I work as a technician on a small stage and often lift these installation amplifiers after independent adjustments. Here are the instructions describing the modes and indicators protect https://manualmachine.com/bellaudio/pa2022/9374031-datasheet/ . Bell has a very sensitive protection system - if you select the wrong bridge/stereo or speaker impedance, it starts up but does not transmit a signal. Oh, and check the input sensitivity and low-cut: when the switch is not in the right position, it seems like it's dead.