I worked on a job in Tampa, that worked flawlessly for hours, and then would overspeed and crap out. Finally caught it with 2 meters. One on the output of the 7EA and the other one on the tach. Tach voltage just disappeared while dictation just kept humming.

Brushes looked good, no grounds, when spun with a drill motor Voltage was spot on.Coupling was good... Go figure...

I just know I changed the tach after my diagnosis and voila..No more problem.

Course we lost the job to a non-union company the next year due to low bid...

It Don't mean a thing if it aint got that swing.

Cool, Free, Johnny Smith courtesy of NPR..HERE
http://www.npr.org/ramfiles/asc/asc25.smith.asx