Circuit Description - Kenwood TK-780 Service Manual

TK-780
Frequency Configuration
The TX-RX unit (A/2) incorporates a VCO, based on a frac-
tional N type PLL synthesizer system, that allows a channel
step of 5, 6, and 25kHz to be selected. The incoming signal
from the antenna is mixed with a first local oscillation fre-
quency to produce a first intermediate frequency of
44.85MHz.
The signal is then mixed with a second local oscillation
frequency of 44.395MHz to produce a second intermediate
frequency of 455kHz. This is called a double-conversion
system. The TX-RX unit (A/2) contains a wide/narrow MCF
and CFs. The transmit signal is produced by the PLL circuit
for direction oscillation and division. The signal output from
the VCO is amplified by a straight amplifier and transmitted.
146~174MHz
1st
MIX
ANT RX
–
SW AMP
190.85~
218.85MHz
POWER TX
AMP AMP
146~174MHz
Fig. 1 Frequency configuration
Receiver System
Outline
The incoming signal from the antenna passes through a
low-pass filter and a transmission/reception selection diode
switch (D211) and goes to the front end of the receiver. The
front-end filter is a variable BPF consisting of three coils and
three varicap diodes (D206, D207, D208) to eliminate un-
wanted out-of-band signal components. The low-noise am-
plifier (LNA) (Q202) uses a bipolar transistor to achieve wide-
band and low-distortion amplification.
The signal passes through the BPF and is down-con-
verted with the first local signal by IC202 to produce the first
IF signal of 44.85 MHz. The first local signal passes through
26

CIRCUIT DESCRIPTION

CF 455kHz
(Narrow)
CF 455kHz
(Wide)
MCF
44.85MHz
MIX/IF/DET
+
44.395MHz
PLL/VCO
VCXO
16.8MHz
ANT
Q202
LPF D211,212 HPF
AMP
ANT
SW
Fig. 2 Receiver system
an LPF and an attenuator to eliminate unwanted harmonics
components and implement the optimum input level to the
mixer, then enters IC202. A DBM is used as a mixer to
achieve a high potential.
The signal output from the mixer passes through two
MCFs (XF1). The signal is amplified by an intermediate fre-
quency amplifier and input to the FM IF IC (IC11).
The first intermediate frequency signal is mixed with the
second local signal of 44.395MHz to produce the second IF
signal of 455kHz.
The unwanted near-by signal components are then elimi-
nated by a wide ceramic filter (CF1) or a narrow ceramic fil-
ter (CF2) and the resulting signal goes back to the FM IF IC.
The signal is quadrature-detected in the IC to produce an
audio signal, which is amplified by a DET amplifier (IC2) and
output to the TX-RX unit (B/2).
Wide/Narrow Changeover Circuit
The W/N port (pin 11) of the shift register (IC7) is used to
switch between ceramic filters. When the W/N port is high,
Q24 turns on and the ceramic filter SW diode (D22, D23)
CF1 turns on to receive a Wide signal. At the same time,
AFO
Q16 turns on and one of the filters is selected so that the
wide and narrow audio output levels are equal.
When the W/N port is low, Q23 turns on and the ceramic
filter SW diode (D22, D23) CF2 turns on to receive a Narrow
MD
signal.
IC11
MB
IF system
C107 Q32 R121
DET
+
OUT
+
DM
Q16
Fig. 3 Wide/Narrow changeover circuit
L202~204 IC202 XF1 Q15
BPF MIX BPF AMP
–
+
X2
1st local 2nd
OSC (HT) local OSC
IFI
C118
MXO
CF1
(Wide)
AFO
CF2
(Narrow)
D22 D23
W/N
IC7 11pin
Wide : H
Narrow : L
Wide : L
Narrow : H
IC2
AMP
IC11
MIX,IF,DET
DEO
CF1/
CF2
5C
Q23
Q24