Philips 20PT643F Service Manual page 119

Protection Events
Several protection events are controlled by the UOC:
BC protection , to protect the picture tube from a too high beam current. The
UOC has the capability of measuring the normal back level current during the
vertical flyback. So if for some reason the CRT circuit is malfunctioning (i.e. high
beam current), the normal black current will be out of the 75 A range, and the
UOC will trigger the power supply to shut down. However, this is a high beam-
current situation, the TV screen will be bright white before the set is shut down.
E/W protection , two protection mechanisms are built in, over-current and over-
voltage.
In case of over-current due to defective parts in the line deflection output
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stage, a high current will flow through resistors 3405//3406. If this current
is large enough to create a voltage drop of 0.7 V across 3405//3406,
transistor Q7606 (in A7 diagram) will conduct and pin 80 of the UOC will
be pulled down. Thereafter, the UOC will shut down the power supply. In
case of further current increase, the fused resistor 3411 is built-in for
double protection.
In case of a high voltage appearing across capacitor 2401 (dependent of
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the tube size), which is high enough to trigger zener diode 6401 into
conduction, transistor Q7606 (in A7 diagram) will conduct and UOC is
triggered to shut down the power supply.
I2C protection , to check whether all I
In case one of these protections is activated, the set will go into 'standby'.
The 'on' and 'standby' LEDs are controlled via the UOC.
C ICs are functioning.
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