I had a quick look at the specs for the Yamaha FB-01, thinking it might be something simple like a volume slider distorting what should be otherwise pure digital sound.
And...there aren't any analog controls on this thing what so ever.
Are there Chris?
Good luck with this one.
Oh, if you ever take it apart, let me know what's inside.
I still get a kick out of old Yamaha FM synthesis components.