Philips 20PT643F Service Manual page 101

The 'MainSupply' line is the reference voltage for the power supply. It is sampled by
resistors 3543 and 3544 and fed to the input of the regulator 7540 / 6540. This regulator
drives the feedback optocoupler 7515 to set the feedback control voltage on pin 3 of
7520.
The power supply in the set is 'on' any time AC power goes to the set.
Derived Voltages
The voltages supplied by the secondary windings of T5520 are:
'MainAux' for the audio circuit (voltage depends on set execution, see table
below),
3.3 V and 3.9 V for the microprocessor and
'MainSupply' for the horizontal output (voltage depends on set execution, see
table below).
Other supply voltages are provided by the LOT. It supplies +50 V (only for large screen
sets), +13 V, +8 V, +5 V, and a +200 V source for the video drive. The secondary
voltages of the LOT are monitored by the 'EHTinformation' lines. These lines are fed to
the video processor part of the UOC IC 7200 on pins 11 and 34.
This circuit will shut 'off' the horizontal drive in case of over-voltage or excessive beam
current.