Hardware Blue light and then off New 3ds LL (Jap. XL) No CFW

[Idrolitina]

Hello everyone!
As for the title, I re-assembled a n3dsLL (xl Jap model) and I have a blue light and a few secs later dead.

Little details
:
A friend gave me his N3DS LL after playing around to fix it (and failing). He broke the flaps that are not flaps (lol) on the MB, the small one of the home button, mic and similar.
Also, it had a fried control pad so it did not turn if not plugged in. But in general, it worked plugged in for the exception of the home button and those ones.

I had mine here with a broken display so I wanted to swap the motherboard and control board.

I did the swap.
However, I am simply not able to understand where I messed up. I have a blue light for a few seconds and then it turns off. The wifi antenna is a bit loose but for the rest, I cannot see problems. There is a small dent in the video cable but nothing serious (pic n. 6).
I suspect this is about cable positioning since the 3ds was working with no error when plugged in before was only the screen. So I reassembled it other 2 times with no luck.

Also funny story, just to test I reassembled his n3ds and it works exactly like it is supposed to (no response when not plugged in and dead home button etc.)
I will post a pic here of the Motherboard hoping one of the many that knows more than I can take pity and save me m(_ _)m !

Please let me know if you need to know anything else.
Thank you for reading!

https://ibb.co/0JwtV75
https://ibb.co/8B37yT0
https://ibb.co/Th1kgdn
https://ibb.co/BnwPbGK
https://ibb.co/wdyM8GX
https://ibb.co/HpC34Xh

 

[IC_]

Do you hear any popping sound from the speakers or do you see one of the screens quickly flashing white when the 3DS shuts off?

 

[Idrolitina]

Thanks for the answer. I tried to carefully listen.
It pops but not clearly like others I have seen.

 

[IC_]

Also I am a bit confused by your post, you got a 3DS that has a faulty motherboard but working screens even with that dent in the screen cable, so you tried to put in a working motherboard from another 3DS with a damaged screen but it does not work like that? Is that right or am I not understanding it?

 

[Idrolitina]

Exactly, I swapped motherboard and controls pad that were not working with some that worked perfectly. Sorry for the messy explanation.

 

[IC_]

Does this working motherboard still work when put back into the 3DS it came from?

 

[Idrolitina]

thanks for the suggestion. After re-assembling again also both 3ds MBs, the same problem emerged on my 3ds despite the motherboard. I supposed it was the cable of the upper the display.. but it is apparently the 'good 'motherboard... I did not touch anything in particular... any idea of what can cause this by the mb? I suspect something really noob like bad cable positioning at this point

I also I tried to hear better, again and again, the sound of the turning off pop is almost inaudible, I am not even sure if it is not only the sound of the display turning off..

Can one of these poor 3DS be fixed in some way?

 

[IC_]

I guess bad contact of some cable is the most likely problem, but it is also possible that you accidentally damaged something on the good motherboard. Make sure all the screen and speaker cables are correctly inserted.