Kenwood TK-2180 Service Manual page 8

TK-2180
3-3. Wide/Narrow switching circuit
Wide and Narrow settings can be made for each channel
by switching the ceramic filters CF201 (Wide), CF200 (Nar-
row). The Wide (low level) and Narrow (high level) is output
from IC502.
When a Wide (low level) is selected, Q200 (N-ch) turn off
and Q200 (P-ch) turn on. When a Narrow (high level) is se-
lected, Q200 (N-ch) turn on and Q200 (P-ch) turn off. D201
and D203 are switched to ceramic filters when a high/low
level is selected.
Q204 turns on/off with the Wide/Narrow and the IC200
detector output level is changed to maintain a constant out-
put level during wide or narrow signals.
IC200
5R
Q200
D201
Fig. 3 Wide/Narrow switching circuit
3-4. Audio amplifier circuit
The demodulated signal from IC200 goes through the
mute switch (Q203), IC600 (1/2) and IC606, high-pass fil-
tered, low-pass filtered, high-pass filtered and de-empha-
sized by IC607.
The signal then goes through mute switch (Q605 1/2), and
an electronic volume control (IC605), and an AF switch (Q618
is on), and is routed to audio power amplifier (IC609), where it
is amplified and output to the internal speaker.
The audio mute signal (AM1) from the IC502 becomes
Low in the standby and Q612, Q613 which are power supply
circuit for IC609 turn off. Also, the audio mute signal (AM2)
becomes High to turn Q605 (1/2) off. When the audio is
output, AM1 becomes High to turn Q612, Q613 on, and volt-
age is supplied to power terminal VP of IC609. Also, AM2
becomes Low to turn Q605 (1/2) on.
The speaker is switched by the logic of speaker switching
terminal SSW on the universal connector. When SP-MIC is
not attached, the logic of SSW becomes High and SW (Q618)
is turned on, and the AF signal is input to both amplifiers of
IC609.
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CIRCUIT DESCRIPTION
Q204
R222
R224
R225
CD200 5C
AFOUT
W/N
W/N
"H" : Narrow
"L" : Wide
CF200
D203
CF201
When SP-MIC is attached, SSW is connected to GND at
inside of SP-MIC. For this reason, Q618 is turned off, and the
AF signal is input only to amplifier for EXT SP of IC609.
Change of INT/EXT SP refer to Fig. 4.
AM1 AM2 SSW
H L H
H L L
L H H
L H L
B
Q613
Q612
Q618
AF SW
AM1
Q616 Q617
(1/2)
SSW
AM2
Fig. 4 Audio amplifier circuit
3-5. Squelch circuit
The output from IC200 enters FM IC again, then passed
through a bandpass filter. The noise component output from
IC200 is amplified by Q201 and rectified by D202 to produce
a DC voltage corresponding to the noise level. The DC volt-
age is sent to the analog port of the CPU (IC502). And IC200
outputs a DC voltage (RSSI) corresponding to the input of the
IF amplifier. The CPU reads the RSSI signal via pin 125.
IC502 determines whether to output sounds from the
speaker by comparing the input voltage of pin 124 and pin
125 with the preset value.
IC200 : FM IF IC
IF AMP
DET
BPF
RSSI
AMP
7
12
Fig. 5 Squelch circuit
SQ close Preset
value
SQ open
ANT input level
Fig. 6 Squelch and RSSI voltage vs ANT input level
VC1 VC2 SP
H L INT
L H EXT
L L MUTE
L L MUTE
VP 5
INT.SP
VC1
2
EXT.SP
VC2
8
IC609
IC605
AF
SW IC607
Q605
IC502
CPU
Q201 D202
NOISE
DET 124
AMP
125
Preset
value
ANT input level