Circuit Description - Kenwood TKR-750 Service Manual

1. Outline
The TKR-750 is a VHF/FM repeater designed to operate
in the frequency range of 136 to 174MHz.
The unit consists of receiver, transmitter, phase-locked
loop (PLL) frequency synthesizer, and control circuits.
2. Receiver Circuit
The receiver is double conversion super-heterodyne, de-
signed to operate in the frequency range of 146MHz to
174MHz (K,E) or 136MHz to 150MHz (K2).
The receiver circuit located in TX-RX unit (X57-626 A/2)
consists of the following : 2-1 front-end circuit, 2-2 first
mixer, 2-3 IF amplifier circuit, 2-4 audio amplifier circuit, and
2-5 squelch circuit.
2-1. Front-end Circuit
The front-end circuit consists of BPF L2/L3, RF amplifier
Q1, and BPF L5/L6/L7. The helical BPF covers frequency
ranges 146 to 174MHz (K,E) and 136 to 150MHz (K2), with a
passband of 3.0MHz.
TX-RX unit (X57 A/2): TX-RX
IF AMP
Q3
RSSI
Q4
SW
DET W/N
NOISE DET
D5
SQL
DET
TX-RX unit (X57 B/2): Control
DEI DEO
RD
IC604
ASQ
CPU
RSSI

CIRCUIT DESCRIPTION

MCF (Wide)
XF2
IF AMP
D2 Q2
D1
MCF (Narrow)
SW
SW
XF1
W/N W/N
44.850MHz
FM SYSTEM IC
IC9
QUD
CF1 (Wide)
SW SW
D4
CF2 (Narrow)
NOISE AMP
455kHz
Q7
W/N
AMP AMP
IC614
IC608 IC619 (B/2) IC619 (A/2)
CODEC
AINR
AOUTR
Fig. 1 Receiver circuit
The BPF L5/L6/L7 attenuates the unwanted signals, and
sends only the necessary signal to the first mixer DBM A1.
2-2. First Mixer
The signal from the BPF is heterodyned with the first lo-
cal oscillator signal from the PLL frequency synthesizer cir-
cuit at the first mixer DBM (A1) to become a 44.85MHz first
intermediate frequency (IF) signal. The first IF signal is fed
through two monolithic crystal filters (XF2; Wide, XF1; Nar-
row) to further remove spurious signals.
2-3. IF Amplifier
The first IF signal is amplified by Q2 and Q3, and then
enters IC9 (FM system IC). The signal is heterodyned again
with a second local oscillator signal (44.395MHz) with in IC9
to become a 455kHz second IF signal. The second IF signal
is fed through a 455kHz ceramic filter, CF1 (Wide), CF2 (Nar-
row) to further eliminate unwanted signal, and the quadra-
ture detection circuit FM-detects the signal to produce a
base-band signal and output it from pin 11.
44.85MHz
BPF
1st MIX
L5,L6,L7
DBM A1
IF AMP
LPF
Q9
X1
44.395MHz
D3
LPF
RXI RXO IC615
IC622
DAC
RA
IC621
AM
SPM
TKR-750
RX ANT
BPF
RF AMP
L2,L3
Q1
RX VCO
A3 (X58-478)
IF AMP
Q8
VCO A
VCO B
f +44.85MHz
RX
IC629
Q606
AF
SW
MUTE
AMP
INV
Q605
INT SP
17