Philips PTV800, Service Manual

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LARGE SIGNAL PANEL
The large signal panel contains the SMPS (Switch Mode Power Supply), Scan-derived Power
Supply, Horizontal and Vertical deflection, High Voltage circuits, and safety shut down circuits.
Switched Mode Power Supply
120 volts AC is applied through F401 to the bridge rectifier D406 which produces
approximately 160 volts DC at TP8
. (Figure 3) The 160 vdc is applied through F402 and the
primary windings of T401 Pin 8 to the drain of Q401. It is also applied through R416 and the
primary windings of T402 Pin 3 to the drain of Q402 the Standby Regulator. 160 volts from
R416 goes through R415 (Start Resistor) and powers IC401 Pin 16 (Controller). The 160 vdc
source also provides power for an audio amplifier, which has its own switch mode regulator
supply.
Stand By Power Supply
As the voltage through R415 charges C414 to 14.5 volts peak at IC401 Pin16, the under
voltage lockout is over come, and the 5 volt power supply inside IC401 switches on starting the
50 kHz oscillator. A 12 vpp drive signal is then output from IC401 Pin 7 to the gate of Q402,
which turns Q402 On. Current through R414, Q402, and the primary winding of T402
develops a voltage across R414. This voltage is sampled at IC401 Pin 6, and is applied to a
comparator within the IC. The sampled voltage is compared to an internal reference voltage.
When the sample exceeds the reference, the reset goes high, turning off the drive signal at
IC401 Pin 7. When Q402 turns off, the voltage drop across R414 goes away. Reset inside the
chip goes Low. It turns on with the next high from the oscillator. IC401 Pin 7 will go High
again turning on Q402.