Hello Minorkey,
Yes, that is what I was talking about. The "D" cells were there for a reason: Hi current requirement. The nine volt battery you want to install would not last a minute if you try to power the keyboard with it.
The current handeling abilities of the "D" cell are around .5 to 1 amp.
The current handeling abilities of the 9 volt battery: 20ma (ie .02 amps)
It wont work. If your keyboard has a power input jack, than get yourself a power supply and use that instead.
The other sugjestion would be to contact Yamaha, check with Tech support and see if your problem can be taken care of and how.
.........................................Pat