Circuit Description; Receiver Circuit; Front-End Circuit; First Mixer - 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 146 to 174MHz
(K,E) or 136 to 150MHz (K2).
The receiver circuit located in TX-RX unit (X57-695 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.
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
IC619 (A/2)
IC608 IC619 (B/2)
CODEC
AINR AOUTR
Fig. 1 Receiver 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) or 136 to 150MHz (K2), with a
passband of 3.0MHz.
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 local
oscillator signal from the PLL frequency synthesizer circuit at
the first mixer DBM (A1) to become a 44.85MHz first inter-
mediate frequency (IF) signal. The first IF signal is fed
through two monolithic crystal filters (XF2; Wide, XF1; Nar-
row) to further remove spurious signals.
44.85MHz
1st MIX BPF
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
11