Yamaha DTXpress/DTXplorer/DTXtreme

I may have a service manual but won't be able to check until Tuesday. A simple bad connection could give you the symptoms you describe if it has a microprocessor with external program memory - it depends on what is inside the box. If there is a socketed EPROM, try removing and reinserting it. Check voltages - I would guess it is 5V inside, the DTXpress is.

What diagnostic equipment do you have?

Regards 

Keith