Andy your question is not clear? Are you saying the body of the microphone and not the capsule has become noisy? Or are you indicating you simply plug the microphone into another mixer and it was fine? Removing the mic electret insert out?? What? The capsule is an electret capsule and not a true non-polarized condenser microphone capsule. The guts inside the body consists of a single FET transistor, a couple of resisters, couple of capacitors and a transformer. And then there is the battery holder. Sometimes having dead batteries and trying to power the microphone from +48 V phantom will cause noise problems. So make sure you don't have a dead battery sitting in the microphone?

The microphones are easy to disassemble provided you have jewelers tools. And you'll likely not be able to completely remove it because there are also switches embedded in the microphone body. So some of these things go through smaller assembly procedures when a button these things closed. Which may require you to un-solder before you can remove everything? My recommendation is to send it in for proper factory servicing since you obviously don't even have a schematic. So why try to repair a microphone where you don't even understand the proper technical jargon to use? There's more than one FET transistor in the world you know? And you need the right one. And generally, professional equipment, requires professional service and maintenance. Not newbie maintenance. I mean I will stop you from screwing up your professional microphone. It's yours to screw around with as you see fit.

So you have been able to establish that there is nothing wrong with the capsule as opposed to the microphone bodies active impedance converter electronics? I mean it could be the capsule? Or it could be the FET in the electronics? Or it could be a bad connection that's dirty or corroded? Or could have been a very humid environment and the electret capsule is shorting out as they are known to do on all condenser microphones in very humid environments.

Get the schematic and manual first
Mx. Remy Ann David

No, the Electret insert is I suspect faulty, I am a qualified electronics engineer, as I said, substituting my M66 with a friends M67 gives same results, and plugging friends M66 on my M67 works FB, so it proves the electret capsule is faulty. Yes, I am curious and just wanted to take a look as buying the electret capsule is fairly cheap, and I do not fancy paying somebody £60 / $95 an hour if I can do it myself.. I have tried to get service manual but no joy.
Thanks for your comments, btw, I fix ribbon microphones!
rgds
Andy

I assume you are talking about having swapped the K6(-P) powering unit on to your friend's mic, and that worked OK. It could be the power conditioning circuit inside the mic head rather than the capsule that is faulty. Since you claim electronic experience, it's worth checking the d.c. conditions inside the head before condemning the capsule.

Yes, good point, the DC o/p from the K6 powering module (sorry gave it wrong name before), is 7.50 v on battery, is this correct as I do not have access to the service manual.
rgds
Andy

That could well be right, but I don't have any service documentation for those products either. Does it give the same result on 48V phantom power?

Yes, and with friends powering module so must be ok..