Stopping the Sony Beep

I've got my Sony head unit installed and it works great. Its not a new unit,
few years old now, model number XR-5800R, and I don't have a manual for it.
When you press buttons on the unit or the remote a loud beep is heard from
the speakers. I know for sure that you can kill the beep on newer models
using a shift button (which I don't have) pressed together with a preset
number. I'm thinking that there must be a method for older models too,
possibly holding a button at power-on?

If anyone has any ideas I would be most grateful. Better still would be if
someone knows where to get old Sony manuals!

Many thanks

Mark