One of the most important skills a sound engineer should develop is troubleshooting. Sometimes it is even more important than the ability to fix the problem. If you can first isolate and then eliminate the problem either by fixing it or just replacing the problem piece with a do-fer until there is the time and/or budget, to correctly fix the problem. A good basis for troubleshooting audio is following the signal path of the sound. for example: CD Player | Cable | Sound Board (input) | Sound Board (output) | Cable | Amp | Cable | Speaker Now generally a good palce to start is the weakest link. And in sound systems that is usually the cables. They are constantly abused and tend to develop problems faster than anything else. But on your average channel of sound you are dealing with three different cables. Which one is dead? First of all both the board and the amps usually have some sort of input signal. Sometimes it is an analog needle and other times it is a series of lights and other times it is just one light that lights up to different intensities as the signal becomes weaker and stronger. So with this knowledge is the board reading some kind of input signal? If so move on to the amps. Are they reading an input signal? If the board is reading a signal and the amps are not. Then start by replacing the cable to the amps. Another thing never to overlook is the basic stupid stuff. Forgetting to power something up or plug something in are two of the leading causes of problems. And sometimes people feel inclined to unplug something for no apparent reason. Trust me nothing makes you want to slap your head harder than spending two hours tracing down the source of a problem, then you discover that it wasn't turned on.