[Harp-L] Harp mic hum problem

[Matt Schick <schickm@xxxxxxxxx>]

I purchased a bullet mic off Ebay years back, it had an RCA shell and a Shure 99A350 element in it.  I only used it for a bit, and then put the harp down totally until early this year.  Upon picking up the mic again and plugging it into a guitar amp, I noticed it has a fair amount of hum.  I've plugged it into various amps, as well as into a high impedance recording device, and still the same noise.  There are two interesting things I noticed about it:

1. When the potentiometer is at 0% (no volume) there is no noise, at 100% (full volume) there is hum, but at 50% the hum almost doubles in volume.
2. The hum quiets down partially when I am holding the microphone in my hand, or touching any metal part of the mic.

I've read that sometimes bullet microphones need to be grounded well, specifically the potentiometer to the shell, and the 1/4 jack as well.  The shell has some type of fancy coat of paint on it that I'd guess defeats the ability of the jack to ground into the shell.  Do you think that'll solve any of these hum issues?  Any other ideas?  

You can find some close up shots of the guts, as well as a little wiring diagram I made so you don't have to squint at the photo here:
http://imgur.com/a/5mTuK

Also, I removed all of the pieces of electrical tape from the places where the wires are soldered together so the picture is clearer.  They will get re-covered of course when the microphone get's reassembled.

Here are some audio samples to hear what's going on.  The first one is done with the bullet mic plugged in direct to an Onyx BlackJack (USB analog to digital converter) with Hi-Z mode enabled for the channel:
http://audiour.com/playlist/0mhxgdyc

This sample was recorded with bullet mic plugged into my little Fender Champ, and then micing the amp:
http://audiour.com/playlist/fvc3xjgq

You shouldn't have to turn up your sound volume too much to hear the hum :-(

Also, I've read that transformers are used to take a Low-Z element and make it High-Z, or vice-versa.  Is that accurate?  Is that what is going on in this microphone, it's a Low-Z element and the transformer is making it a High-Z output?

thanks for taking the time to read this and possibly respond to this very long post,

- Matt Schick
- Chicago, IL