My iPhone 4 keeps shutting down / restarting (stuck on the apple)

Question

Hello everyone,
I've got a serious problem since yesterday.
Everything was going nice on my iPhone 4 (bought in 2010) and in all of a sudden my phone shut down and since then no way to start it back up.
I think I've tried everything. First off I told myself maybe that was the screen that gone to Heaven, but I asked someone to call me, nothing, busy, as if the phone was off. Then I tried using the vocal assistant to make sure that was really not the screen, to make sure I couldn't hear something through the speakers, but nothing. First conclusion : the phone is off.
Then I thought my battery was gone (the battery was loaded by 85-90% at the moment it shut down), but anyway I took my girlfriend's battery (she's got the same phone) to make sure the problem was not the battery, so I switched them. My battery works perfectly in her phone, hers doesn't in mine.
Still black screen. Impossible to start it either by the power button or by the power+home buttons.
Then, I decided to plug it in my computer to check if the things could get better if I link it into iTunes. And YES : the apple appeared on the screen the second after pluging in the USB cable. A few seconds later : the apple disappeared, and reappeared another few seconds later. And since then, that's the best I could get. This apple keeps appearing and disappearing on the screen, without any way to start it up.
The SECOND I take the USB cable off the phone, the screen goes black and no way to start it up as well, just like before (as if the battery was totaly empty). I plug in the cable another time, the apple comes back in, and disappears, and so on. I've tried several other cables, same result.
The apple appears when I plug it in the computer, and also in sector. But without any cable, no way to make it move, even a little.
So I tried anyway to put it in DFU mode, I could make it. I could hear the USB connection sound in the computer. iTunes has found it in Recovery Mode (but it doesn't find anything without DFU mode).
I'm then trying to restore the phone, iTunes downloads the latest iOS version, everything goes well, it goes extracted, it begins to restore, then I got several errors (1600, 1602, 40xx....) everything saying me it cannot be restored.
I've searched many hours on the net, I could figure out that during the restoring operations, iTunes shuts the iPhone off to start it back to do whatever it wants. I'm pretty sure the problem comes from here, because at the second you cut the USB cable power, the phone goes off (as well as the DFU mode).
- Some of you will probably tell me it's coming from the power/usb dock ? I bought a new one several weeks ago, changed it myself and worked perfectly (I changed it because the home button wasn't working anymore, but this problem was fixed).
- This is NOT the battery, because it's working fine in my girlfriend's phone, and hers doesn't in mine, just like I said before.
- I don't think it's a software problem, since I haven't done anything special, I was just messing around the web and texting when it suddenly got shut off.
I've disassembled the whole phone to check if there was a connector unplugged, to check for dust, nothing. As I said, it was working perfectly.
The phone NEVER got in contact with water.
So my other conclusion is : nothing happens in the phone if no USB cable is plugged in. The apple appears when plugged in the computer, and when plugged in sector. But when it is unplugged, radio silence.
So.... since I can't find a way to start it, since I can't find a way to restore it.... I don't know.... I'm now thinking about the motherboard.... But how could I make sure that's it ? Have you got any idea ? Please help me !
Sorry for the novel.
Thanks in advance for your help.
Phil

Answer 1

I'm not looking for any warranty help or something, I just want a diagnosys of my phone's situation....
My phone is acting as if there was no battery in the phone. When there is no cable plugged, nothing happens. When there is a cable, something happens (not a lot though....).
Could it be the battery port on the motherboard that's down ?
PS : I've taken a Genius-bar appointment because I can't stand in that situation for years, so I'm not absolutely looking to gain money and avoid Apple's official help.