Hi there, I have worked on these Mackie units, and I would say the supply is going into protect mode, due to a short in the power amp section. They usually use a digital amplifier IC as the output, and these go down very easily. Not easy to replace unless you have a rework iron with a heat gun unit to extract the old one, even then be careful of board track damage. Best leave this job to a pro workshop, sorry, probably not what you want to hear. I am not a fan of these digital power amp modules, or the switchmode supplies used in musical equipment these days, nut they are lightweight, which is what the consumer wants, and cheaper to produce, less copper etc that a conventional transformer, but more difficult to repair when they turn up their legs so to speak.