Philips H8 - Power Down and Front Detect (Figure 4)

Power Down and Front Detect (Figure 4)
The Power-Down and Front Detect circuits
signal the Microprocessor in case power is
removed from the set. The Microprocessor
will store the customer settings before shutting
the set down. Diode 6942 produces a negative
voltage to turn transistor 7987 On, keeping the
Power-Down line High. This keeps transistor
7277 turned Off. If power is removed from the
set, the negative voltage will discharge before
the operating voltage for the set falls.
Transistor 7987 will turn Off. 7277 will then
turn On, placing a High on the RGB CRT drive
lines, blanking the CRT. Transistor 7277
will continue to conduct until Capacitor
2284 has discharged. In the same manner,
7990 will be turned Off, turning 7991 Off,
which will turn 7992 On, causing the Front-
Detect line to go Low. This will prevent the
Microprocessor from receiving any
commands while the set is being shut
down. If there is a problem in this circuit
which causes the Front Detect line to go
Low, the Microprocessor will not respond to
any commands from the Keyboard or
Remote.
Figure 3 - 5 Volt Switch
Page 6
Figure 4 - Power Down and Front Detect