Kenwood TK-3148 Service Manual page 12

TK-3148
3-4. Audio amplifier circuit
The demodulated signal from IC200 goes through the mute
switch (Q204) and is amplified by IC300 (2/2), high-pass filtered,
low-pass filtered, high-pass filtered, band-eliminate filtered,
and de-emphasized by IC500.
The signal then goes through an AF amplifier IC305 (2/2),
an electronic volume control (IC307), and an AF switch (Q308
is on), and is routed to audio power amplifier (IC313), 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 Q302 and Q305 which are power
supply circuit for IC313 turn off. Also, IC500 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 Q302 and Q305 ON, and voltage is supplied to
power terminal VP of IC313. Also, IC500 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 (Q308)
is turned ON, and the AF signal is input to both amplifiers of
IC313.
When SP-MIC is attached, SSW is connected to GND at
inside of SP-MIC. For this reason, Q308 is turned OFF, and
the AF signal is input only to amplifier for EXT SP of IC313.
Change of INT/EXT SP refer to Fig. 4.
AM SSW
H H
H L
L H
L L
12
CIRCUIT DESCRIPTION /
VC1 VC2 SP
H L INT
L H EXT
L L MUTE
L L MUTE
SB
AF
AM
Q305
SSW
Fig. 4 Audio amplifier circuit / 图４ 音频放大器电路
电路说明
3-4．音频放大器电路
来自于IC200的解调信号进入静音开关 （Q204） ， 并且被IC300
（2/2）放大，被 IC500 高通滤波，低通滤波，高通滤波，带阻滤
波和去加重。
然后信号通过音频放大器 IC305（2/2） ，电子音量控制器
（IC307）和音频开关（Q308 接通） ，并且发送到音频功率放大器
（IC313） ，被放大后输出到内置扬声器。
来自于位移存储器的音频静音信号（AM）待机时变为低电
平，给 IC313 供电的电路 Q302 和 Q305 断开。同时，IC500 按照
来自于微处理器的数据被设定为低功率模式，音频信号处于被
静音状态。 当输出音频时， 音频静音信号变为高电平并接通Q302
和 Q305，并向 IC313 的功率端点 VP 提供电压。同时，IC500 取
消低功率模式。
扬声器被通用接口上的扬声器开关端点 SSW 的逻辑电路开
启。 当扬声器-话筒不接上时， SSW的逻辑电路变为高电平且SW
（Q308）被接通，同时信号被输入到 IC313 的两个放大器。
当扬声器 - 话筒接上时，SSW 与扬声器 - 话筒内的地线连接。
因此，Q308 断开，音频信号只输入到 IC313 的 EXT SP 放大器。
有关 INT/EXT SP 的变化参见图 4。
AM
H
H
L
L
Q303
Q302
VP 5
SW
VC1
2
VC2
8
Q307
IC313
SSW VC1 VC2 SP
H H L INT
L L H EXT
H L L
静音
L L L
静音
INT.SP
EXT.SP