How to Fix Hum / Radio / Inteference in the MXL 770 Condenser Microphone

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Friday, July 10th, 2015, 5:03am
Categories: Journal | Tech / Nerdery

Hey peeps; just another nerd post. I'm going to attempt to explain the cause of, and provide a solution for, a problem I had with my microphone awhile back, where it was picking up the radio and interference.

MXL 770
One of the microphones I frequently use for my music is the MXL 770. It's a consumer level cardioid condenser microphone with an XLR output; really great for home recording on a tight budget. Dollar for dollar it's a huge value. I plug it into my little usb audio interface and make tracks for iTunes or youtube.

But eventually I noticed it had one huge problem: The microphone would hum. Like ... a lot. Sometimes I could actually hear a radio station coming from the microphone. And I don't mean it was picking up a radio somewhere. I mean the actual microphone body was somehow acting as a radio antenna, picking up signals, and transmitting them down my microphone cable and into my computer. The worst part was - the station it picked up wasn't even that good! (lol).

So anyway. I wracked my brain trying to figure out if it was indeed the microphone (as opposed to the cable, my interface, the usb port, or some other wonky component), and if so, what part was picking it up - and would I be able to fix it?

I love the internet, because eventually I found the answers in the form of online forum complaints and debates, and even a comment on an online shopping website. Eureka!

It was the damn paint

To briefly summarize the solution these internet citizens came up with: The microphone sometimes ships with it's non-conductive paint, painted on parts of the microphone that aren't supposed to be painted; Specifically the inside rim on the top half of the microphone casing. When this happens, the top half of the microphone's case doesn't bond electrically with the bottom half of the case, and thus does not benefit from the fact that the bottom half of the case is electrically grounded via the XLR jack's pin #1. Since the top half of the microphone case is supposed to provide shielding for the actual mic capsule, and is unable to do so (due to not being grounded), the microphone ends up receiving interference.

Whew! Okay, so we kinda know what's going wrong. Now what do we do?

Scrape the Paint Off

Disclaimer: Nothing I say here or anywhere else is to be taken as any kind of guarantee or acceptance of liability on my part ... or should even be taken seriously at all. Follow my advice at your own risk; It could ruin your equipment, void the warranty, or end your life in a huge ball of flame.

The basic trick that I followed, was pretty simple: I removed the top half of the case, noted that there was paint on the inside rim (the side that's supposed to touch the bottom half of the case), and sanded it off with sandpaper. It was kinda nerve wracking, as I'm no expert in these kinds of ordeals, but in the end pretty quick and painless.

Here's my microphone before doing anything
MXL 770
-My MXL 770 condenser microphone, before repairing for interference.
Image By: Mike Peralta

Carefully unscrewing the base and pulling it off
MXL 770, Removing Base
-My MXL 770 condenser microphone, as I remove the base.

Next, pulling off the bottom half of the case
MXL 770, Removing Bottom
-My MXL 770 condenser microphone, as I remove the bottom half of the casing.

Note that there's a little gold ringy thing in between the top half of the base, and the bottom. You'll need to remember to put that back on, later
MXL 770, bottom half of base removed (gold ring)
-MXL microphone, gold ring between top/bottom of base

And here's the gold ring removed
MXL 770, Gold Ring between base halves removed
-MXL 770 condenser microphone, with bottom half of base and gold ring removed

Top and bottom halves of the microphone case compared. Holy crap ... notice how one of them has black paint on it, in the wrong area. Dohhhhhhh #Fail
MXL 770, Black Paint on Top Half of Casing
-MXL 770 condenser microphone, top and bottom half of casing compared, to show black paint incorrectly applied to the top half!

I will remove this paint, if it's the last thing I ever do !!!!!!!!!!!
MXL 770 before sanding off black paint
-MXL 770 condenser microphone, black paint incorrectly applied to the top half (before sanding)

Very gently sanding off the black paint with just the tip of my finger. I'm only removing the paint on the inside, where the two halves need to connect to each other, but leaving the rest so the microphone still looks nice. Many things could have gone wrong here - a wrong push to a wire, a static shock to a circuit, the bending or breaking of a board. I probably should have grounded myself during this whole process so as not to shock the circuits.. I got lucky. Be careful!
MXL 770, Sanding off black paint from casing
-MXL 770 condenser microphone, gently sanding off black paint (top half of case)

After sanding is finished, you can see that both the top and bottom insides of the casing can electrically connect with each other, because that black paint is no longer in the way.
MXL 770 Microphone, After Sanding Away Black Casing Paint
-MXL 770 condenser microphone, after sanding away black paint on inside of case. Radio interference is probably fixed.

I'll spare you the photos of me putting it back together, as you can easily just look at the ones already posted above, in reverse. I'm happy to report that my microphone no longer suffers from interference or radio hum, and works flawlessly as I had first imagined when I got it. Knowing how to fix this issue, I can confidently say I'd buy another one of these suckers if I needed a replacement mic, or even just for fun, since these are so cheap for the great quality you get (after repairs anyway, hahahaa).

Remember that by doing this, you risk breaking your microphone, and probably immediately void the warranty. If you decide to try this out to fix your own, I wish you luck!

For those of you interested, here's a video of me using this very same microphone (post-repair) to sing a cover of "Photograph", by Ed Sheeran.

Stay tuned for future episodes where I attempt to upgrade the entire audio circuit using instructions from some dodgy DIY website, and inevitably catch my shirt on fire.

~Mike Peralta

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Mike Peralta is an independent musician and you should follow him on Twitter (@MikePeralta) and Instagram (@MikePeraltaMusic), then subscribe to his blog just a bit below this text!
Current avatar photo by Heinz von Bockelmann (CC by 2.0, modified by Mike Peralta)

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