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iPhone 4S NO Backlight RepairiPhone 4S LCD backlight is couple tiny LEDs. In order to light up these LEDs at the same time, a higher voltage drop is neccessary. When "PP_VCC_MAIN" goes thru circuit components L18, D1, C134, FL4, and J4 Pin 2 to LCD Panel, U5 Power IC Pin P13 "WLED_LX" provides a chain of on-off pulses which is a switching signal to boost up this voltage so that all LEDs on the LCD panel have enough voltage drop to light up at the same time. Switching Boost Circuit diagram is shown below. NO light, NO display or dim backlight is caused by the defective of one of the component in the circuit or defective LCD panel.Repair example 1: if there is no beep on the filter coil FL4 or FL4 is damaged or missing, just simply put one jump from J4 Pin 2 to C134 and the repair is done. Repair example 2: if on-off pulse at U5 Pin P13 is not found by an oscilloscope, U5 Power IC needs to be replaced and the repair is done. Source of all circuit diagram are from Apple's schematic. iPad, iPhone, iPod, Macbook are trademarks of Apple company. |