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This lesson talks about how to repair the failure of no service,

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that is, dial-up failure or a prompt to turn on the antenna when dialing.

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These failures are actually caused by problems in the 2G frequency band.

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The first step is to check the operator

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First judge and distinguish whether it is a transmission problem or a reception problem

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That is, the failure of the transmitting circuit or the failure of the receiving circuit causes no service.

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Find the operator in the mobile phone, and set the operator option here to manual search

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Its default is that this button is turned on automatically

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After setting it to manual, if you can search for China Mobile China Unicom China Telecom,

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it proves that there is no problem with reception

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On the contrary, if you turn off the automatic here, that is, when you change to manually search the Internet

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These items are not displayed here, then there is a problem with the reception of this machine

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But if there is no problem with the reception, and your machine has no service,

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it must be the fault of the transmitting circuit

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When there is no problem with the reception, the mobile phone has no signal,

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which proves that the transmitting circuit of the mobile phone is faulty.

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That is, no service caused by the transmitting circuit of the mobile phone

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This is the difference between receiving and transmitting

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After distinguishing and receiving and transmitting, it is necessary to proceed to the second step

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First visually inspect the transceiver channel of the 2G signal

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Are there any corroded parts of these components (that is, are there any missing components)

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But these components include chips and series capacitors or series inductors, or inductors with one end grounded

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The things to note here are:

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Mainly observe these chips that contain the channels that the 2G frequency band passes through

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Generally, it depends on the power supply chip of the 2G power amplifier and the 2G power amplifier chip

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or the chip containing the 2G power amplifier.

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That is, for some models, the 2G power amplifier is included in the low-band power amplifier.

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At this time, you have to observe whether the low-frequency amplifier has been corroded by water.

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It is mainly to observe whether there is water corrosion on the bottom of these chips and the surrounding area.

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If the chip is corroded by water, it is generally re-soldered first, and the faults of many models can be repaired.

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If the power-on test fails after re-soldering, consider replacing the water-corroded chip at this time

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Then there is this small component to pay attention to here.

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The dropped parts here usually refer to small component dropped parts

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It may be that this machine has been repaired by the previous family or by others,

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and some key small components have been touched.

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So which small components are critical

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It is a series of small components, these small components cannot be lost

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The easiest way for us to judge these small components in series is that both ends are not grounded,

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and they are connected in series in the signal channel.

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It is a component that is not grounded at both ends, and generally can be short-circuited.

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It's the kind that you don't need to find a small component

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with the same parameters to replace it and solder it

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You only need to short the pads at both ends of the small component

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Of course, here is a problem to pay attention to, what small components cannot be shorted

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One end is grounded and the other end is connected to the signal. These small components,

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whether it is a capacitor or an inductor, must never be shorted.

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After you short-circuit, it is equivalent to directly grounding this signal line, so this is where you need

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That's all for this lesson