Circuit Description - Alinco DJ-191 Service Manual

CIRCUIT DESCRIPTION

1) Receiver System
1. Front End
2. IF Circuit
3. Demodulator Circuit
4. Audio Circuit
The receiver system is a double superheterodyne system with a 21.7 MHz
first IF and a 450 kHz second IF.
The received signal at any frequency in the 130.00- to 173.995-MHz range
is passed through the low-pass filter (L102, L103, L104, C113, C107, C116,
and C114) and tuning circuit (L112 and D107), and amplified by the RF
amplifier (Q107). The signal from Q107 is then passed through the tuning
circuit (L109, L110, L111, and varicaps D104, D105 and D106) and
converted into 21.7 MHz by the mixer (Q106). The tuning circuit, which
consists of L112, L109, varicaps D107 and D104, L110, L111, varicaps
D105 and D106, is controlled by the tracking voltage from the CPU so that
it is optimized for the reception frequency. The local signal from the VCO is
passed through the buffer (Q108), and supplied to the source of the mixer
(Q106). The radio uses the lower side of the superheterodyne system.
The mixer mixes the received signal with the local signal to obtain the sum
of and difference between them. The crystal filter (XF101, XF102) selects
21.7 MHz frequency from the results and eliminates the signals of the
unwanted frequencies.
signal of the selected frequency.
After the signal is amplified by the first IF amplifier (Q105), it is input to pin
16 of the demodulator IC (IC104). The second local signal of 21.25 MHz
(shared with PLL !C reference oscillation), which is oscillated by the internal
oscillation circuit in IC102 and crystal (X101), is input through pin 1 of
IC104. Then, these two signals are mixed by the internal mixer in IC104
and the result is converted into the second IF signal with a frequency of 450
kHz. The second IF signal is output from pin 3 of IC104 to the ceramic filter
(FL101), where the unwanted frequency band of that signal is eliminated,
and the resulting signal is sent back to the IC104 through pins 5 and 7.
The second IF signal input via pin 7 is demodulated by the internal limiter
amplifier and quadrature detection circuit in IC104, and output as an audio
signal through pin 9.
The audio signal from pin 9 of IC104 is compensated to the audio
frequency characteristics in the de-emphasis circuit (R162, R161, C172,
C173) and amplified by the AF amplifier (Q109). The signal is then input to
pin 2 of the electronic volume (IC103) for volume adjustment, and output
from pin 1. The adjusted signal is sent to the audio power amplifier (IC105)
through pin 2 to drive the speaker.
The first IF amplifier (Q105) then amplifies the