A few simple suggestions. Check for a shorted or loose battery cable. That can cause a pop when it arcs.
Check the starter leads. Check the starter motor relay. Check the regulator/rectifier. Possibly a bad starter.
My first inclination is that the starter motor relay is bad if it pops under the seat when you hit the starter button. It has it's own 20a fuse. You should be able to get it tested at a parts store or at the dealer.
Here is the electrical stuff at Ron Ayers:
Electrical
Good luck!