Hardware Old 3DS c-stick not working top-bottom axis

[yaum]

I have this old 3DS XL which had multiple problems preventing it to boot (speakers not working, broken digitizer) that I fixed.
I happily tested it, everything seemed back in order, but I noticed that the cursor was circling down for a few seconds just after booting it. After 5-10 seconds, the cursor just stops.

I searched for a few seconds and discovered that the C-stick is responding only right and left, and not up or down. Easy fix, I thought: replace the C-stick, and voila!

Few minutes later, exactly the same behavior with a known working C-Stick: cursor keeps moving down for 5-10 seconds. No response from C-stick on a vertical axis,only horizontal. It is working both axis with the pad however.

I know that if you boot without the stick, cursor moves right for a few seconds, and it felt like the system is doing the same (as if detecting only partially the c-stick).
I decided to clean the contacts: same behavior.
I unplugged the c-stick and booted again: I expected cursor to move only right. Cursor moved diagonally down and right before stopping after few seconds!

I'm a bit lost now on how to proceed with this one... Any ideas on what could cause this ? A faulty contact?

Thanks!

 

[IC_]

Yes, it sounds like a bad connection to the vertical axis. Do the connector and other things around it on the motherboard look fine?
Are you talking about the small C-Stick on the right side that doesn't move or the normal big circle pad on the left side? Both of them probably send the signal in the same way, but the old 3DS doesn't have the small C-Stick on the right.

 

[yaum]

The connector looks ok, no damage to it. You can have a look to the following pictures top & bottom.

Of course, I'm talking about the circle pad, sorry about the wrong term...
What's bugging me is that cursor moves down and then stops... and that it modifies the normal behavior when I disconnect the circle pad..

 

[IC_]

What's bugging me is that cursor moves down and then stops... and that it modifies the normal behavior when I disconnect the circle pad..

I'm understanding that the circle pad moves down and stops when it's connected, and moves down/right and stops when it's disconnected? That's normal behavior, the 3DS will see the maximum values from the circle pad if it's not connected which causes it to move diagonally for a few seconds until some part of the software or hardware realizes that this isn't right and stops it. In your case the horizontal axis still works normally but the vertical does that behavior so it sounds like the vertical is not making a proper connection but the other pins are working normally.

 

[yaum]

By any chance, would you know which pair of contacts are responsible for vertical input?
I'm going to try sticking a bit of paper above the ribbon to force contact on the pins tonight.

 

[IC_]

By any chance, would you know which pair of contacts are responsible for vertical input?
I'm going to try sticking a bit of paper above the ribbon to force contact on the pins tonight.

There are 4 pins on the connector, 2 for ground and 5V, and the 2 other ones for vertical and horizontal input.
You can see a pinout of it in this picture: https://bitbuilt.net/forums/index.p...k-gamecube-joystick-finished-diagram-png.125/

 

[yaum]

Haha, problem solved thanks to a tiny bit of paper! I didn't think it could have been this with only one direction failing...
@IC_ Thanks for your advice and for the pinout picture!