Alinco DR-03T, Service Manual

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C I R C U I T D E S C R I P T I O N
1) Receiver System
The receiver system is a double super-heterodyne system with a 10.7M H z first IF and a 450kH z
second IF.
1. Front End
The received signal at any frequency in the 28.000MHz to 29.695MHz
range is passed through the low-pass filter (L115, L114, L113, C204,
C203, C202, C216 and C215) and tuning circuit (L105 and D105), and
amplified by the RF amplifier (Q107). The signal from Q107 is then
passed through the tuning circuit {L104, L103, L102, and variable
capacitor D104, D103, D102) and converted into 10.7MHz by the mixer
(Q106). The tuning circuit, which consists of L105, variable capacitor
D105, L104, L103, L102, variable capacitor D104, D103 and D102, is
controlled by the tracking voltage from the VCO. The local signal from the
VCO is passed through the buffer (Q145), and supplied to the source of
the mixer (Q 106). The radio uses the upper side of the super-heterodyne
system. .
2. IF Circuit
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 10.7 MHz frequency from the results and eliminates the signal of
the unwanted frequencies. The first IF amplifier (Q105) then amplifies the
signal of the selected frequency.
3. Demodulation Circuit
After the signal is amplified by the first IF amplifier (Q105), it is input to
pin16 of the demodulator IC (IC108). The second local signal of 11.15MHz
(shared with PLL IC reference oscillation), which is oscillated the external
oscillator X601, 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), where the
unwanted frequency band of that signal is eliminated, and the resulting
signal is sent back to the IC108 through pin 5. The second IF signal input
via pin 5 is demodulated by the internal limiter amplifier and quadrature
detection circuit in IC 108, and output as an audio signal through pin 9.
4. Audio Circuit
The audio signal from pin 9 of IC 108 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 the pin 1 to drive the speaker.
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