Circuit Description - Alinco DR-235 TMk III Service Manual

CIRCUIT DESCRIPTION

1 ) R e c e i v e r S y s t e m D R - 2 3 5
The receiver system is a double superheterodyne system with a 30.85 MHz first IF and a 455 kHz
second IF.
1. Front End
2. IF Circuit
3. Demodulation Circuit
4. Audio Circuit
The received signal at any frequency in the 216.000MHz to 279.995MHz
range is passed through the low-pass filter (L116, L115, L114, L113, C204,
C203, C202, C216 and C215) and tuning circuit (L105, L104 and D105,
D104), and amplified by the RF amplifier (Q107). The signal from Q107 is
then passed through the tuning circuit (LI 03, L107, L102, and varicaps
D103, D107 and D102) and converted into 30.85 MHz by the mixer
(Q106). The tuning circuit, which consists of L105, L104, varicaps D105
and D104, L103, L107, L102, varicaps D103, D107 and D102, is
controlled by the tracking voltage form the VCO. The local signal from the
VCO is passed through the buffer (Q145), 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 (XF101A, XF101B)
selects 30.85 MHz frequency from the results and eliminates the signals
of the unwanted frequencies. The first IF amplifier (Q105) then amplifies
the 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 (IC108). The second local signal of 30.395 M H z,
which is oscillated by the internal oscillation circuit in 1C108 and crystal
(X104), is input through pin 1 of 1C108. Then, these two signals are mixed
by the internal mixer in IC108 and the result is converted into the second
IF signal with a frequency of 455 kHz. The second IF signal is output from
pin 3 of IC108 to the ceramic filter (FL101 or FL102), where the unwanted
frequency band of that signal is eliminated, and the resulting signal is sent
back to the IC108 through pins 5.
The second IF signal input via pin 5 is demodulated by the internal limiter
amplifier and quadrature detection circuit in IC108, and output as an audio
signal through pin 9.
The audio signal from pin 9 of IC108 is amplified by the audio amplifier
(IC120:A),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 (IC120:B). The signal is then input to volume
(VR1) . The adjusted signal is sent to the audio power amplifier (IC117)
through pin 1 to drive the speaker.