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Hey everyone,

I have a U87ai that works fine 85% of the time, but the other 15% it's been a little frustrating. The signal will sound great but then gradually fade to silence followed by a THUMP!... 15 seconds later the signal gradually fades back in. The mic will work for days on end before deciding to crap out on me...

This kind of behaviour rings like a bad connection within the microphone to me... usually tempermental problems like this are... but I really haven't a clue.

Anyone have any ideas?

Thanks!

i don't know jack about this mic

but that won't stop me

does the signal distort as it is fading out?

if so, do the mic electronics contain electrolytic capacitors (the can-type things with polarity markings--since they are polarized)

if so, i would replace those bitches with new caps and see what happens

I would agree with the moisture, and dirt idea. Unfortunately dirt can also get under the diaphragm and create what looks like little bumps. These are called "tents." Under the right moisture circumstances (someone breathing on it) that dirt can become conductive and short the diaphragm to the back plate. This can happen over a varying period of time. On a dry day you may not notice a problem. We've had this happen to a few different mics here (altec 175s, geffel 582s, schoeps 221s). Cleaning this is a real trick. You need some serious tools and OCD to do it right.
If you can open the mic up to look at the capsules (front and rear) check for "tents" and corrosion by the center screw. If it looks perfect, then work your way down to the body. Look for component leads being to close to one another, loose solder joints, half broken wires and such. After that, check the xlr connector for oxidation, and if there are any switches exercise them. If after all that the same problems persist have Neumann worry about it and drop the $100 to $300 for a refurbishing.

Thanks for your help everyone! I think you're right about the microphone being dirty, I'll just ship it off.

Thanks again