I changed the battery quickly in my Sennheiser ME64 and put the battery in the wrong way. When I next turned on the camera (with the K6 capsule actually not even switched on) I got a terrible HISS that I haven't yet been able to resolve.

I've taken the battery out, turned it the right way around and put it back in, but the hiss won't go away.

Please don't tell me I cooked something ...

I've attached the .wav file here if anyone can listen (it's only 7 seconds).

Strangely enough, the hiss disappears when I unscrew the K6 capsule almost all the way off. It's only when screwed in halfway or more that there's hiss. Some connector ... And perhaps a call to sennheiser.

Try cleaning all the contacts, those mics are VERY sensitive to corrosion and humidity with the change of seasons. Use of a spray electronic contact cleaner is often good. Been there done that....
Can you run the mic on phantom power rather than batteries in your use?

Other mics that is suffers similar problems are AKG 451's (with the fine connection thread) and Sony ECM lapels

It could be dirty contacts between the capsule and the K6 as Brian suggested, so definitely use a good quality electronic contact cleaner and a lint-free cloth that you haven't handled much with your fingers.

Clean the contacts first, then clean the threads inside and out.

The K6 switch only activates the battery, so if you got a loud hiss without the switch being on, then the input to the camera may have been phantom powered. In other words, the mic works even if the switch is OFF but it's receiving phantom power.

You should also remove the battery completely and test with a known good phantom powered input.

And replace the battery completely too, as well as clean the battery contacts inside the K6. I also try to be careful when installing a battery to clean the battery's contacts first and then not handle them as I put the battery in.

Good luck, hope you can get it solved.

Thanks guys. There is a bit of corrosion on one end of the K6 (the side that connects to the capsule). There looks to be a pin on the capsule side that plugs into a tiny "bowl" in the K6. That could be where the problem lies.

The hiss is there with or without the battery in place, and with or without the K6 turned on. Battery is new. XLR is phantom powered (tested on both my marantz recorder and the XLR of my Canon C100).

Strange thing is that it was all working perfectly until i removed the old, slightly corroded battery (at behest of a rude customs agent when flying out of the Dominican Republic).

I fear that you may have fried the transistor in the impedance converter when you applied reverse-voltage.

Thanks Richard.

Played around a bit (haven't had time to take to a camera store yet).

If I start to screw the capsule on the mic sounds fine until I get halfway threaded. Once halfway threaded then the static starts.

Hopefully the mic has a diode to prevent problems with reverse-voltage.

Emailing Sennheiser now.

Thanks Richard.

Played around a bit (haven't had time to take to a camera store yet).

If I start to screw the capsule on the mic sounds fine until I get halfway threaded. Once halfway threaded then the static starts.

Hopefully the mic has a diode to prevent problems with reverse-voltage.

Emailing Sennheiser now.

Then it sounds like a contact problem with the capsule as Jay said so try some cleaning spray on the threads and contacts , this was a common fault on mic's like the AKG 451.

I doubt if a 1.5v battery is strong enough to fry anything made by sennheiser but Richard may be right although I have been hot plugging all sorts of mic's to 48v phantom power for decades and have yet to fry any of them.

Thanks Gary (and Jay). I'm visiting a camera store back here en "el norte" this weekend so will see if I can buy / borrow some kind of cleaning spray.

Appreciate all the friendly advice!

Methylated spirits on a cotton bud, WD40, CRC type of cleaners will work fine, Spray electrical contact cleaners are what you need for this job.
I have found that camera stores tend to focus on dust removal cleaners (compressed air type) for lenses which won't do any good in this case.

I'm not sure how I feel about using WD40 on electrical contacts.

My favorite electronic contact cleaner is Cramolin, followed closely by Deoxit. Both are made by Caig Laboratories. The CRC electrical cleaners, perhaps more readily available as they're found in hardware and auto parts stores, are also quite reputable.

In a pinch, isopropyl alcohol will remove a fair amount of dirt, perspiration, even some aluminum oxidation products. Use the 91% strength, as lower strengths contain a fair amount of water which you do not want.

Lots of contact cleaner and patience seems to have worked.

However, I have to have the capsule screwed in VERY tightly to avoid static from resurfacing. I think the corrosion might have reduced a tiny bit of the metal in either the pin on the mic or the "cup" it fits into on the K6 capsule as there's a slight wiggle if its not screwed in really tight, which then causes audio breakup.

I guess when I opened the mic I broke the corrosion "bond" that the old battery had left. Teaches me to leave a battery in a compartment unused (I always power off the camera).

The K6 is now empty. No reason to use the battery. And will buy a new mic in the coming months, in case the wiggle room reappears.

Thanks for all advice.