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This lesson is about maintenance of unreadable SIM cards

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The first step is to rule out the cause of flashing

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If the mobile phone has been flashed,

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it is necessary to re-flash the mobile phone into the original system package to troubleshoot.

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Sometimes some systems will cause the mobile phone to not recognize the card.

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The second step, if the phone has not been swiped through the system,

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then check the SIM card seat

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The card seat is welded on the tail plug small board

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Like the socket on the left in the picture, it is welded on the tail plug small board

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In this case, the tail plug needs to be replaced

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After the replacement, the mobile phone can read the card normally,

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which means that the failure of the small board in the tail plug caused the card not to be read.

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If the card is still not read after replacement, you need to proceed to the next step

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The third step, after replacing the tail plug small board,

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if it still doesn鈥檛 work, or the card seat is welded on the mainboard,

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we need to measure the clock, reset and ground value of the data line of the SIM card

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It should be noted here that

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the multimeter should not touch the shell of the card slot around the hole when measuring,

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because this shell is the ground wire

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For example, in the picture, be careful not to touch the white iron piece when measuring,

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because it is the grounding pin

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Measure the ground value of the pin

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If OL appears, you need to re-solder the chip to which the pin leads

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Generally, these pins lead to the baseband CPU or large CPU

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If the measured ground value is zero, the anti-static diodes or capacitors on the line should be removed first,

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especially for machines with water ingress, and they should be removed first

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Remove and measure again

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If it is still a short circuit, re-solder the chip that the pin leads to,

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which is actually the baseband CPU or the big CPU

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If there is still a short circuit after re-soldering, the fault is irreparable

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If there is no short circuit after re-soldering, the fault is repaired

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If the ground value of the above pins is not abnormal, you need to proceed to the next step

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The ground value of these pins is generally above 400

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That's all for this lesson