Sony ICF-7800 - SECTION 1 CIRCUIT DESCRIPTION; Electronic Timer Functionality

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