Mic Parts 251 build issue…

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redddog

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Feb 19, 2009
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47
Hey guys. First build from a ACM 1200 mod. Did the whole shebang….RK capsule, circuit, switches.

I’m getting sort of a low rumble out of it. It seems to lessen the when I burn it in for an hour or so but when I go back it still has the rumble.

The mic parts dude told me to refer to the troubleshooting guide (which I have done - it’s not that robust).

Wondering if a “low rumble” would give someone with your level of expertise something of a starting point to look at first. All joints are good and there’s no real obvious build issues.

Thanks guys
 

kingkorg

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Apr 15, 2017
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Is it constant? 50/60hz? Changes when you move the mic? Any change if you blow gently towards the capsule?
 

kingkorg

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Apr 15, 2017
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Check if the whole body is properly grounded, grille as well. You haven't removed the inner, fine grille mesh?
 

redddog

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Feb 19, 2009
Messages
47
totally. Understood. I’m wondering if that rumble could be a grounding issue?
 
Last edited:

TLRT

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Jan 9, 2014
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Ground the tube grid,listen the noise, or replace the capsule with a 50-100pf capacitor. Sounds like a capsule issue.
 

Flatpicker

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Jun 3, 2004
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I'm also thinking it related to the capsule and will add to double check that the capsule mounting screws and capsule holder mounting screws are tight.

In my experience a mechanical grounding issue results in a hum or buzz, not a low rumble.
 
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