Philips PTV800, Service Manual

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The pulse from T402 Pin 1 is rectified by D405 and provides the run voltage for IC401 Pin 16.
This rectified run voltage is also applied to IC401 Pin 4 to adjust the chip's internal reference,
regulating the duty cycle at IC401 Pin 7. The duty cycle controls the on time of Q402 to
regulate the output of T402. This maintains the DC voltage on Pin16 of IC401 between 12 and
13 volts.
The voltage from T402 Pin 9 is rectified by D403 to provide a 15vdc standby source voltage.
T402 Pin 7 is rectified by D407 and is applied to IC408 Pin 1, a 5 volt regulator and IC402 Pin
10. The voltage from D403 is also applied to IC406 Pin 1 (Opto-Isolator), IC405 Pin 2 (Opto-
Isolator).
Reset from IC402 Pin 11 is coupled through Opto isolator IC403, Pins 4 and 5, to Q315 on the
Small Signal Board. When ac is first applied to the set, a momentary High from IC402 Pin 11
is coupled through IC403 turning transistor Q315 off, causing the Microprocessor, Pin 17, to go
Low to provide reset. After reset, IC402 Pin 11 stays Low, turning Q315 On providing a High
to the Microcomputer, Pin 17, in normal operation. IC 400 monitors the 160 volt supply for a
loss of AC power. In normal operation, IC400, Pin 2 will have a higher dc voltage than Pin 3
causing Pin 1 to go Low. This turns the LED inside Opto isolator IC403 On. If the 160 volt
supply drops below 50 volts, IC400 Pin 1 goes High turning IC403 Off. The 5 volt standby
through R446 turns Q315 Off, causing the Microcomputer, Pin 17, to go Low, resetting the IC
which turns the set Off. Voltage from Pin 1 of T402 is rectified by D420 and filtered by C483 to
provide power for IC400 and IC403. This voltage is also used to provide a dc reference for Pin
3 of IC400.
When AC is applied to the set, IC402 Pin 16 is held high by the 5 volt standby supply through
R448. The High on the input of the NOR gate inside IC402 switches Pin 8 Low turning On the
LED of Opto-Isolator IC406. An approximate 1.5 volt drop across Pins 1 and 2 of IC406 will
indicate that the IC is On. The photo transistor inside IC406 will forward bias, pulling IC406
Pin 5 Low holding IC401 Pin 12 Low. With IC401 Pin 12 at ground, the internal reference on
the comparator will be negative with respect to the non-inverting input of the comparator,
causing the reset of the flip flop to be high. This will keep IC401 Pin 10 Low, keeping the full
power supply turned off. With no 130 vdc from the full power supply, there will be no voltage
present at IC402 Pin 7. IC402 Pin 9 will go Low, turning the Opto-Isolator IC405 On, driving
Pin 6 Low and also driving IC401 Pin 13 Low.
Full Power Supply
When any button on the front of the set or the Power button on the remote control is pressed,
the microcomputer IC300, Pin 52, on the Small Signal panel goes Low, pulling IC402, Pin16,
Low. IC402, Pin 8, goes High, turning Off IC406, removing the low from IC401, Pin 12. The
output of the comparator connected to Pin 12 goes high, allowing the flip flop inside the IC to
drive Pin 10 High turning Q401 On. Energy is stored in T401 while Q401 is on and is
transferred to the secondary when Q401 is Off. Current through Q401 develops a voltage
across R407, R408, and R466 and is monitored by Pin 11 of IC401. If this voltage exceeds
the internal reference of the IC, the output on Pin 10 will shut Off.
Voltages from separate windings of T401 are rectified and filtered, producing 130 volts dc, 15
volts dc, 10 volts dc, -32 volts dc, and 32 volts dc. The 130 vdc is sampled through a voltage
divider network, R453, R454, and R455 and fed to IC402, Pin 7, where it is compared to an
internal 3.5 volt reference. As the 130 volt supply reaches the correct voltage, the voltage at
IC402 Pin 9 will increase, which will reduce the brightness of the LED inside IC405. IC405
controls the voltage on IC401 Pin 13, controlling the duty cycle at IC401, Pin 10. This controls
the On time of Q401 to maintain the 130 volt power supply at the correct voltage. The 15 volts