PHILIPS 55P8288
Manual de Servicio

7615 Adjustments page 19

Troubleshooting Tips
The Power Supply Circuit
When the set is turned On, the Main Switch Mode power supply is turned On. This supply produces the +130 volt,+32 volt,-32 volt, two +15 volt, and Audio supplies. When IC 7302 develops a normal steady state operation, the 130 volt supply is sampled by resistors 3324,3323,and 3322. This sample voltage is then sent to Shunt regulator 7303 which drives the feedback optoisolator 7303.The feedback voltage on Pin 14 is then compared with an internal 2.5 volt reference by comparator "C". Comparator "C" then sets the reference voltage on the inverting side of comparator "B" to control the On time of the drive at Pin 3.If the voltage on Pin 14 increases due to the 130 volt supply increasing, the On time of the pulse on Pin 3 will be reduced. If the 130 volt supply decreases, the voltage on Pin 14 would decrease, causing the On time of the pulse on Pin 3 to increase. This is used to keep the 130 volt supply at the correct voltage. Variable resistor 3322 is used to adjust the 130 volt supply to the correct level. The overvoltage protection circuit of the IC will shut the IC down if the VCC level on Pin 1 exceeds 17 volts. To troubleshoot this circuit, first check the On/Off line from the Small Signal Panel to ensure that it is going Low. Then check for the presence of startup voltage on Pin 1 of the regulator IC 7302.If the IC is working correctly, this voltage will be changing from 9.4 to 14.5 volts. If the startup voltage is not present, check the startup resistor 3300 and the bridge rectifier. If the voltage on Pin 1 is changing, check the drive signal on Pin3.Then check for signal on the gate and drain of 7301.An excessive load on the secondary, a short on the 130 volt line for example, would cause the supply to pulse with little or no voltage on the secondary.

Picture problems
Picture too dark or too bright Press �Smart Picture� button on the remote control. If this improves the picture, increase / decrease the brightness value or increase / decrease the contrast value. Enter the Customer Service Mode. If this improves the picture, exit Customer Service Mode, then Increase / decrease the brightness value or increase / decrease the contrast value. White line around picture elements and text Press �Smart Picture� button on the remote control. If this improves the picture, decrease the sharpness value. Enter the Customer Service Mode. If this improves the picture. Decrease the sharpness value. Snowy picture Snow is an antenna or antenna connector problem. Check tuner and AGC. Black and white picture Press the �Smart Picture� button on the remote control. If this improves the picture, increase the color value. Enter the Customer Service Mode. If this improves the picture. Increase the color value. Menu text not sharp enough Press the �Smart Picture� button on the remote control. If this improves the picture, decrease the contrast value. Enter the Customer Service Mode. If this improves the picture. Decrease the contrast value. Sound problems No sound or sound too loud (after channel change / switching on) - Enter the Customer Service Mode. If the volume is OK, increase / decrease the volume level.