This has nothing at all to do with comptuers, and is probably a Laust question besides...
I discovered last night, after burning my hand on the back of it, that the JD-990 I bought second-hand a couple years ago is an AC 100V unit (probably Japanese).
Fixing this is easy - Inside the unit is a universal transformer. I'm just going to move the hot lead from "100V" over to "120V."
What I'm wondering, however, is if any permanent damage may have been done to the synth after using it for so long on ~120V current. I've never noticed anything out-of-the-ordinary, other than the excessive heat... but then, I've no basis for comparison!