Alinco DX-70, Service Manual

Page: 8 / 116

I. S Meter, Squelch
The output of Pin 1 and RF meter output are combined in the diode, then it is sent
to the front CPU to display the meter. The output signal of Pin1 is fed to Pin6 of
IC14:A. The voltage of Pin5 is determined by the squelch VR of front unit. Com­
paring with this voltage, the squelch is opened or closed.
While the check operation the CPU output decreases the voltage of squelch VR in
front side to open the squelch forcedly. The squelch output controls IC13, at the
same time it is provided to the front unit to light RX LED and led to CPU unit.
m. N oise Blanker Circuit
This circuit eliminates the pulse noise of a car, etc. Because the noise emitting
time is short, in this duration the operation of receiver is stopped to prevent the unit
Irom emitting a noise. The pulse noise is delayed when it is passed through the
narrow band filter, and the emitting time becomes longer. It makes difficult to
eliminate the noise, so it is necessary to eliminate the noise in the earlier stage.
A part of the second mixer output, whose band width is limited, is amplified in Q20,
Q19, 018, and Q16. The signal is detected in D33 and D34, and the AGC voltage
is applied to 019, Q18 and Q16.
The charge time constant of this AGC is determined by R82 and C128, and also
the discharge constant is determined by R81+R82, C128. The voltage of AGC
does not rise suddenly because of the charge constant, so that this voltage is not
applied to almost all the short signals such as pulse noise, but is applied to the
continuous signals such as receiving signal and amplilier gain is decreased.
While emitting the pulse noise, the AGC voltage does not follow the pulse noise,
so the detected voltage is high, then 015 is turned ON in that time.
On the contrary, as for the continuous signal, the detected voltage of D33 and D34
is fixed by AGC, so 015 is turned OFF because of the emitter bias of R85 and
R84.
Namely Q15 is turned ON only the time of the pulse noise, then 021 is turned
OFF. The source of IF amplifier of 022 is biased through R98 and R 102 so that
the gain is decreased and the signal is blanked. When the emitter of Q15 is
biased to high, the Noise Blanker is turned OFF.