signal with the 38.4MHz of the second lo-
cal oscillator output (TCXO: X1) and produces the second IF
signal of 450kHz.
The second IF signal is passed through the ceramic fi lter
(CF200) to remove the adjacent channel signal. The fi ltered
second IF signal is amplified by the limiting amplifier and
demodulated by the quadrature detector with the ceramic
discriminator (CD200).The demodulated signal is routed to
the audio circuit.
2-4. Audio Amplifi er Circuit
The demodulated signal from IC200 is amplified by
IC303, IC302 and goes to AF amplifi er through IC307.
The signal then goes through a volume control (VR300),
and is routed to an audio power amplifi er (IC307) where it is
amplifi ed and output to the speaker.
To output sounds from the speaker, IC401 sends a high
signal to the SPMUT line and turns IC401 on through Q301,
Q302, Q303, Q305 and Q306.