How to fix iPhone 6 LCD screen no display


We got a damaged iPhone 6, the customer described the failure is cannot turn on. Remove the motherboard and connect it to DC power supply , we found that the process of current jump is normal, which shows that iPhone 6 is not unable to boot, but not display.

The initial judgment of the iPhone 6 motherboard without maintenance, water damaged, fall pieces and so on, carefully check the display adapter, there is no obvious fault. Now we measure the data of display adapter (reference schematic diagram and phone repair system), the resistance data of display adapter all is normal. As shown in picture 7-1-1 and 7-1-2.

Then use multimeter  to measure the power supply, it shows +5.7V,-5.7V, the backlight boost circuit is only about 4V. However, on the normal case, the backlight boost circuit should have a voltage of about 11-17V. As shown in picture 7-1-3.

Because the backlight boost circuit is not normal, so we need to check the circuit and find the problem. As shown in picture 7-1-4,7-1-5,7-1-6.

Working principle and analysis of display screen 

1. The machine is powered on 1.8V, through a small inductor to reach the display screen.

2. The CPU detects the display screen through the MIPI line.

3. The CPU sends the reset signal to display screen

4. The display screen sends the power supply signal to the display screen power supply IC.

5. The CPU outputs the video signal through the MIPI line, the LCD screen can display.

The backlight power supply is abnormal, but the +5.7V is normal, this situation is usually caused by damaged backlight IC, damaged backlight chip peripheral components and backlight IC solder pad disconnect or virtual welding.

Check the perimeter first with the exclusion method, finally check the chip. Measure whether the peripheral inductance of the backlight IC  is open or not, whether the resistance has become larger, and whether the diode breakdown. However, none of this has found fault. As shown in picture 7-1-7 and 7-1-8.

Then measure the data of each pins on U1502  backlight drive IC, all is normal. As shown in picture 7-1-9. So we suspect the chip itself is damaged, replace the new chip.

After measure the voltage of C1503 with multimeter, it has 16V, which is the normal data. This error repair completes. As shown in picture 7-1-10, 7-1-11 and 7-1-12.

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