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Apple iPhone audio part mainly has audio decoding, U0900, ringtone (speaker), amplifier. Check iPhone schematic diagram on WUXINJI Dongle, we can see that this iPhone audio IC has 4 circuits, one is VCC_MAIN, which is 3.7V, the other three are 1.8V. After check iPhone power supply part, then we check the iPhone schematic diagram of iPhone audio channel section to check the direction of the audio signal.

First look at the normal call mode:

Two microphone signals LOWERMIC1_TO_CODEC_N_CONN, LOWERMIC1_TO_CODEC_P_CONN through iPhone tail plug flex cable and 17/18 pin of tail plug interface J1817, after the inductance FL1881, FL1882, finally through the capacitor C0922, C0923 send to audio IC-- U0900.  The 1.8V power supply signal PP_CODEC_TO_MIC1_BIAS that was sent from audio IC U0900, through FL1 and 21 Pin of interface J1817, after through tail plug circuit, the final supply to the microphone. This is the process of microphone signal.

The earpiece signal that from the audio IC U0900 K7,L7 pin through inductance FL1151, FL1152 send to 1/3 pin of interface J111, and then through earpiece (sensor and iPhone front camera) circuit line to earpiece. 

The audio channel for speaker mode:

the transmitter signal FRONTMIC3_TO_CODEC_N_CONNFRONIMIC3_TOCODEC_P_CONN through interface J111 and pass inductance FL1103, FL1101, finally send to iPhone audio IC, U0900.The speaker signal from audio amplifier U1601, and then through tail plug interface J1817, finally send to speaker.

In addition, we need to noticed that there is a another mode that the iPhone microphone is on the boot circuit when it in the state of the video. The recording signal through the boot line interface, and then after the inductance FL0801 FL0802 sent to U0900, which means that if an iPhone has other voice but the video was silent, then you can fix it by replacing boot line or boot line interface. 

If the iPhone is no sound at all, firstly check the several power supply circuit line on U0900, if the power supply is normal, then re-install it or replace the audio IC U0900 by using hot air gun.

If the sound of a certain circuit is abnormal, such as there was a handset, but no sound, or other handset no sound issues, we need to check whether the inductors or resistors that pass through the channel are corrosive, check earpiece circuit line or microphone circuit line. After excluding these, if the failure still exists, then you need re-install or replace U0900.

If there's a voice when it on the phone calls, but no ringtone and no speaker, mostly it is the iPhone amplifier IC problem. When face this problem, first exclude the tail plug line issue, measure the battery power supply of C1695 to check if it is normal, check inductance L1604 for damage, check C1630 1.8V for normal, after excluding above issues, replace U1601.

If no sound on the phone calls but the recording is normal, not only is the damaged or virtual welding audio IC can cause this problem, but the virtual soldering of the iPhone baseband CPU can also cause this problem.

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