Welcome to our community,
@Th.B.
As Max has pointed out, it seems you have no signal coming into the mixer. So your signal chain into the Model 12 needs to be addressed before considering the Model 12's internal signal flow.
Since you're brand new to the Model 12, we need to start with the basics, so it would help if you would confirm for us:
1. If you've followed the Owner's Manual (OM) steps in Section 5, "Basic Recording";
2. If you've looked over the OM Block Diagram's signal routing paths; and
3. If you've consulted the "No Sound is Output" segment found in the Owner Manual, Section 16, "Troubleshooting".
Having that information from you at the outset provides us with a way to go forward if you've already consulted/followed that guidence with no joy; and saves us the time of repeating what's already available in the OM, which us where we would otherwise need to start.
So, the more information and interaction with the community, the better the likelihood we can help you get to making music.
