Hi, We`re new owners of an AP450 and didn`t think it would be a problem if our little grandkids played it. It looks like they`ve pressed some buttons and now one note is off. It has no touch response. Unplugging doesn't help. The manual suggests turning off backup, but even though we`ve read through the manual a few times, we can`t figure out how to turn off backup. Could someone walk us through it step by step?