Alinco DX-70, Руководство по обслуживанию

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The isolation is required the value of guaranteed attenuation of each filter (approx.
70dB) or more. The diode connected in parallel in front and rear of no used filter is
short and the diode connected in series is open. The combination of open and
short is used to get the high isolation.
The modes, transmission/reception and wide/narrow of this filter are selected by
Q36-Q46, D79, D80, D82, D83, D84.
e. The Second IF Am plifier Circuit
f. Demodulation Circuit
g. CW Audio Filter
After passing through the filter, the signal is led to the transmission/reception
switching diode D49, and amplified in Q23 and Q24, then buffered in Q25.
The AGC voltage is applied to the second gate of Q22, Q23 and Q24.
The output level of 025 is fixed because the AGC voltage is added to the receiv­
ing signal.
This output signal is used for the demodulation in SSB, AM and CW modes and
AGC detection.
In the FM mode, after passing through the transmission/reception switching diode
of D49, a part of receiving signal is fed to 1C7(MC3357) from C221, then it is IF-
amplified and demodulated. C214 is connected in parallel to the feedback resister
R182, and the resister is de-emphasized. Even in the FM mode, Q23, Q24 and
Q25 are active, also AGC is operated.
In SSB and CW modes, the following local oscillating frequency is supplied from
PLL unit to IC3 balanced mixer, then the signal is demodulated.
The receiving signal is fed to Pint, and local oscillation Pin3, then picked up the
demodulation output of approximately 100mV from Pin7.
USB 456.5kHz +IF SHIFT
LSB 453.5kHz +IF SHIFT
CWU 455.8kHz
+1F SHIFT
CWL 454.2kHz +IF SHIFT
The output is led to the switching circuit of each mode, and to the CW audio filter.
In AM mode, the signal is detected in D51, then led to IC5.
In FM mode, the signal is demodulated and de-emphasized in above-mentioned
IC7, then led to IC5.
IC4 is the active filter, which consists of the combination of low-pass filter and high-
pass filter in the operational amplifier. It has the band width of about 600Hz (-6dB)
centering the trequency of about 800Hz.
h. AF, AGC Tim e Constant Selection
1C5 is the analogue multiplexer which has 2 circuits with 4 contact points, and
switches the demodulation output in every mode and AGC time constant. The
voltage combined in D55 and D56 is input to Pin9 and Pinl 0, then the output of
1C3 (SSB, CW-W), the modulation output of FM/AM and CW audio filter output
(CW-N) are selected. The voltage of 8V is applied to Pin6 (INHIBIT) when
transmitting, and the modulation output is turned OFF unconditionally.
GO