No matter what electronic products are repaired, the first step is to understand the structure of the machine, and then differentiate the faults. For example, Apple iPhone is divided into two main parts:
1. AR part is the boot part.
2. BB part is baseband part, which belongs to the phone call part.
Now the fault of this iPhone 5s is it can boot, just sometimes it can call out, but cant receive calls. We can determine the fault is in the baseband section and then start measuring.
1. Enter *#06#, we can see IMEI number, which means the baseband has no problems.
2. The problem of reception and launch is generally RF power supply.
3. We can detect the SIM card, which means receive part is normal, now we need to test the launch part.
4. Open the schematic diagram, and then find the RF power supply IC U11_RF. As shown in
picture 7-7-1 and 7-7-2.
Find this chip on iPhone 5s motherboard, as shown in picture 7-7-3.
Measure the power supply and output of the chip with multimeter. Fist we measure the PP_BATT_VCC_CONN on C1215 capacitance, it has a voltage. But PP_PA has no output voltage, so we suspect the RF chip is damaged.
This chip can be replaced or directly connected (solder the jump wire between two pins under the stereo microscope). As shown in picture 7-7-5 and 7-7-6.
After soldering jumpwire, assemble the iPhone 5s and test it, everything is normal.
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