Alinco DR-135 Service Manual page 4

4. Audio Circuit
5. Squelch Circuit
6. AIR Band Reception (T only)
7. WIDE/NARROW Switching Circuit
4
The audio signal from pin 12 of IC 108 is amplified by the audio amplifier
(IC104:A, IC120), and switched by the signal switch IC (IC111) and then input
it to the de-emphasis circuit.
And is compensated to the audio frequency characteristics in the de-emphasis
circuit (R203, R207, R213, R209, C191, C218, C217) and amplified by the
AF amplifier (IC104:D). The signal is then input to volume (VR1). The adjusted
signal is sent to the audio power amplifier (IC117) through the pin 1 to drive
the speaker.
The detected output which is outputted from pin 12 of IC108 is inputted to pin
19 of IC108 after it was been amplified IC104:A, IC120 and it is outputted from
pin 20 after the noise component was been eliminated from the composed
band pass filter in the built in amplifier of the IC, then the signal is rectified by
the internal diode in IC108 to convert into DC component. The adjusted voltage
level at VR101 is delivered to the comparator of the CPU.The voltage is led to
pin 2 of CPU and compared with the setting voltage. The squelch will open if
the input voltage is lower than the setting voltage.
During open squelch, pin 30 (SQC) of the CPU becomes "L" level, AF control
signal is begin controlled and sounds is outputted from speaker.
When the frequency is within 118.000 ~ 135.995MHz, Q110 automatically
turns on, pin 14 of IC 108 becomes "H" level and the condition becomes in AM
detection mode.
The receiver signal passed through the duplexer is let to the antenna switch
(D107, D101). After passing through the band-pass filter, the signal is amplified
by RF amplifier Q112. Secondly the signal is mixed with the signal from the
first local oscillator in the first-mixer Q106, then converted into the first IF. Its
unwanted signal is let to pin 24 of IC106. Then converted into the second IF.
And is demodulated by AM decoder of IC106, and is output from pin 13 as the
AF signal.
The second IF 450kHz signal which passes through filter FL101 (wide) and
FL102 (narrow) during narrow, changes its width using the width control
switching IC103 and IC102.