I just got a Casio CTK 2400 last night and I noticed my right earbud on my headphone wasn't working. I tried various headphones and they still didn't work. I tried the stereo standard plug (the adapter) on my guitar amplifier and it was working. The headphones I used were the ones that came with my Apple phone. I would like to fix this problem asap so please don't hesitate to help me out.