Phone not turning on.

Hello ifixit Community,

My trusty old GT-I9100 sadly stopped working today. Read the news on it this morning, switched it off and went under the shower. When I came back the display kept dark. I was late anyway and so just let it at home the whole day.

Back at home I tried the usual. Remove battery, wait, put it back in and power on. -Didn't work

So I replaced the battery with another one I had laying around and still no sign of life.

Maybe it was broken, too? Luckily, I had another used GT-I9100 at hand and tested both batteries: The first battery seemed to be broken indeed, the second battery was fine.

I tried it once again with the second battery. Phone did still not start.

After some research I found a bunch of other guys having a similiar problem. The fix was to break out a capacitor on the phone's mainboard. Since this capacitor was only needed for Functionality like USB-Tetherhing anyway, I gave it a shot and removed it. But nothing changed.

OK, maybe the phone was never broken the first place, but the display was? Since the Galaxy S2 has no status led's whatsoever, it could have been just the display. Again pulling out the second Galaxy S2, I connected the display of my broken phone to the mainboard of the working one. And apperently the display works.

... and at this point I am running out of ideas. The only (horrible) option left is that something on the mainboard broke. Which usually means that the whole phone is dead.

But I am not giving up yet. This phone, my very first phone I got for my birthday over six years ago, accompanied me through tough highschool and (thus far) awesome college. It is (was) running on its third battery, second back-lid and, thanks to Cyanogenmod, Android 6.

So is there anything left to try? A way to identify the problem on the mainboard and sovle it? Or maybe the error lies somewhere else where I haven't checked yet?

Any help appreciated - thanks in advance