[Harp-L] bullet mic hum

[Greg Heumann <greg@xxxxxxxxxxx>]

>

Hey, John

From the pics, it looks like your mic is properly wired. I see only  
one thing that looks suspect - the mic hot lead looks like it might  
be touching the case of the pot? If so, that's bad - but I don't  
actually think that's your current problem.  The end leg of the pot,  
the ground of the element, the connector exterior and the shell case  
all need to be connected to the ground of the cable. You're on the  
right track in wondering if the connector itself isn't grounding to  
the mic shell. I can't see that part in the pic so it is hard to say  
- if it was there originally, I doubt that's the problem. If it was  
installed later, then depending on how it was installed that could be  
part of the problem. I've seen a lot of installations of screw-on  
connectors using glue and/or set-screws-  these are ground faults  
waiting to happen. When you say "if I grab the mic body, it hums  
badly; if I touch only the cable connector, it's quiet as can be"  
that's a good clue that the ground of the connector isn't well  
grounded to the shell. Have you loosened and retightened the screw  
that holds the ground lug to the case? I would look there and clean  
that connection. If you have an ohm meter check for resistance  
between all possible ground points - if you detect any at all (more  
than 0.3 ohm or so) then you have a line on the problem.

FYI - that is not like any green bullet shell I've ever seen. Looks  
like an Astatic shell to me.


/Greg

http://www.blowsmeaway.com
http://www.bluestateband.net





> From: "john" <jjthaden@xxxxxxxxx>
> Date: August 2, 2007 7:54:54 PM PDT
> To: "harp-l@xxxxxxxxxx" <harp-l@xxxxxxxxxx>
> Subject: [Harp-L] bullet mic hum
>
>
>      Man have I enjoyed playing through the bullet mic pictured  
> disassembled here  http://www.flash.net/~jjthaden/ 
> bulletMicInnards.jpg.  You'll see it's got an Astatic element in a  
> Shure green bullet body, along with a military-spec 4 megohm pot I  
> got from a junked oscilloscope, bridged by two 47 picofarad  
> capacitors in parallel (sort of a Rod mod).  The brown braided wire  
> is the mic hot lead to the cable.  White wires go from the pot to a  
> grounding screw inside the casing and to the element.
>      Problem is, if I grab the mic body, it hums badly; if I touch  
> only the cable connector, it's quiet as can be.  The connector is  
> fused directly into the casing, so this has got me confused.  Could  
> the grey metal of the casing be a bad enough conductor that the  
> potentiometer and element are essentially ungrounded?  Should I try  
> to run a wire from the grounding screw directly to the cable  
> connector ground?  Could the hum have increased as the casing paint  
> job wore off?  Could the fix be as easy as a coat of paint?
>      I'd sure be obliged for some hints on how to fix this problem.
> Thanks!
> -John Thaden (ublokr)
> rocklittlerock.com