Philips 20PT643F Service Manual page 118

Y/C or CVBS source is present. The presence of an external Y/C source makes
this line 'high' while a CVBS source makes the line 'low'.
SEL-MAIN-FRNT-RR. This is the source select control signal from the
microprocessor. This control line is under user control or can be activated by the
other two control lines.
Power Supply Control
The microprocessor part is supplied with 3.3 V 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' (3.3 V) 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.