Libra colour bricked

Hi,

I bought a used kobo libra colour which was stucked on the home screen.
I tried several reset, soft and hard, tried to unplug and reconnect the battery, without success.
I put another battery in it (from my personal working libra colour) and it finally shows an empty screen then a yellow one. If I reboot it, same pattern, and it ends up to a yellow screen, without text nor image.

It is not detected by my computer.
I guess it is a firmware issue, but the motherboard being without sd-card, how to solve it ?

Do you have any idea ? :help:
Many thanks