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[J.J. Blair]

I have a C12A here that is making a sort of howling / whistling sound.  I've swapped PSUs and tubes with another unit, and the noise stays with the mic.  Also, this mic is noticeably quieter than the other C12A.  Maybe 3 to 6 dB?

I've never had a capsule make that noise before.  Before I go through the pain of swapping in a different capsule, what are the chances that it's the culprit?  What are the chances that it's one of the caps? 

[J.J. Blair]

BTW, looks like that 5,000pF cap is ceramic.  A likely culprit?

[klaus]

Capsule motor-boating is usually descending in pitch, even if slowly.  And it will always respond in pitch or intensity to close-up vocals, even just popping of the capsule.

I have not encountered any high-pitched sound emanating from a capacitor in that mic, including ceramics. Could this be an RF event? Does it change when you move the mic around in the room?

Can you upload that sound?

[J.J. Blair]

I suspected RF, but wasn't sure.  Seemed pretty steady.  I'll record it and upload it tomorrow. 

On a side note, what size are the screws that go i the bottom of the mic? 

[klaus]

I think they are the same flathead size and length as the three screws on C12/ELA M251 swivel mounts. 

[gtoledo3]

This is a kind of "no duh" comment, so I apologize for saying it...

But you might want to check all associated cable down the line.

I recently had an experience troubleshooting something, and for some reason, even though I had switched every other thing out, I didn't switch output cables on one of the channels of the preamp I was using. Which was the exact problem.

Since it sounds *sort of RF* related, it seems like any ground/shield/shorting problem with any cable down the line, could be one possible cause. Just switching PSUs wouldn't necessarily catch that problem of course.

[J.J. Blair]

Ghost in the machine.  Problem is gone, but that mic is still 6dB quieter than the other mic. 

[klaus]

Ghost in the machine.  Problem is gone

A bit more info please!

[J.J. Blair]

I mean that noise has disappeared.

[klaus]

Should it return, please post a file.
KH

[rsmor]

Should it return, please post a file.
KH

Hello Klaus:  I am so new at posting that I was only able to open a quote window.  Sorry but I will learn the correct way.

I wondered if you could tell me how to find the year of manufacture for my U87.  Serial 25945. It still sounds wonderful all these years since I bought it in the mid 80's.   The only quibble is that turning the ring to change pick-up patterns is very stiff and it has a bit of trouble changing patterns. 
Would it be safe to carefully spray some lubricant on the ring, do you think?

Thank you for any assistance or advice,

Roger

Ps: I already posted this by quoting a person you helped but thought you might not see it.  Sorry.

[J.J. Blair]

I finally had a chance replace the capsule that was 6dB quieter with another CK12, and now the gain is the same.  I have two questions: Could something be wrong inherently with the capsule that made it quieter?  Or, if the lead that connects the backplate was touching the metal mount base, could that have robbed the mic of gain?

Also, this CK12 has something I've never seen.  The orange paint is applied rather haphazardly.  Its even all over the metal mount.

[klaus]

In my experience, AKG has never released Golden Gate-painted CK12 capsules in as sloppy a condition as yours. It's most likely an aftermarket attribute, which may also point in the direction of the 6dB gain loss compared to other CK12, which, for a stock capsule, is extreme.

The capsule would need to be taken apart to analyze whether all of the components are genuine AKG, period- and model-correct, and assembled correctly.

P.S.: Do I assume correctly that with "quieter" you mean "lower in output"?

[J.J. Blair]

Yes.  Lower in output. 

Upon some further testing, it was not contact with the base that was causing the lower output, but it is still there.  That capsule has lower output for some reason.