AKG c414 hum.

[jakobz]

I am trying to fix an old AKG c414 p48, The mic is in good condition, but has a low hum. The noise disappears when I touch the housing or the windshield. I have read several places that it might be caused by a bad connection from ground to the windshield, that needs to be resoldered So I removed the grill to have a look, but as far as I can tell , this mic does not seem to have any wires going to the shield. So I am wondering is there supposed to be a wired connection from the pcb to the grill?

 

[radardoug]

No wired connection, but you need to make sure there is electrical contact. Scrape off the dirt maybe? A good cleaning?

 

[Sorr]

Think the grille is supposed to make electrical connection to the case.

 

[Whoops]

The noise disappears when I touch the housing or the windshield.

That means that the housing and the windshield (grille) are not making contact to a ground connection.

 

[jakobz]

Thanks for replying  as far as i can tell,  the housing connects to pin 1 on the xlr, with a screw throught the pcb.
http://www.sdiy.org/oid/mics/AKG-C414-p48.pdf
I tried reflowing the solder and loosing and tightening the screw, but i still get the hum.  Is there any other critical i should look into.

 

[Bo Deadly]

Use a meter to measure the resistance between the housing and pin 1 of XLR. It should read near 0 ohms.

Note that if the enclosure is anodized, it can be difficult for the DMM probe to make contact. Use a gator clip for pin1 so that you can hold the mic steady in your hand and then, with a pointy-tipped probe, stab the body somewhere where you get good electrical contact.

Also measure / check the cable.

 

[jakobz]

Use a meter to measure the resistance between the housing and pin 1 of XLR. It should read near 0 ohms.

Note that if the enclosure is anodized, it can be difficult for the DMM probe to make contact. Use a gator clip for pin1 so that you can hold the mic steady in your hand and then, with a pointy-tipped probe, stab the body somewhere where you get good electrical contact.

Also measure / check the cable.

I tried that already actually, and measured 1,4 ohm. I assumed it was low enough, but maybe it should be lower?

 

[Bo Deadly]

I tried that already actually, and measured 1,4 ohm. I assumed it was low enough, but maybe it should be lower?

Actually no, it might not be low enough but more important, it should be lower [1].

Where is the resistance? Now open it up and try testing things like the connection through the PCB screw. Try body to body just to make sure you're getting through any plating. Narrow it down with your meter.

You can also try to bypass the connection with a jumper. Pull the shell of the XLR back and attach an allegator jumper to pin 1 and try grounding places on the mic body and see if the noise goes away (this might not work though - opening the XLR and adding a long wire to the whole thing might also just make the noise worse). But if it does go silent, you know the problem is just a back connection.

[1] Check to see what your meter reads when the probe tips are connected directly together and then subtract that from whatever value you get. That number should be no more than 0.1 ohms or so.

 

[jakobz]

Ok, i think  maybe my dmm was not being accurate. I borrowed a Fluke and now i measure around 0.2 ohm between pin1 and the case and grill. I still have the faint hum  :-[ it raises in volume when my hand gets nearer, and disappears completely when i touch the grill.

 

[radardoug]

Are you using a system where you have no ground on the microphone? For instance, a laptop, an interface with phantom, and a laptop computer? The symptom you describe indicates that the mike is not grounded, but since it needs phantom to run, there must be a ground connection to your interface.