Philips 7629 Service Manual page 92

Power Supply Control
The microprocessor part is supplied with 3.3V and 3.9 V both derived from the
'MainAux' voltage via a 3V3 stabilizer (7560) and a diode.
Two signals are used to control the power supply:
Stdby_con This signal is generated by the microprocessor when over-current
takes place at the 'MainAux' line. This is done to enable the power supply into
standby burst mode, and to enable this mode during a protection. This signal is
'low' under normal operation conditions and goes to 'high' (3.3V) under 'standby'
and 'fault' conditions.
POWER_DOWN This signal is generated by the power supply. Under normal
operating conditions this signal is 'high' (3.3 V). During 'standby' mode, this
signal is a pulse train of approx. 10 Hz and a 'high' duration of 5 ms. it is used to
give information to the UOC about the fault condition in the Audio amplifier
supply circuit. This information is generated by sensing the current on the
'MainAux' line (using voltage drop across R3564 to trigger Q7562). This signal
goes 'low' when the DC-current on the 'MainAux' line exceeds 1.6- 2.0 A. It is
also used to give an early warning to the UOC about a power failure. Then the
information is used to mute the sound amplifier to prevent a switch off noise and
to solve the switch-off spot.
Protection Events
Several protection events are controlled byte 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. highbeam
current), the normal black current will be out of the 75 µA range, and the UOC