Sony ICF-6800W - Page 14

ICF-6800W |
In this manner, the oscillating frequency is varied
by varying the capacitance of the variable-capacitance
diode by the AC components. However, if the
response speed of this loop is faster than the varia-
tions of the AC components, then the locking will
take place instantaneously.
Through these means, variations in frequency
are kept within the accuracy of the crystal for the
1 MHz pulse generation.
VT1 is provided for making adjustments so
that the functioning of each of the FETs of Q53
is equalized.
Functions of Q58—Q61 (sweep circuit)
(Refer to Fig. 29)
The sweep circuit is provided to carry out a
locking function so that the oscillations of VCO2
do not drift when the SW BAND SELECTOR control
is turned and the oscillating frequency of VCO2
does not get into the capture range and is unlocked.
If it unlocks from a locked condition because of
temperature variations, then this would be an in-
stance of it getting beyond the locking range, and the
sweep circuit would not be of any use at all.
When VCO2 is in a locked condition, a signal
is not fed to Q58, and so this sweep circuit does
not function.
The frequency at which VCO2 is locked has
a width of several hundred kHz plus or minus the
intended frequency; when within this range, it is
pulled into the intended frequency much like an
AFC circuit.
When not in this range, the lock is released,
and the PLL circuitry is no longer capable of stable
reception of the receiver.
Suppose that the lock is released. Now a beat
component is fed to the base of Q58, as explained
in the section on the phase detector.
The signal amplified by the AC amplifier Q58
is rectified by D31 and fed to Q59. This serves to
make Q59 ON.
When Q59 goes ON, C203 will now be grounded,
and this will disconnect AC-wise the negative feed-
back circuit of R221.
The signal from the collector of Q61 that has
passed through R224 and C204 is fed back to the
base of Q60 through a positive feedback circuit, and
because the negative feedback circuit is cut out, Q60
and Q6] start oscillating at low frequencies.
The output of these oscillations is extracted and
fed to the variable-capacitance diode of VCO2, to
slowly change the capacitance of the variable capaci-
tance diode.
—14-
When the frequency of VCO2 is moved up to
the width of its locking range by this sweep circuit,
it will try to lock in a certain frequency.
When VCO2 locks in, there will no longer be
a beat component at Q58, and so Q59 goes OFF, and
the oscillations of Q60 and Q61 will stop.
The sweep circuit functions in this manner so
that VCO2 will lock quickly in a certain frequency.
Fig. 29
Q48 and Q49 (display switch)
(Refer to Fig. 30)
The ICF-6800W is equipped with a counter that
provides a digital readout of the frequency being
tuned to. This part of the receiver has a high
current drain (approximately 100mA), and so it
is equipped with a display switch to enable it to be
switched off when it is not needed. Fig. 30 is a
schematic of the display-switch circuit.
Q48 and Q49 constitute a bistable multivibrator;
when the power switch is ON, Q49 will be OFF and
Q48 will be ON. (In an inactive state, because of
R196, Q49 goes OFF.)
When Q48 is OFF, a high voltage is fed to the
base of Q47, and so it goes ON.
When Q47 goes ON, it lowers the base potential
of Q46, and so Q46 also goes ON, and a B+ voltage
is supplied to the counter section. Q46 is a PNP
transistor, and it goes ON when its base potential
drops in more than 0.6 V below the emitter.
When S7 is switched ON, the positive end of
C255 is grounded so the potential at point @) drops
quickly. This amounts to the same thing as feeding in
a negative pulse signal.
When in an inoperative state, C179 and C180
are maintained at the potentials shown in Fig. 30.
When a negative-pulse signal is applied, it passes
through C179 to the base of Q48, and Q48 turns
OFF. There is no voltage difference between the two
ends of C180, and so the negative-pulse signal will
not pass through it, while the pulse signal is applied
to Q49 after passing through R198, and soit become
an extremely small pulse.