One of my beloved 414 EB's - with the brass diaphragm ring, has developed a hum in the cardioid position.

I can use it in hyper cardioid and omni without any problems, but both Fig 8 and Cardioid are producing what must certainly be a ground issue.

I don't think that this is a diaphragm tension - related issue, as it sounds crystal clear in hyper cardioid and Omni... I'm thinking it could be the polarity switch ?

Does anyone here know anyone who works on vintage mics - and who can be trusted to not send this Vintage EB back to me with one of the newer teflon-ringed diaphragms?