New 3DS XL turn on with black screen, damaged flex cable

Hi, recently I've bought a new 3DS XL from ebay sold as broken. The seller couldn't tell me what happened, but since the price was right, I decided to give a try. The console charge battery and turn on without any problem, I can hear the background sound of what I think is the menu (but it seems unrensposive to any button), but both the top and the bottom screen are black (also the backlight). This initially led me think to a softbrick, so I tried using a flashcart with ntrboot but it didn't work. Then I tried entering in safe boot mode, but the console shut off itself with a pop noise from the speaker and a very rapid flash of the bottom screen backlight. I started to think that it could be a hardware related problem. So I opened it and found something strange: the two water sensitive sticker were ok (image below), but I found corrosion in the camera flex and speaker flex. After cleaning it, I discovered that the corrosion had destroyed at least one pad in both of the two cable. The corrosion was also in the ZIF connector, but using alcohol I think I have removed: using multimeter to check continuity it seems that there is nothing destroyed.

There are no sign of liquid damage in other section of the motherboard (althought the console - I think only the bottom part - was surely opened by someone else in the past, one screw was missing and the top screen flex cable was disconnected and I don't know what have been done) and the console itselft it's pretty new since it has been bought last year.

Do you think that the simptoms could be related the missing pad on the flex cable (image below)? It's however strange since there are no problem to the bottom screen flex, but the screen does not turn on.

There are some tests I can do to check if there is other issue or to check if motherboard is working fine?

 

I've managed to repair the flexs damaged by corrosion, now the 3DS fine.

The thread can be closed.

 

Can you please explain how you were able to fix the flex cable? I'm really curious especially because it has to fit in the connector.

 

Well, as you could image by viewing the pics above, the fix consist in bridging the gap that the corrosion has caused. So, as first thing, I had to remove a bit of the flex's plastic coating to expose the copper of the interrupted track, then I've "simply" bridged it with solder. I said "simply" because the contacts are indeed very small, so I don't think it's something that can be easely achieved by someone who is novice at soldering. I've some experience, but I'm not a professionist and I know that I had a bit of luck managing to fix (that's why I hesitate at the beginning). The risks of damaging the flexs are high, infact while I was repairing one of the two flexs I've accidentally broken the little track between two contacts and so I had to rebuild it using a 0.1mm copper wire. After this first attempt, however, I had some concern about the thickness of the wire that could cause issues fitting the ZIF connector. You can see it in one of the pics I've attached to this post. So, using tweezers I have flattened the wire (and refined soldering) with a satisfactory result (though I've not taken a picture with the microscope to show you). This way I had no problem fitting the flexs in their connectors and, as said, the 3DS is resurrected.