Kenwood TK-290 Service Manual page 19

TK-290
3-4. Wide/Narrow changeover circuit
Narrow and Wide settings can be made for each channel
by switching the ceramic filters CF300, CF301 (Wide),
CF302, CF303 (Narrow).
The WIDE (high level) and NARROW (low level) data is
output from IC400 (I/O port expansion) pin 4.
When a WIDE (high level) data is received, Q303 turn off
and Q304 turn on. When a NARROW (low level) data is re-
ceived, Q303 turn on and Q304 turn off. D302, D303 are
switched to ceramic filters when a high/low level data is re-
ceived.
Q305 turns on/off with the Wide/Narrow data and the
IC300 detector output level is changed to maintain a con-
stant output level during wide or narrow signals.
5R
Q302
5R
Q303
D302
Q304
Fig. 3 Wide/Narrow changeover circuit
3-5. Audio amplifier circuit
• TX-RX unit
The demodulated signal from IC300 goes through IC301,
and is amplified by IC601 (2/2), high-pass filtered, low-pass
filtered, high-pass filtered, band-eliminate filtered, and de-
emphasized by IC607.
The signal then goes through an electronic volume con-
trol (IC603), an AF amplifier IC604 (2/2), and an AF switch
(Q8 is on and Q7 is on of the control unit), and is routed to
audio power amplifier (IC1 of the control unit), where it is
amplified and output to the internal speaker.
18
CIRCUIT DESCRIPTION
Q305
R322
W/N "H" : Wide
"L" : Narrow
IC300
FM IF SYSTEM
W/N
CF300 CF301
D303
CF302 CF303
• Control unit
The audio mute signal (AMP SW) from the microproces-
sor becomes Low in the standby and Q5, Q6 which are
power supply circuit for IC1 turn off. When the audio is out-
put, AMP SW becomes High to turn Q5, Q6 on, and voltage
is supplied to power terminal VP of IC1.
Speaker switching is done from IC403 (TX-RX unit) by INT
AFC or EXT AFC. First, the logic level at the speakers
switching terminal (SSW) on the universal connector is input
to the microprocessor (IC406 TX-RX unit). The microproces-
sor then outputs data to IC403 based on this input.
When there is no SP-MIC installed, this logic level is high.
When the INT AFC is high, the EXT AFC goes low, so the AF
signal is only input to the amplifier for the internal speaker
(INT SP) of IC1. However, when a SP-MIC has been in-
stalled, this logic level is low, so the INT AFC goes low and
the EXT AFC goes high. So that the AF signal is input only to
amplifier for the external speaker (EXT SP) of IC1.
IC601 (2/2)
AF AMP
IC301
IC300
SW
MIX, DET, IF
CF300
X300
CF301
2nd local
OSC
CF302
CF303
2 1
IC607
HPF LPF HPF BEF
5
Fig. 4 Audio amplifier circuit
3-6. Squelch circuit
It amplifies the demodulated noise signal from FM IC
(IC300) after filtering through BPF circuit. Then, the ampli-
fied signal is coverted to DC signal by the detection circuit.
The coverted signal is feeded through the microprocessor
(IC 406) and the voltage level is detected by ADC (A/D con-
verter) to control the squelch circuit.
AF
OUT
9 8 IC300
Q301
7
NOISE NOISE
AMP AMP
DET
OUT
Fig. 5 Squelch circuit
SB
Q6
Q5
AMP SW
IC1
5
Q8 Q7
13
AF
4 16
SW SW
2
INT AFC
Q15
9
6 12
SW
8
EXT AFC
IC604 (2/2)
IC603 AF AMP
DE-
EXP MUTE VOL
EMP
41
IC406 IC403
D300
DET
OUTPUT
BUSY
42
EXPANDER
CPU
INT.SP
EXT.SP
AF
AMP SW
INT AFC
EXT AFC