Mic Parts 251 build issue…

[redddog]

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]

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

 

[nashkato]

you cleaned all the sensitive high1impedance-circuitry from any soldering residue ?

 

[redddog]

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

Yes, no, yes. Lol

 

[redddog]

you cleaned all the sensitive high1impedance-circuitry from any soldering residue ?

Yeah. Cleaned thoroughly.

 

[kingkorg]

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

 

[redddog]

Y’all think It may a grounding issue? I have not removed inner mesh.

 

[redddog]

Would grounding present as a rumble?

 

[kingkorg]

The entire body needs to be grounded.

 

[redddog]

totally. Understood. I’m wondering if that rumble could be a grounding issue?

 

[TLRT]

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

 

[Flatpicker]

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.