Sony ICF-6800W - Page 12

ICF-6800W |
Role of Q72 (manual gain control) (Refer to Fig. 23)
The antenna band-pass filter has a wide band-
width of around 1MHz to 30MHz, and so both
high-level and low-level signals come in together.
In the case of the situations where the frequen-
cies of the incoming signals are very close to each
other, such as in amateur radio, if there should
be a station with a high-level signal in the immediate
vicinity of the frequency the listener wishes to tune
in to, the lower-level signal will receive interference
from the high-level signal, and this could cause
cross modulation.
The role of this circuit is to adjust the level of
the input signal coming in from the antenna so that
the high-level signal will not interfere with the low-
level signal, to reduce the effects of this cross
modulation.
FROM BPF
IN ANTENNA
CIRCUIT
Fig. 23
IC2 (% divider) (Refer to Fig. 24)
IC2 is a % divider, to demultiply the frequency
of the input to %.
In order to reduce the input frequency of IC3
to
the
order
of
several
MHz,
it
goes
through
a
%
dividing process by means of IC2, and the pulse
signal is then fed to the input of IC3.
A signal comprising a signal consisting of a mix-
ture of the signals of VCO1 and VCO2 to which a
10 MHz signal has also been mixed is fed to the
terminal @) ‘
The
signal
from
the
VFO
is
fed
to
terminal
Each of the signals are converted into pulse
signals divided by 2 before being fed to the input of
IC3, In order to ascertain whether IC2 is defective or
not, all that has to be done is to ascertain whether the
signals fed to terminal B) oT are divided by 2
when they emerge at terminal @) or
—12—
Fig. 24
When
actually
carrying
out
this
work
on
the
printed
circuit
board,
each
of
the
pins
of
IC2
should
be
raised
from
the
pattern,
a
test
circuit
as
shown
in
Fig.
25
prepared,
and
signals
fed
to
it,
then
con-
firm
the
frequency
and
waveform
of
the
output.
4Vv
signal
generator
Connect to pin @) or @ and
check the frequency and waveform
at pin or
Fig. 25
eS
3Vp-p
_
Balanced-Type 1 st Mixer (Q2 and Q3)
(Refer to Fig. 26)
Q2 and Q3 are referred to as a balanced-type
mixer, and comprise a circuit that is used to extract
only the necessary IF signal.
The signal from the antenna passed through
the band-pass filter, then passes through the RF
amplifier, again passes through a band-pass filter,
and goes to the Ist mixer.
However, if the signals that have to be eliminated
by these RF filters should be excessively powerful,
the RF filters will be unable to cope with them and :
they may reach the Ist mixer.
If these unwanted frequencies should be the
same as the lst intermediate frequency, with the
single-type conventional mixers used heretofore,
they would go right through the mixer into the Ist
IF amplifier circuit.
If that should happen, since it will not be pos-
sible to eliminate these unwanted signals in the cir-
cuits that follow the 1st IF amplifier, they will
necessarily remain in the form of interference.