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1-2.
CIRCUIT DESCRIPTION
(1)
Electronic Timer
This radio receiver uses an electronic timer which
one IC contains the blocking oscillator, the counter,
the
digital-analog
(DA)
converter,
the comparator,
the
switching
circuit,
the
remaining-duration-time
detector,
the
voltage
regulator
and
the
current
detector.
This
blocking
oscillator
produces
a reference
signal for the timer.
The
timer
selector
S2-1 (96
minutes/8
hours)
selects
time
constant
of
the
oscillator
to change the oscillation frequency.
The
oscillation can be checked by observing the sawtooth
wave on the oscilloscope connected to terminal 2 of
IC8.
The cycle period of sawtooth wave should be
0.35156
seconds
for 96-minute
timer
and
1.7578
seconds
for 8-hour
timer.
The
acurate frequency,
however, can not be read on the oscilloscope, since
the oscilloscope connection changes time constant of
the oscillator.
The sawtooth wave produced by the oscillator is
demultiplied
into a pulse wave
at the counter and
then
converted
into
the
analog signal by the DA
converter.
The circuit of this converter is as shown in
POWER
SUPPLY
POWER SWITCH
B+
| ICF-78(
Fig. 1-2.
The pulses supplied at @ charges capacitor
C step by step and gradually increases the voltage at
®.
In the next stage, the comparator,
the voltage at
is compared with the voltage determined by VR4
(TIMER SET CONTROL).
When
those
two
voltages
becomes
equal,
the
comparator
actuates the switching circuit.
Then the
switching circuit turns Q4 ON or OFF to switch B+
circuit ON or OFF.
On the other hand, the difference of those two
voltages is detected
by the remaining-duration-time
detector and is indicated as remaining-duration-time
on the meter.
The voltage regulator supplies stable
reference-voltage
to the comparator
through
VR4.
The current
detector detects output current
change
of the voltage regulator caused by turning $3-2 ON or
OFF and actuates the switching circuit.
That is, if the POWER
switch S3-1
is turned ON
and
the
timer
is preset,
the radio
section
will be
automatically switched
ON at the preset time, and if
the POWER
switch S3-1 is turned OFF and the timer
is preset, the radio section will be switched OFF at
the preset time.
Pulse wave
|
Voltage at
c
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