Why does CAP5 protection keep activating on my 851A when I turn the volume up?

If the amplifier is continually driven too hard, the amplifier's protection circuitry will kick in to prevent damage to either the amplifier or the speakers.

The protection circuitry monitors things like clipping and temperature, and if you continually drive your amp into clipping or into high temperatures then the protection circuitry will activate and either turn the volume down automatically or switch the amplifier off before any damage occurs.

If the CAP5 protection continues to cut in, it is advisable to check your speaker connections to ensure that there are no loose strands that could cause a short circuit.