Circuit Description; Receiver Circuit - Kenwood TK-690H Service Manual

1. Transmitter Circuit
1-1. Microphone amplifier section (X57 B/2)
The audio input from the microphone is attenuated by
VR501 and passes through the active high-pass filter (pre-
emphasis circuit) in IC505, the compressor circuit in IC504,
the IDC (limiter circuit) in IC505, the summing amplifier cir-
cuit in IC510, the active low-pass filter in IC510, the sum-
ming amplifier circuit in IC513, and the D/A converter in
IC512, and is output from the CN502 to the CN202. Q504 is
used as a microphone mute switch.
to TX VCO
MO
IC512
PC
DAC
IC513 IC510 IC510 IC505
SUM SUM
LPF IDC
AMP AMP
CONTROL UNIT X57 B/2
Fig. 1
1-2. Modulation section (X57 A/2)
The signal from the MO terminal of the CN202 goes to
D305 in the VCO, and the VCO is directly modulated. The
transmit signal output from the VCO passes through switch
D306, is amplified by Q203, and is output from the CN203 to
the CN1.
TX-RX UNIT X57 A/2
D306
SW
TX-VCO
MO
Fig. 2
1-4. Final amplifier section (X45)
The signal from the DO terminal of the CN1 is applied to
drive two transistors Q1 and Q5, and the output is divided
into two signals which are amplified by Q6 and Q7. The
signals are mixed and the resulting signal is fed through
transmit/receive switching diode; D2, D3, D4 and D10, low-
pass filter, and CM coupler to the antenna connector.
CM coupler is a coil for detecting traveling and reflected
waves. Traveling waves are detected by D9 and fed through
the APC control to differential amplifiers Q8, which compare
the signal level with the reference voltage of PC tuned.
The output is amplified by DC amplifier Q7 to control APC
DC amplifier Q6. Q7 controls the power supply voltage for
pre-drive amplifier Q203 on the TX-RX unit (X57-560 A/2),
keeping the transmitter output constant. To protect the
transmitter power amplifier stages, there are two protec-
tion circuits which one is against abnormal antenna loading
and the other is against overheating.

CIRCUIT DESCRIPTION

IC504 Q504
IC505
VR501
MUTE
MIC
COMP HPF
SW
Q203
DO
AMP
APC
8T
TK-690H
If an abnormal antenna load is connected, the reflected
wave level increases. Reflected waves are detected by D6,
D5 and the output level is fed to the differential amplifier Q8,
leading to the transmitter output power being reduced in the
way already described. If an abnormal high temperature is
detected by the thermistor TH1, DC SW Q2 is made to re-
duce the reference voltage of the PC tuned. This also leads
to the transmitter output power being reduced. Antenna
switching is done by four diodes; D2, D3, D4 and D10 with
8T.
FINAL UNIT (X45)
Q1 Q5 Q9
DO AMP AMP AMP
B
Q10
Q6
APC
AMP
DB
drive
Q7
DC
+B
AMP
Q8
Differential
AMP
Fig. 3

2. Receiver Circuit

The incoming received signal from the antenna socket
passes through the low-pass filter, the transmit/receive
switching diodes (D2, D3, D4 and D10), and the second low-
pass filter, then is fed to CN104 in the TX-RX unit.
The signal passes through the band-pass filter in the TX-
RX unit, is amplified by RF amplifier Q101 and Q102, then
passes through the second band-pass filter (BPF). The
tuned varactor BPF (L101, L102, L104 and L105) changes
the cut-off frequency according to the receive frequency.
After the signal has passed through the BPF, it inters RX
first mixer Q110 to Q113. The local signal (received fre-
quency + 10.7MHz) from the PLL also enters the first mixer.
The first mixer is a double-balanced type consisting of four
FETs, which converts the receive signal to the 10.7MHz first
IF signal.
The 10.7MHz IF signal passes through NB (noise blanker)
unit A102 and IF amplifier Q103, and enters NB gate Q105
and Q106. The pulse noise in the IF signal is detected by the
NB unit, and removed by the NB gate. The IF signal passes
through the monolithic crystal filter (XF101), IF amplifier
Q301, monolithic crystal filter (XF102), and enters FM IF
system IC (IC101).
The signal is heterodyned again to 455kHz using the sec-
ond local signal generated by the crystal oscillator
(11.155MHz), at the second mixer in IC101. The second IF
signal is filtered by CF101 and CF102 (ceramic filter), ampli-
fied by limiting amplifiers in IC101, and is detected as an FM
signal.
The output AF signal from IC101, then passes through
the control unit's HPF and D/A converter IC. After entering
the AF power amplifier (IC522), the signal is output from the
speaker.
(
D2,3,4,10
ANT
LPF CM coupler
SW
D5,6 D9
8T REF FWD
DET DET
Q2
VR1
DC
8T
SW
TH1
Q4
DC
PC
AMP
B )
25