I have a "96 1100 Yamaha Waveventure in which the

take out the batteries for starter.

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This sounds like a loose connection at the starter, Follow the positive battery cable down to the starter. Check the connection for being hot (rusty color). There are two large connections on the starter check both. If one or the other looks discolored the starter may be damaged. Be sure the engine is not locked up. Try to turn the engine by hand at the main crank bolt.

That's the thing with today's electronic and why warranty length is so important. Where did you buy your keyboard? It's strange that they won't fix it if it's still under warranty. My Casio CTK-2100 came with 1 year free service and they fix one key that stopped function in few days. I'm pretty sure Yamaha has the same warranty length. You sometimes have to be very persistent with them though...

Those are the typical symptoms of a weak battery. Try either to charge the battery, try a jump start or replace the battery. Lights or horn working do not give an indication of the battery charge status.

you could be right about the starter relay, start with a test light and check the power at the relay. then if so check power at the switch if good make sure that the power is getting back to the relay if so replace your relay. let me know what you find and if you need help

Check all ground wires and make sure they are tight. Start at the battery, starter, solenoid switch.

The starter circuit is completing somewhere. Have a look at your start button and/or solenoid. It could be mucky contacts, if so give them a clean.