Dreamcast lost imagen after a few second after poweron

[CARROMATO]

First of all sorry my bad english.
Im new in this site. Is my first post.
Need help with my dreamcast.
here a video showign the problem.https://youtu.be/NppIlKYMnX0

As you can see. the imagen lost after a few seconds.
Is no the fan, its works perfect.
The power supply is a pico cpu. and the power conector in the motherboard was clened .


Any can giveme a clue abaut were y must start to check ?
Thanks. !!!