I don't have the book on the SC-802, but the instructions and function diagrams on the Sc 710, Sc 811, and SC 815 do not show a Loudness button ( although I remember it as being there on many of the earlier series of head units). This function may have been taken over by the ASC (active sound control) function, which increases the volume of the radio depending on the speed of the vehicle, and is controlled by a lead to the speedometer. If you have that function, you should be able to activate or deactivate it by pressing on the "cd" button--"ASC" will show on the screen. To a certain extent, the "loudness" function is also performed by the "dolby" circuitry in the radio; there may not have been any need to have a separate "loudness" function on the newer radios because of the improved dolby set-ups. Sorry I can't help more.
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We have met the enemy and they is us. [Pogo] S70 cop car : Rough Rider tires& suspension, Walmart fog lights, eBay speakers, ambiance by Pall Mall, trim by Le Duc d'Tape, 8-channel THD by OEM amps