Transmitter System - Alinco DJ-175 Service Manual

4. Audio Circuit
The audio signal from pin 9 of IC103 is amplified by the AF amplifier (Q120).
The signal is then input to pin 16 of the electronic volume (IC105) for volume
adjustment, and output from pin 14. The adjusted signal is sent to the audio
power amplifier (IC104) through pin 2 to drive the speaker.
5. Squelch Circuit
The signal except for the noise component in AF signal of IC103 is cut by the
active filter inside IC. The noise component is amplified and rectified, then
converted to the DC voltage to output from pin 14 of IC103. 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.

2) Transmitter System

1. Modulator Circuit
The audio signal is converted to an electric signal in either the internal or
external microphone, and input to the microphone amplifier (IC110). IC110
consists of two operational amplifiers; one amplifier (pins 12, 13, and 14) is
composed of pre-emphasis and IDC circuits and the other (pins 5, 6, 7, 8, 9,
and 10) is composed of a splatter filter. The maximum frequency deviation is
obtained by IC105 and input to the cathode of the varicap of the VCO, to
change the electric capacity in the oscillation circuit. This produces the
frequency modulation.
2. Power Amplifier Circuit
The transmitted signal is oscillated by the VCO, amplified by the pre-drive
amplifier (Q101) and drive amplifier (Q106), and input to the final amplifier
(Q105). The signal is then amplified by the final amplifier (Q105) and led to the
antenna switch (D101) and low-pass filter (L104, L102, L101, C119, C118,
C117, C127, C126, C125, C124, C108, and C101), where un-wanted high
harmonic waves are reduced as needed, and the resulting signal is supplied to
the antenna.
3. APC Circuit
Part of the transmission power from the low-pass filter is detected by D108,
converted to DC, and then amplified by a amplifier (IC101). The output voltage
controls the bias voltage from the source of Q105 and Q106 to maintain the
transmission power constant.