Circuit Description - Kenwood TK-280 Service Manual

TK-280
1. Overview
This transceiver is VHF/FM portable transceiver designed
to operate in the frequency range of 146 to 174MHz (E, T).
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 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 44.395MHz second local
oscillator output to produce the 455kHz second IF. This is
detected 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.
TX/RX : 146~174MHz (E,T)
ANT
1st MIX
ANT RF
SW AMP
44.85MHz
190.85~218.85MHz (E,T)
PA TX
AMP AMP
146~174MHz (
Fig. 1 Frequency configuration
3. Receiver System
3-1. RF unit
An incoming RF signal from the antenna terminal is passed
through the antenna switch (D12, D14, D22 and D401 are off)
and then the bandpass filter (L403,404,405,406). The bandpass
filter is adjusted by a variable capacitor. The input voltage to
the variable capacitor is regulated by the voltage output from
ANT
L403,404,
L40,42, D12,14, 405,406
43 D15,401 BPF
ANT
SW
2
IC13
HPF LPF
5
20

CIRCUIT DESCRIPTION

CF
455kHz
MCF
FM IF AF
SYSTEM AMP
44.395MHz
PLL
MIC
VCO
AMP
)
E,T
Q24
IC18
RF L27,29
L31,33
AMP BEF 1st MIX
BPF
1st Local
OSC (PLL)
1
HPF BEF
DE-
EXP
EMP
Fig. 2 Receiver section
the D/A converter (IC8). The signal is amplified by RF amplifier
(Q24), and passed through the bandpass filter (L31,33) and
band-eliminate filter (L27,29) to remove the spurious signal
again. The resulting signal is applied to the first mixer (IC18),
where it is mixed with the first local oscillator signal output
from the frequency synthesizer to produce the first IF
(44.85MHz). The 1st mixer uses the GaAs IC.
3-2. IF unit
The first IF signal is passed through a crystal filter (XF1) to
remove a adjacent channel signal. The filtered first IF signal is
amplified by the first IF amplifier (Q12) and then applied to the
lF system IC (IC12). 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 first IF signal with the 44.395MHz of second
local oscillator output (crystal unit X2) and produces the second
IF signal of 455kHz.
The second IF signal is passed through the ceramic filter
(CF1; Wide, Semi wide, CF2 ; Narrow) to more remove the
adjacent channel signal. The filtered second IF signal is
amplified by the limiting amplifier and demodulated by the
SP
quadrature detector with ceramic discriminator (CD1). The
demodulated signal is routed to the audio circuit.
3-3. Wide, Semi wide/Narrow changeover circuit
MIC
Narrow and Wide, Semi wide settings can be made for each
channel by switching the ceramic filters CF1 (Wide, Semi wide),
CF2 (Narrow).
The WIDE, SEMI WIDE (high level) and NARROW (low
level) data is output from IC19 (microcomputer) pin 99.
When a WIDE, SEMI WIDE (high level) data is received,
Q6 turn off and Q7 turn on. When a NARROW (low level)
data is received, Q6 turn on and Q7 turn off. D5, D7 are
switched to ceramic filters when a high/low level data is
received.
Q9 turns on/off with the Wide, Semi wide/Narrow data and
the IC12 detector output level is changed to maintain a constant
output level during wide or narrow signals.
Q12
XF1
MCF
1st IF
IC12
MIX, DET, IF
X2
2nd local
OSC
IC7 (2/2)
AF AMP IC8
MUTE
VOL
41
IC4 (2/2)
AF AMP
Q15
SW
CF1: Wide,
DM DM
Semi wide
CF2 : Narrow
IC300
AF PA
Q310
AF AF
SW
VC1
SSW
VC2
INT.
SP
EXT.
SP