Condenser microphone repair

[gdgross]

Ok, new guy here.  I'm pretty familiar with electronics but this will be my first attempt at a condenser mic repair. 

I have a large diaphragm condenser microphone that has developed a buzz.  It gets worse when i move my hands closer to the grill.  The buzz is not terribly loud, but it pretty much means i can't use the mic to record quiet sources.

I can't find a repair manual at all for this mic, a lewitt 640, and lewitt directed me to a repair shop that will at minimum cost me a couple hundred $, so I'm attempting the repair myself, without a schematic or repair manual.  Worst case, i end up sending it in later.   (It's out of warranty anyway.)

To me it sounds like a grounding issue, something is picking up the noise from my hands and the environment.  I've already opened the mic up and there are a few circuit boards inside, as this mic has selectable attenuation and polar patterns and phantom power.  Currently figuring out how to disassemble the PCBs, and being very careful. 

Do you all concur with my intuition?  I expect that the grill should be grounded, or perhaps there is something else around the capsule that is not grounded properly or has come loose?

Thanks,
Geoff

[SilverSolder]

Just thinking out of the box here, have you tried it on a different amp,  i.e. 100% sure it isn't something to do with the amp?

[gdgross]

yeah, already proved it was the mic unfortunately.  (I've swapped it out in the same cables/preamp/etc with another of the exact same mic which works fine and has no buzz.)

[Audiorepair]

As you suspected, the grille is probably not grounded correctly.

The fact that when you move your hands near the grille you get more hum would indicate this grille is not acting as a Faraday cage, as it should.

Try connecting the grille temporarily to pin 1 of the XLR connecting to the mic with a wire, and see what happens.
(You might need to make sure the outer grille conducts, and is not coated in something insulating)


Edit:  The ground connection from pin 1 of the XLR to the grille, is usually just mechanical.  This contact connection does fail, I have seen it a few times before.
Stick a multimeter on it, it should be zero ohms Pin 1 to grille.

[drussell]

Is your "buzz" at 120 Hz like mains hum?

Otherwise, also check the grounding around as well as bypass and filter capacitors in whatever power supply runs whatever microcontroller thingy they're likely using to do the front panel buttons and attenuation switching, etc., be sure the uC noise isn't getting picked up somehow, and then investigate what kind of power supply they're using for the condenser power supply itself, since I kinda doubt it's running directly off the incoming phantom power either.

[nali]

Is the output balanced (differential)? If so perhaps one side of the pair may be faulty or disconnected.

[DaJMasta]

If it's not related to grounding, it could also be the condenser itself being damage.  I poked around in a dead NT1 a few months ago with similar symptoms.  Tried chasing down the amplification chain and cleaning/replacing the first fet and making sure the area inside the guard trace was all clean, but couldn't get it to work.  After a few more attempts of things, I lifted the leg of the condenser from the FET input and the hum went away entirely, even without the metal housing.  At that point it probably needs a replacement, at least, I don't know how one would refurbish a damaged condenser without specialty equipment, and they weren't available for sale in my case.

[bob91343]

Perhaps the phantom supply is poorly filtered.

[drussell]

Perhaps the phantom supply is poorly filtered.

I thought that at first also, but the OP said that another of the same model microphone works correctly on the same cables and phantom supply.

[gdgross]

Well, i fixed it guys. 

Adding a discrete ground wire from the grill to the XLR pin1 solved it.  What's strange is that I wasn't able to find where the break in the chassis connection was, but no matter.  Works fine now. 

As audiorepair suspected, it seems that this mic does rely on a mechanical connection to ground the grill, somehow i  suppose that had worked it's way loose, although as i mentioned it was difficult to find where with the mic disassembled. 

In retrospect, I should have taken pictures.  Considering opening up the mic again soon just to do that, haha.  I was pretty proud of my handiwork. 

[Audiorepair]

The grille sits in the upper part of the chassis.
You would expect it couldn't possibly NOT provide a good connection to the metal chassis it sits in.

But then you could be wrong.

[Jakobz]

Hi There.
Sorry for bringing back this old thread  I am having trouble disassembling this same microphone, and was hoping you  could offer some help. I have removed the 3 screws by the XLR connector, and thought i would be able to wiggle the microphone apart, but it is not happening, and  do not want you use too much force. Am i missing a screw (perhaps behind the grille or something.