Kenwood TK-782 Service Manual page 37

TK-782
" 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, 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
signal.
"
IC11
IF system
MXO
C107 Q32
R121
DET
AFO
OUT +
+
DM
Q16
W/N
IC7 11pin
Wide : H
Narrow : L
Fig. 3 Wide/Narrow changeover circuit
" AF Signal System
The detection signal (DEO) from the TX-RX unit (A/2) goes
to the audio processor (IC504) of the TX-RX unit (B/2). The
signal passes through a filter in the audio processor to ad-
just the gain, and is output to IC502. IC502 sums the AF
signal and the DTMF signal and returns the resulting signal
to the TX-RX unit (A/2). The signal (AFO) sent to the TX-RX
unit (A/2) is input to the D/A converter (IC5). The AFO output
level is adjusted by the D/A converter. The signal output from
the D/A converter is added with the BEEP signal (BPO) and
the resulting signal is input to the audio power amplifier (IC10).
The AF signal from IC10 switches between the internal
speaker and speaker jack (J1) output.
36
CIRCUIT DESCRIPTION /
IFI
C118
CF1
(Wide)
CF2
(Narrow)
D22 D23
Wide : L
Narrow : H
"
TX-RX UNIT (B/2) : Control section
IC504
AUDIO
DEO
PROCE.
Fig. 4 AF signal system /
5C
Q23
" Squelch Circuit
The detection output from the FM IF IC (IC11) is amplified
by IC2 and the signal (DEO) is sent to the TX-RX unit (B/2).
Q24
The signal passes through a high-pass filter and a noise am-
plifier (Q503) in the TX-RX unit (B/2) to detect noise. A volt-
age is applied to the CPU (IC511). The CPU controls squelch
according to the voltage (ASQ) level. The signal from the
RSSI pin of IC11 is monitored. The electric field strength of
the receive signal can be known before the ASQ voltage is
input to the CPU, and the scan stop speed is improved.
"
IC2
IC11 AMP
IF
SYSTEM
Fig. 5 Squelch circuit /
IC502 IC5
AFO
SUM D/A
AMP CONV .
DTMF BPO
TX-RX UNIT (B/2) : Control section
IC503 Q503
AMP NOISE AMP D509
DEO
HPF DET
RSSI
IC10 SP
AF PA
IC511
CPU