Circuit Description Dr-135; Receiver System - Alinco DR-135 Service Manual

CIRCUIT DESCRIPTION DR-135

1) Receiver System

The receiver system is a double superheterodyne system with a 21.7MHz first IF and a 450kHz second IF.
1. Front End
2. IF Circuit
3. Demodulator Circuit
4. Audio Circuit
The received signal at any frequency in the 136.000MHz to 173.995MHz 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 (L103, L102, and varicaps D103 and D102) and con-
verted into 21.7MHz by the mixer (Q106). The tuning circuit, which consists of
L105, L104, varicaps D105 and D104, L103, L102, varicaps D103 and D102,
is controlled by the tracking voltage form the VCO. The local signal from the
VCO is passed through the buffer (IC112), 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 (XF102, XF101) selects 21.7MHz
frequency from the results and eliminates the signals of the unwanted frequen-
cies. The first IF amplifier (Q105) then amplifies the signal of the selected fre-
quency.
After the signal is amplified by the first IF amplifier (Q105), it is input to pin 24 of
the demodulator IC (IC108). The second local signal of 21.25MHz (shared with
PLL IC reference oscillation), which is oscillated by the internal oscillation cir-
cuit in IC116 and crystal (X103), is input through pin 1 of IC108. 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 450kHz. 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 12.
The audio signal from pin 12 of IC108 is amplified by the audio amplifier
(IC104: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 (IC104:D). 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.
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