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We continue the lesson of repairing the fault of no display

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When we test that the MIPI bus is not abnormal,

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we need to test its other working conditions, such as reset signal,

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detection signal

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Well, let's take a look at this phone, Xiaomi 9 PRO

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We open the bitmap

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The bitmap is equivalent to an actual mobile phone,

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which corresponds to the circuit components one-to-one

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Let's zoom in on it and find its reset,

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this is the reset of the touch screen

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For example, pin 22 is reset

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Let's see where the detection is

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This is 3 volts power supply, 1.8 volts power supply

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Pin 16 is its detection, LCD_ID_DET

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OK, let's see how to analyze

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We measure the ground value of the detection pin

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Let's first see where the detection pin goes, we light it up,

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and we can see that it goes straight to the CPU without going through any components

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If we measure the ground value to be normal,

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it means that there is no problem with the detection signal,

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and we don't need to care about it

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If the measured ground value is infinite,

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we can see if the connector is falsely soldered

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There are only two possibilities, either it has been repaired,

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or it is corroded by water, and there will be false welding

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We take the corrosion treatment, re-solder the seat, or replace the seat

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If it is not the problem of the connector,

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it means that there is a problem with the CPU

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We have to remove the CPU and re-solder it, and it can be repaired

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If we re-solder the CPU and measure the ground value, it still shows infinity

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We need to measure the signal point on the CPU to see if there is a ground value,

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or there is a problem with the board layer of the mainboard

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If there is a problem with the board layer,

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it is difficult to repair,

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basically, the machine can be returned to the customer

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Of course, in this case it is rare

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Here's how we can fix it if something goes wrong with the detection pin

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Ok, that's all for this lesson