I am guessing that you are using something like this? http://www.world-import.com/tc-3000a.htm#sthash.cqKkiUKm.dpbs
Pretty much every "World voltage converter" I have come across is a highly under-rated piece of sh*t that uses far too thin wire for the winding's along with paper insulation.

Use a REAL 120/240V industrial rated transformer. Yes they are about 2-3x the cost but they are bulletproof. Proper size wire and core construction, generally potted in resin, UL-rated, etc...