After replacing the cracked display on a customer's smartphone, you place a test call. The caller reports complete silence, and a short voice-memo you record immediately afterward also contains no audio. The device's speakerphone plays music and ringtones at normal volume, and calls made with a Bluetooth headset work fine. Which internal part was most likely left disconnected or damaged during your repair?
Because both the voice-memo app and standard voice calls pick up no sound, the problem is with the handset's audio input path, not its speaker. The fact that speakerphone audio and a Bluetooth headset work proves the cellular radio, speakers, and software are functioning. The component common to direct handset input is the primary microphone assembly (often a small microphone on a flex cable or daughterboard near the charging port). If that cable is pinched, torn, or left unplugged during a screen replacement, callers will not hear anything and voice recordings will be blank. A faulty earpiece speaker would affect audio output, not input; a proximity/light sensor controls the display during calls; and a vibration motor only provides haptic feedback. None of those parts would stop the phone from capturing audio.
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What does the primary microphone flex cable do?
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How can damage to internal parts occur during a screen replacement?