If the engine is running fine, then it will be either the armature or field, or possibly the electrical wiring coming out. Whilst it is not running and you've fully earthed it, take a look at the wiring behind the output socket(s) {110v/240}, there is also a rectifier in there which may have perished. If everything is in order, then its possibly a problem with the armature or field. These are only repairable/replaceable in an workshop...
Luke M Moore, April 2010
