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Turn off your unit at the power button. Check speaker outputs for a short circuit - also check other end of speaker cables for a short. If you can't find an obvious problem then disconnect all speakers and power up the unit. If all is OK then connect speakers one at a time (this should be done with the unit powered OFF) and power up after each conection. This should identify which output is causing the problem.
If that does not work then it's an internal fault that will require a service engineer to identify - but I hope that is not the case!