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Small current when iPhone is turned on

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In this lesson, we mainly talk about the maintenance process of small current

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Maintenance process step 3

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If the power supply and clock signal of the CPU are normal,

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we need to measure the reset signal of the CPU

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We first cut off the power, measure the resistance,

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and rule out short circuit and disconnection problems

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If the measured value is relatively high,

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it may be that the fuse of the circuit is damaged,

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or the board layer is broken

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If the measured value is relatively low,

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it means that the capacitor or chip connected to the line is damaged,

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If the measured voltage is low,

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it is also caused by the capacitor connected to the line or the chip damage

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Next, let me demonstrate how to measure the reset of the CPU

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Let's take the iPhone12 model as an example, we first find the reset measurement point

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In the bitmap, we can see that there is a measuring point under the mainboard

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We can also see that the reset signal of the CPU will also be connected to a resistor,

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which belongs to the reset pull-up resistor

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Its other terminal voltage is 1.2V, and the reset voltage of the iPhone 12 model is 1.2V

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Now let's measure it

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After the power is turned on and the switch is triggered,

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let's measure the reset power supply

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The voltage value is about 1.2V, indicating that it is normal

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Next, let's measure the reset diode value

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After the mainboard is powered off,

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the multimeter is adjusted to the diode level,

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the red marker is grounded, and the black marker is used to measure

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The measured value is 296, indicating normal

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The mainboard of the iPhone 12,

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its reset signal is sent directly to the CPU by the power supply,

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without connecting other chips

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If we detect a short circuit, we can only replace the power supply

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After replacing the power supply, if it is still not normal,

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it means that the CPU is damaged

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But in some models, the reset of the CPU will be connected with the reset of other chips

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At this time, we can't replace the power supply first,

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we can remove other chips and then measure it

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If not normal, go to replace the power supply

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If it is not normal after the power supply is replaced,

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it means that the CPU is damaged

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Maintenance process step 4

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If all the working conditions of the CPU are normal,

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but the current is still small when starting up,

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it may be because the 32.768M crystal oscillator is damaged

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we can replace it

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If it is abnormal after the replacement,

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it may be that the CPU is damaged or there is a problem with the bottom of the board

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At this time, we need to move the board and move the CPU to the bottom of the good board

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If it is still a small current, we can judge that it is caused by CPU damage

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So, this is the end of the maintenance process of small current