network Ro46, Ro44, Ro50, Ro45 and Ro42. By adjusting
the trimmer potentiometer Ro45, the output voltages of
the power supply can be controlled.
In order to avoid excessive decreasing of +7 V during re-
cording mode, diode Do20 is connected from the above
mentioned resistor network to +7 V. If the voltage level
drops too far below +7V, the diode conducts causing a lower
voltage level on pin 5. The controller then generates wider
pulses from the output pin 3 and the supply voltages of
the power supply will be increased.
The width modulated drive pulses are output from pin 3
via the pulse transformer Mo3 to power supply controller
pin 2. The rising edge of the drive pulse causes the switch-
ing transistor to conduct and the falling edge, which is syn-
chronized to the line flyback pulse, switches off the tran-
sistor.
This arrangement prevents disturbances caused by the
switching-off time of the transistor from upsetting the
screen display.
Under / over voltage detection
The power supply controller has an internal monitor for
both under and over supply voltage on pin 16. The under
voltage threshold level is typically +8.5 V. Lower voltage
levels disable the output pulses.
The over voltage threshold is typically +15.7 V. Higher volt-
age levels disable the output pulses.
Restarting requires that the voltage level on pin 16 is first
decreased below +8.5 V and then increased to +11.8 V,
unless the voltage level across capacitor Co22 has reached
+2.5 V. In this case, circuit operation is completely stopped.
Current limitation
As mentioned above, the source of the switching transis-
tor is connected to ground via resistor network Ro18/19
and Ro21/22. The measured result is fed to input pin 3 of
ICo1. A double threshold system is used, first limitation
level (+0.6 V) against momentary overloads and a second
limitation level (+0.8 V) against very strong overloads.
When the first threshold level is reached, the switching tran-
sistor stops conducting until the end of the period, and a
new pulse is needed to start it conducting again. During
the first threshold period, capacitor Co22 is charged. If the
voltage level across Co22 reaches +2.5 V, the output will
be disabled. This system is called "repetitive overload pro-
tection". However, if the overload subsides before +2.5 V
is reached, capacitor Co22 will be discharged and normal
operation will continue.
If a very powerful overload causes the second threshold
level to be reached, the output will immediately be disa-
bled.
If the power supply stops because the first threshold has
been exceeded, it can be restarted by decreasing the sup-
ply voltage on pin 16 below +8.5 V and then increasing it
to +11.5 V.
If, however, the power supply stops due to exceeding the
second threshold, the circuit is stopped completely and can
only be restarted with mains switch.
Regulators / voltage switches
ICo4 regulates the +5 Vstby, which is always available when
the power supply is operating . Due to the micropower
system, there are no voltages available in standby mode.
ICo3 regulates the +12 Vr and +12Vp supply voltages. The
+12 Vr is available in normal and recording modes, but not
in service standby mode.
The +12 Vp can be switched off in recording mode by the
microcontroller. In this case the P_on line is high, transis-
tor to7 conducts and transistor to4 is switched off.
The +12 Vp and +8 Vp voltages are absent, and therefore
the horizontal deflection stage will not operate.
ICo6 regulates +5 Vr, which is available in normal and re-
cording modes, but not in service standby mode.
The tuning voltage +30 V for the tuner is regulated from
+130 V. The circuitry is located in the horizontal output stage
consisting of resistors Rk11...Rk114 and zener diode ZDk1.
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