Just to assure you that an external transfer works OK.I come from the UK and I temporarily went out to USA to live 5 years ago with a KN1500 240v UK keyboard which I used via a transformer(240v stepped down to 120v - 100w) until I purchased a KN7000 3 years later.
When I returned to UK I used the same transformer but the other way around (120v stepped up to 240v intake. I originally bought the transformer at Fry's and they have an internet shop - Outpost.com (Heavy item so postage could be expensive)I have never had any problems with my external "black box".
When I returned to UK to live, I did enquire about having the internal transformer changed and I would have been charged about 100 dollars for the part, plus a labour charge. However I stay with the premise "if it works Don't fix it" so I have left it as it is.