Vortex Sound Research

Hi, so I recently acquired my dad's old TX81Z, and whenever I hold down a key for about 1 second, it plays the right note, but then it start doing some other note that seems like has nothing to do with the current one. I looked up a manual and didn't get anything out of it relating to this problem. Can anyone help?

​

BTW I am super new to this stuff so if it is a simple button press please don't be annoyed at me. Thank you! The keyboard I am using is a Kawai MDK61 II.