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This lesson is about the maintenance steps of the unreadable SIM card

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After excluding the above 3 steps, it is necessary to proceed to the fourth step

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Install the SIM card, do not need to turn on the mobile phone,

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to measure the ground value of the detection signal of the SIM card

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What you need to know here is that after the SIM card is installed in the mobile phone,

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the card detection signal will no longer be grounded,

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and there will be a normal ground value, generally above 300

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In the real thing, this copper piece is its detection pin

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When the SIM card is not inserted,

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the detection pin is in contact with the grounding shrapnel

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Look at the picture on the right

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The shrapnel below is the ground wire, and the shrapnel above is actually the detection signal

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When the SIM card is not inserted, the two shrapnels are in contact,

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and the detection signal is actually grounded

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Look at the two pictures on the left, when the SIM card is inserted,

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the ground wire and the detection signal are disconnected

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Check out this enlarged image

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This is the detection signal, and the one below is the ground wire

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The shrapnel of the ground wire is pushed away, and the detection signal is no longer grounded after the push-off,

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and it has a normal ground value

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So when we measure the ground value of the detection signal, we must insert the SIM card

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If there is no abnormality in the ground value measured, proceed to the next step

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If there is a short circuit to ground, first remove the capacitor or diode connected to this line

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Generally, diodes are rarely designed for this circuit.

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After excluding the above components, consider the chip that the detection signal leads to

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So where does the heartbeat go?

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Generally it leads to baseband CPU or big CPU

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When it is still short-circuited after removing the capacitors and diodes

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it is necessary to re-solder the baseband CPU or large CPU

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If there is an infinite OL, it is an open circuit

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At this time, first observe whether the seat has been welded, and there are false welds

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If it has not been soldered, you need to re-solder the baseband CPU or the main CPU at this time

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If it is invalid after soldering, basically either the board layer is disconnected,

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or the CPU or baseband CPU is damaged

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If the CPU and baseband CPU are damaged, it is very serious and basically irreparable

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That's all for this lesson