Kenwood TK-385 Service Manual page 7

3-3. Audio amplifier circuit
The demodulated signal from IC12 goes through the mute
switch (Q15) and is amplified by IC4 (2/2), high-pass filtered,
low-pass filtered, high-pass filtered, band-eliminate filtered, and
de-emphasized by IC13.
The signal then goes through an AF amplifier IC7 (2/2), an
electronic volume control (IC8), and an AF switch (Q310 is
on), and is routed to audio power amplifier (IC300), where it is
amplified and output to the internal speaker.
The audio mute signal (AM) from the shift register becomes
Low in the standby and Q304, Q305 which are power supply
circuit for IC300 turn off. Also, IC13 is set to the power down
mode according to data from microprocessor, and the AF signal
is muted. When the audio is output, AM becomes High to turn
Q304, Q305 ON, and voltage is supplied to power terminal VP
of IC300. Also, IC13 is canceled out of the power down mode.
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 (Q310)
is turned ON, and the AF signal is input to both amplifiers of
IC300.
When SP-MIC is attached, SSW is connected to GND at
inside of SP-MIC. For this reason, Q310 is turned OFF, and
the AF signal is input only to amplifier for EXT SP of IC300.
Change of INT/EXT SP refer to Fig. 3.
AM SSW VC1
H H H
H L L
L H L
L L L
SB
Q305
Q304
AF
SW
AM
Q308
Q301
SSW
Fig. 3 Audio amplifier circuit
CIRCUIT DESCRIPTION
VC2 SP
L INT
H EXT
L MUTE
L MUTE
VP 5
INT.SP
VC1
2
EXT.SP
VC2
8
IC300
3-5. Squelch circuit
The output from IC12 enters FM IC again, then passed
through a band-pass filter. The noise component output from
IC12 is amplified by Q4 and rectified by D4 to produce a DC
voltage corresponding to the noise level. The DC voltage is
sent to the analog port of the CPU (IC19). And IC12 outputs a
DC voltage (RSSI) corresponding to the input of the IF amplifier.
The CPU reads the RSSI signal via pin 93.
IC19 determines whether to output sounds from the speaker
by comparing the input voltage of pin 91 and pin 93 with the
preset value.
IC12 : FM IF IC
IF AMP
DET
Q4
RSSI BPF NOISE
AMP AMP
7
12
Fig. 4 Squelch circuit
1
SQ close 5
SQ open 9
ANT input level
Fig. 5 Squelch and RSSI voltage vs ANT input level
4. Transmitter System
4-1. Microphone amplifier
The signal from the internal microphone goes through the
mute switch (Q300).
When the SP-MIC is not attached, the microphone switching
terminal (MSW) on the universal connector becomes High, and
mute switch (Q300) is turned ON. When the SP-MIC is
attached, MSW is connected to GND at inside of SP-MIC. For
this reason, Q300 is turned OFF, the internal microphone is
muted, and only the input of the external microphone is supplied
to the microphone amplifier of the TX-RX unit.
The signal from microphone passes through the limitter
circuit in D8, Mic mute switch (Q17 is off in TX) and through
the low-pass filter (IC25 : 1/2), the high-pass filter, the ALC
circuit, the low-pass filter, the high-pass filter, and pre-
emphasis/IDC circuit in IC13. When encoding DTMF, mute
switch (Q13) is turned OFF for muting the microphone input
signal.
The signal passes through the D/A converter (IC8) for the
maximum deviation adjustment, and enters the summing
amplifier consisting of IC7 (1/2), and is mixed with the low speed
data from the CPU (IC19) and 9600bps DATA from Optional
Board Terminal.
TK-385
91
IC19
CPU
D4
DET
93
Preset value
ANT input level
7