After motherboard-replacement no reaction from the upper screen

Hi,

I bought two broken 3DS (eBay) to assemble a working on. The first on (in white) got damage by cleaning it with to much water. When it arrived I started checking it by pressing the power button, no reaction. Then I put the power-cable in and it started charging the battery. Again pressed the power-button and it worked. Both screens turned on and the menu appeared. The lower screen had white blotches because of the water-damage. So I thought it is a battery-problem. But after turning it off I couldn't turn it on again (no blue LED), even after hours charging the battery no matter with or without the power-cable. So I disassembled it and checked the fuses, but both showed continuity.

So I bought a second damaged 3DS (in blue). It was broken into two parts, in a upper one with the 3D-screen and the lower one with the touchscreen and the buttons.

My plan was to replace the battery and otherwise to replace the motherboard in the white one. First I turned the blue lower part on. The blue LED turned on and the screen flashed for a second. Also charging worked. After replacing the battery the white one worked again. So I just changed the lower LCD with the blotches. Unfortunately the cameras didn't work. But the bigger problem was, that sometimes I could not turn it on. No matter which battery is inserted or the power-cable. But both batteries can be used to turn the blue 3DS lower part on so they have to be both ok.

So I replaced the motherboard of the white one. But after pushing the power-button only the lower screen flashes up and no reaction from the upper screen, only a short noise from the speakers.

But after replacing it with the original motherboard it booted as it should. The 3DS was working again. After putting it in standby-mode it turned itself after some minutes off. But I could turn it on. Everything seemed to be ok. But the next day, I wanted to turn it on and again, no reaction. Again I replaced the motherboard and again only the lower screen flashes. I also tried to clean the contacts of the ribbon-cable for the upper screen but with no success. There seem to be some oxidations causing the problem, that the upper screen do not turns on.

Now I have no real idea what to do to get a working 3DS without waisting to much money.

I hope somebody can help.

BTW: The upper part of the blue one is of no use, because all the ribbon-cables except the one of the speakers are broken.