Fundamental question about dynamic mic troubleshooting: chassis/handling noise

[thecr4ne]

Fundamental question about dynamic mic troubleshooting: chassis/handling noise
I'm troubleshooting a dynamic mic (sennheiser md441) and when plugged in I only hear buzzing kinda noise, and handling noise that I'd describe as either the chassis being microphonic or responsive to touching the chassis with my bare skin, or both in some cases. I've heard the same thing with busted ribbon mics.

Basically, I want to know what, electronically speaking, is behind this symptom, and is it a clear indicator of a common fault?

 

[kingkorg]

Watch the whole series of his dynamic mic repair videos. Should provide some context.

 

[thecr4ne]

Already watched that vid, and have gone through other repair docs. I've disassembled the mic and visually everything appeard to be correct.

My question was less about the mic itself, and more about why the chassis of a dynamic is microphonic/noisy when the mic doesn't work.

 

[ccaudle]

more about why the chassis of a dynamic is microphonic/noisy when the mic doesn't work

A dynamic mic works by the diaphragm causing the wire coil to move in a magnetic gap in response to sound vibrating the diaphragm. If something damages the connection between diaphragm and coil, then sound will no longer cause the coil to move, but physically moving the mic body could still cause the coil to move in the magnet gap and generate output.

 

[Khron]

... Or the center pole piece has become detached from its mounting, and caught the voice coil between it and the outer pole piece. You might still get some output from sound waves, but low output and bass-less.

Seen that happen in a couple older mics (a Beyer M201 and possibly in an MD441 - memory's a bit fuzzy, it's been quite a few years since).

 

[thecr4ne]

What about the noise changing when touching the chassis? Seems like a capacitance thing, but I'm kinda fuzzy on why.

 

[Khron]

Humbucking coil disconnected? Or is it the only one still connected?

 

[thecr4ne]

Humbucking coil disconnected? Or is it the only one still connected?

I bypassed the humbucking coil altogether and the result was the same either way.

 

[JohnRoberts]

check for a bad solder connection?

JR

 

[thecr4ne]

Every solder point looks good. Even on the board inside the lower tube. I've been lucky enough to have revived 2 other MD441's but one was just a physical break, and the other was some broken solder connections. Neither of those gave me the noise this one does.