Your best alternative to the Step down route, is to purchase a Buck/Boost transformer to 'buck' the 117/120 voltage down to 100 voltage. It has to be matched for wattage and amperage to the Yamaha unit. I use one for my Accuphase units. You could wire it yourself,(or ask an electrician) all come with instructions in the box
and the transformers can be found on Ebay all the time.
and the transformers can be found on Ebay all the time.