Protect circuit activated........where to look for defects?

Typically a shorted speaker output. CHeck where your speaker wires are and see if there's just one wisker of wire touching the adjacent terminal. Check the back of the amp (easier said than done) and then check the back of the speakers themselves. Second most common issue is overheating. People leave an LP covering the ventilation openings on top of the amp and wonder why it overheats, so make sure you have air ventilation. Speaker wiring, then ventilation. Unplug unit, plug back in again, and power on and see if your problem goes away; if not, come back and we'll see what else we can try for you - okay?