The typical cause of this is a blown tweeter or the tweeter has broken off of the horn and fallen inside the cabinet. Remove the screws from the horn and carefully remove the assembly. Try the speaker at low volumes to see if you hear any sound at all. Next, remove one of the wires and using a 9Volt battery quickly touch the 2 leads on the tweeter. If you don't hear a click, the tweeter needs to be replaced. Contact Mackie service for a replacement.

If you hear a click, then the problem is in the crossover or amp and further investigation willbe required.

Dan