Alinco DJ-G1T Service Manual page 4

4. Audio Circuit
5. Squelch Circuit
The 455kHz second IF signal is output from Pin3 of IC301, and unwanted
frequency band of the second IF signal is eliminated by a ceramic filter
(FL301). The resulting signal is led to the Pin5 and 7 of IC301.
Demodulator of FM
The second IF signal of Pin7 of IC301 is led to the internal limiter amplifier,
quadrature detection circuit. The audio signal is output from Pin 12 of IC301.
Demodulator of AM
The second IF signal of Pin5 of IC301 is led to the internal AM amplifier, AM
detection circuit. The audio signal is output from Pin 13 of IC301.
The AGC circuit is used to obtain a stable audio output level even if the input
level is changed. The forward AGC signal is output from Pin 18 of IC301,
and input to the AGC amplifier (Q326). The gain of the first IF amplifier is
controlled by the AGC signal.
FM Mode
The audio signal from Pin12 of IC301, which is pre-emphasized on transmit­
ting, is led to the emphasis circuit (R334 and C338) and compensated for
the audio frequency characteristics. The audio signal is amplified by AF
amplifier (Q306), led to the audio H.P.F. (Q310, C346, C347 and R346).
The audio signal is input to the volume (VR311), and input to the power
amplifier Pin3 of IC305, and output from Pin6 to drive the speaker.
AM Mode
When you select the AM mode, switching transistors Q324, Q308 and Q309
become ON, and then Pin14 of IC301 goes to Low (AM detection circuit is
active), Q307 is active and Q306 is not active. The audio signal from Pin13
of IC301 is amplified by AF preamplifier (Q307), led to the audio H.P.F.
(Q310, C346, C347 and R346), as well as in the FM mode, then input to the
volume (VR311) and the power amplifier to drive the speaker.
Note
The IC301 (TK10930) of the FM detection circuit is active even in the AM
mode, FM detector circuit is active. Q306 connects the base of the FM
audio preamplifier Q309 to the ground so that the FM audio signal by the FM
detection is cut.
The noise in the audio signal from Pin 12 of detection IC, IC301 is passed
through the squelch control variable resistor (VR311) and input to the noise
filter amplifier consisting of C323, C322, C325, R317 and R316, internal
noise amplifier IC301. The desired noise of the audio signal is output from
Pin20 of IC301 and so amplified by noise amplifier (Q305). The amplified
noise signal is rectified by D302 and then input to Pin21 of IC301. When the
voltage of Pin21 is above 0.7V, Pin22 of IC301 goes to "low" (squelch circuit
is active). When the voltage of Pin21 is below 0.7V, Pin22 of IC301 goes to