Usually, a 60-cycle hum is caused by a ground problem. However, if you haven't done anything with the wiring, it's possible that a filter capacitor has failed in the converter. Unfortunately, unless you have some electronics troubleshooting experience, you may have to have a tech take a look at it; and that could be costly, even though it's not difficult to fix.
If you have been messing with the stereo or converter wiring, it's possible that you either left something disconnected (open ground) or you have two leads reversed. Sorry, I can't be more helpful; but a stereo shop or a friend with some electronics background can probably figure out what's wrong.