If Unit; Audio Amplifier Unit; Squelch Circuit - Kenwood TK-768 Service Manual

TK.76B|H
2-2. lF unit
The first lF signal then passes through a four-pole
monolithic crystal filter (XF1). The signal is amplified by
first lF amplifier O16 and goes to the second lF unit.
The second lF unit consists of an lF system lC (lC7)
and the second mixer, second local oscillator, second
lF filter, and FM detector. lC7 mixes the signal input to
it with the 44.595MH2 second local oscillator output of
the crystal oscillator (X2) to produce the second lF of
455kHz.
The 455kHz signal then goes through 455kHz ce-
ramic filter CF201is amplified by the limiting amplifier,
demodulated by the quadrature FM detector (in the
same lC), and output to the receive audio amplifier.
2-3. Audio amplifier unit
The demodulated signal is amplified by lC2 (212t',
and goes through a low-pass filter consisting of 1C403
(1121, a high-pass filter consisting of 1C403 (212l,, and a
BEF consisting of 1C405 (l2l to remove the unwanted
audio signal.
The signalthe passes through the de-emphasis cir-
cuit consisting of the AF switch (0406 on) and 1C405
(2/21, and the volume level is adjusted by the ICO D/A
converter. The resulting signalgoes to audio amplifier
lC1 1, is amplified, and is output to the speaker.
2-4. Squelch circuit
The detector output is amplified by lC2 (2121 and
passes through a high-pass filter consisting of 1C401 (2/
2), which filters the noise components from the signal.
0420 converts the noise pulse level by hysteresis and
applies it to the CPU (1C409).
The CPU counts the pulses, integrates them, and
turns the squelch on and off according to the calculated
value.
VOL
FlS.2 Receiving system
2-2. +ffi+78
ffi-+ffiÈ+iÉi$-+ 4 tesffi,í+ffitÉ{* ()G1 ),
lffilÊtr Ê!'íÈ€ hffi-+ffiffi1{Ë (Qt6 ) ilt,
f*trriËÀ
ffi=+ffi+n"
ffi =+ W+ttffi&lL,ft + ffi^^,Rfr Bffi ( I c
7) , @ffiÊH-tflffi#, H:+ffi, H:+tnlRW{#fE
sffi{* (ffiiE) , &Tc7+, ffii^Éoffi-+ffi,Íà€5ffi
lfrffiffi# (x2) ffiHÊí 44.5e5MHz
ÊlH:AffiíË€fE
m, f +.455KH2
tuïffi -+ ffil;àE "
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(cF201) ffii&E, &pE0Eilt#ÈÍt, f trË^êÊ
+f'+&W#Êtsffi*F ití1ffi1fl ( frlFJ -+*frÊfiS
F ), ffiiEtrÊ!ffi SíË'€iËít&'ltÈffiÈIt*F.
2-3. Ëffiffi*+7E
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FÉ u
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tÊ, ffihrLffwÈffir-"+"
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ffttrÊ!ffiH tra?tË)tthË#"
2-4. ffiErgffi
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iÍ-+É rc401( a2) tqffiffiHiÉlÊw+Ë, Á,íà€+€E
fi 4ËÈFr|, IBÈFàBË'íàëÉ Q420 È&ÈBËftïl4€
+, gÊwÊFLËjËít cpu(rc40e) (ffi4b4{* ) .
cPU xf ËÏ Wi.t W* ttT; B,iïËH, CPU ffi ffi i.tH Ë
trfr (4*tïJ ) #4.
tc7
MIX,DET,IF
tc405Ql2l
l c 1 1
AF PA
GIRGU
IT DESCRI PTI ON/ Êffii,I, HË
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a
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xF1 016
MCF 1st lF
L208
D20,24 BPF
0208
1205-207
Q207
RF AMP BPF 1st MIX
lst LOCAL OSC (PLL)
tc103l2l2l tc405 (1/2)
HPF BEF
0406
rc2w21 1C403|.U2',,
AF AMP
LPF
38
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