I need to do some more investigating but I ran into this today

We had a Sennheiser ME66 / K6 on a boom pole plugged into a EX1 that was running on battery power. There was a hum in the audio signal, and when you touched the mic it would get substantially louder.

Switched AA batteries in the mic, changed XLR cables, turned off mic battery power & tried 48v on the camera (made it worse). Only thing that worked was plugging the mic into a wireless cube transmitter.

Any ideas what could cause this?

thanks

In my experience, hum in audio is almost always from a grounding issue. Going wireless seems to point to that too. Sounds counter intuitive but I might have tried switching the camera to use mains power. <grasping at straws here> Did the boom operator have a personal monitor inline? Was the op suited up with other electrical equipment? Was the ground wet?

Going to shore power.... hmmm... didn't think to try that. Does seem counter intuitive, but wouldn't be the first time that something works by doing something that shouldn't make it work.

Boom was on a stand, shoot was in doors, so no water around & mic was wired directly into the camera.

I also tried a ground lift XLR in line filter, which didn't help. I searched some more around google & someone had a similar problem that there was a loose screw in the power unit. So I'm going to try to take a look at that.

Were you using balanced XLR cables?

I had a simliar problem with a Sen mic.Turns out it was a simple fix in that the "Philips Head" screw was very loose on the mic causing an earth problem. Check the screw.

Just out of curiosity, is there any other cables running out of the camera? S-Video, HD-SDI, component etc? These can create ground issues as well, especially when the camera is running on AC as well.

I've heard of a few people that travel a lot having issues with screws coming loose on shotguns and causing rattles - so double check your mics and tighten everything up.

I've had instances where a ground type hum can emanate from a mic cable being in close proximity to a power amp or other power supply generating a strong electromagnetic field. A laptop PS was the culprit in one instance, a audio power amp in the other. Moving a cable eliminated the problem. The cables belonged to a hotel's A/V dept. and were of unknown quality.

Yes, if the small Philips screw on the body by the XLR input is loose, you'll get an earth hum on the output. Tighten this and the hum disappears.

Only time I've experienced this with the ME66 is when the capsule wasn't screwed into the power module tightly.