Blow it through with compressed air first.
If still trouble (and assuming it an open type and not the modern miniature enclosed type)
you will have to determine where the partial short is taking place by placing a thin insulator and move it along vane by vane to isilate where the problem is. Then you will have to straighten it if possible.
The really old one's (not ali) are a
to straighten as they are very hard metal and contain magnesium I have been told, so don't use much heat.