Nokia 447ZiPlus Service Manual page 14

17” colour monitor
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When the display signal enters the " off " state of power saving which both H. sync and V. sync off,
the voltage at pin31& pin 29 of I501 are low, then Q820 is off and Q822 is off too, there is no 8V &
14V Voltage for the circuit.
The overall power dissipation is significantly reduced to less than 5 watts.
To recover from off state to normal state is automatic since the recoved H. sync and V. sync will
trigger I501 via Q550 &Q556, The voltage on pin 29 & 31 will go to high again and turn on Q822,
.
Q820 via Q823, Q821
Heater voltage
The heater element in the CRT is supplied with 6.3+0.3V during normal operation of the monitor and
5.0+0.5v during suspend mode. To achieve the goal, Q851, D871 and associate circuit are introduced
when at normal state, a Horizontal pulse is fed to D871 via P807, Q851 is on, the feedback voltage on pin
2 of I862 is normal, the output voltage of the secondary side of T801 is normal the heater voltage is 6.3V +
0.3V.
Horizontal output is shut down by making Q820 off during suspend state. There is no Horizontal pulse
then Q851 is OFF. The feedback voltage is higher than the normal state. this will force the output voltage
of the secondary side of T801 drop down. With the right value of R800, R832, R833, the heater voltage
will be 5.0+0.5V
6-3. Automatic and Manual Degaussing Circuit
When power switch getting started, MCU #38 will send out a "high" signal to turn on Q801. Simultaneously
a current flows from B
degaussing coil L901 constitutes an automatic degaussing circuit to demagnetize the shadow mask and
the internal shield of the CRT in order to protect the display screen from color impurity.
The resistance value of the posistor is as low as 14 ohm during power off.
Immediately after the display turned on, a large AC current flows through the resistor into the degaussing
coil for demagnetization.
AT the same time, the resistor value of the posistor increase rapidly due to
the rise of its temperature, the degaussing current thus decreases to less than 100mAp-p in a few
seconds so that the display screen settles quickly, when the degaussing function is finished.
+
(14V) via R807 and SR801 to make the relay on, the posistor R806 and the
12

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