No speakers makes it a bit more difficult to diagnose, but not impossible. If you're playing a Tyros, there have been some reports from other forums of cold solder joints at the output jacks. If it drops out again, try gently wiggling the plug right at the output jacks and see if that solves the problem. If it does, it's a cold solder joint and can easily be repaired.

If this does not solve the problem, try turning the volume control back and forth and see if this changes things. If so, bad master volume control would be the likely suspect.

If neither solves the problem, there's always a possibility of a bad cable going to the amp, but if you're firing into a pair of Barbettas it's unlikely that both are defective.

Again, I'm working in the dark on this one, but hopefully you'll find the culprit and it won't be an expensive repair.

Good Luck,

Gary