Mr. Brotherton, my thanks for your perseverence. I know you had better things to do on a summer Saturday afternoon than to educate us about OVPs!! As promised, I'm through! I will however, monitor this thread in case someone else asks a question like "which terminals do you bridge to test the OVP? As best I remember, the only other answer was wrong and the student that submitted it had to stand in the hall for 15 minutes!!:p :p