Using the site referenced above, I tried conversions of several tunes from Jack Campin's "chalumeau.abc" with little success. Since I'd trust Jack's abc transcriptions more than most anybody's, I have to suspect the converter's at fault.

Although it involves additional steps of converting an abc file into midi, opening in NWC and exporting the resulting score to nwctxt format, I've had good luck creating musicxml files with http://nwc2musicxml.appspot.com/