Anyone good with audio electronics or iPhone repair stuff? Something strange ...

[kári]

A couple of weeks ago the left audio channel of the audio jack on my iPhone 4S stopped working. There was no sound whatsoever coming out of it. I have no idea how it happened, there was no water damage or stuff like that.

Anyway, I had it repaired by a third party (since I didn't want to risk doing it myself). They replaced the audio jack thing with a new part.

I went to pick it up and everything was fine. When I came home however I plugged my headphones in and after a few seconds very loud static was coming out of the left channel (overlaying the music).

I went back to the repair place and they replaced the part again. When I went to pick it up again everything was fine, but when I came home the same thing happened.

I did some experiments and concluded the following:

- the static only comes out of the left channel, right channel is perfect

- the static disappears when the volume slider is above about 90% or below about 10% of the total range.

- when the slider is anywhere else after 20 seconds the static will reappear and stay until I slide the volume either to max or min, when I slide it back to normal levels the sound will be fine again for 20 seconds or so and then the static will return.

I went back to the repair place and they were as amazed as me, they have no idea how something like that could happen.

I hope anyone has any suggestions on what might be wrong.. The chance that both parts were defective is pretty slim, no?

(I also made a topic on ifixit about this: https://www.ifixit.com/Answers/View/115396/Very+strange+stuff+regarding+audio+jack+of+iPhone+4S. )

[Fiery Winds]

Sounds more like a defective audio chip within the iPhone rather than the jack itself.

[kári]

The guy from the fixing store said that the only other thing he knew of that could cause it would be a bad motherboard connection but that would result in no audio at all, rather that some. So he had no idea.

But it's obviously something.

[Fiery Winds]

Yeah, if it was all or nothing, or unpredictable, I'd bet on a loose connection too. But the fact that the problem is repeatable at two ends of the volume scale lead me to suspect the amplifier. At some ranges, it's fine, but at others, it's introducing noise into the process. 

I actually built a small stereo amplifier at university, and all the issues I had while building it were nothing as unique as yours. A bad filter capacitor would affect all ranges, and a poor connection would result in intermittent sound regardless of the volume.

[kári]

Yeah, my guess would be the amp as well. It would make perfect sense that it's the amp if there would be no static at low levels, and static at high levels of volume. But since there is also no static at max volume, I don't know anymore. Also the fact that it works for like 10 seconds and then the static comes seems very strange to me.