Circuit Description - Kenwood TK-290 Service Manual

1. Overview
The KENWOOD model TK-290 is a VHF/FM hand-held
transceiver designed to operate in the frequency range of
136 to 174MHz, the unit consists of a receiver, a transmit-
ter, a phase-locked loop (PLL) frequency synthesizer, power
supply circuits, a control unit.
2. Circuit Configuration by Frequency
The receiver is a double-conversion superheterodyne
with a first intermediate frequency (IF) of 44.85MHz and a
second IF of 455kHz. Incoming a signals from the antenna
are mixed with the local signal from the PLL to produce the
first IF of 44.85MHz.
This is then mixed with the 45.305MHz second local os-
cillator output to produce the 455kHz second IF. This is de-
tected to give the demodulated signal.
The transmit signal frequency is generated by the PLL
VCO, and modulated by the signal from the microphone. It
is then amplified and sent to the antenna.
ANT
TX/RX
: 136~174MHz
MCF
1st MIX
44.85MHz
ANT RF
SW AMP
XF301
180.85~218.85MHz
PA TX
AMP AMP
136~174MHz
Fig. 1 Frequency configuration

CIRCUIT DESCRIPTION

CF300,301
CF
455kHz
FM IF AF
SP
SYSTEM AMP
45.305MHz
PLL
MIC
MIC
VCO
AMP
3. Receiver System
3-1. Front-end RF amplifier
The signal are then passed through an antenna matching
coil, where the high-frequency components are amplified by
a GaAs FET (Q200). The signals are then fed into band-pass
filter that uses varactor diode tuning to reject unwanted sig-
nal components, and is fed to the 1st mixer.
3-2. First mixer
The 1st mixer uses the GaAs IC (IC200). The 1st mixer
mixes the signal with the 1st local oscillator frequency from
the VCO, and converts it to the 1st IF (44.85MHz).
The signal then passes through monolithic crystal filter
(XF301) to remove unnecessary nearby frequency compo-
nents. The signal from the MCF is used as the 1st IF signal.
3-3. IF amplifier
The 1st IF signal is amplified (Q302) and fed into IC300 in
the FM IF IC. The IF signal is then mixed with the 2nd local
oscillator frequency of 45.305MHz to generate the 2nd IF of
455kHz. The 455kHz signal is then passed through a ce-
ramic filter (CF300, CF301 ; Wide, CF302, CF303 ; Narrow)
and fed back into IC300 for additional amplification.
L201 Q200 L202 L205
BPF RF AMP BPF BPF
MCF Q302
XF301 IF AMP
X300
45.305
MHz
Fig. 2 Receiver section
Item
Nominal center frequency
Pass band width
Attenuation band width
Ripple
Insertion loss
Guaranteed attenuation
Terminating impedance
Table 1 Crystal filter XF301 (L71-0588-05)
TK-290
IC200 D602
L206
1st MIX D603
BPF
T/R
SW
IC300
FM IF SYSTEM
CF300,301
Wide
CF302,303
Narrow
Rating
44.85MHz
± 5~7kHz at 3dB
± 25kHz or less at 30dB
1.0dB or less
4dB or less
80dB or more at fo± 910kHz
40dB or more within fo± 1MHz
350Ω / 4.5pF
17