According to the schematics, only the European model has multiple taps for voltage selection, so if you know it's the Japanese model and not instead the European model set to 100V then chances might be slim anyway. Might be easier to just find a replacement U.S. transformer instead of a step down. ...

That sounds like it's a Japanese region DX7. I'm not super-familiar with it, but I think at least some of those models have transformers which can be selected between 100V and 120V. I know that the European model is designed that way, so it could be that yours can be switched -- also, possibly not....