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Clipping does not mean shorted speaker connections. Clipping refers to the cutting off of the tops of the output signal waveform (as viewed on an oscilloscope) and resultant distortion when an amp is driven into overload.
Martinluke, if the problem is not shorted speaker wires, then the receiver could be going into protection mode due to some internal problem. Unless you have troubleshooting and repair experience, IMO a service manual is going to be of little or no help to you.