AT4040 LDC repair - capsule DCR expectations

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pjd3

Member
Joined
Sep 21, 2017
Messages
5
Hello,

My friend sent me his AT4040 mic for an attempted repair. While I've successfully built a couple of quality condenser mics, I've never done any troubleshooting on a faulty one. The claim was that the mic was simply no longer outputting any sound at all. After opening it up and giving it a visual inspection I checked for continuity between the pcb and the capsule. To my surprise, the resistance to the capsule measured less than 2 ohms. I was always under the understanding the the diaphragm and backplate were physically separated, as in a capacitor. This mic appears to be a fairly standard cardioid only mic, which I believe would be a single membrane and a backplate.

Is there anyone that is familiar with the expected straight DCR resistance of a capsule as found in the AT4040? I did disconnect one of the capsules leads from the board to make certain I was looking at the capsules resistance and not the pcb circuit. So yeah, 2 ohms across the 2 leads that connect to the capsule.

thank you!
Best,
Phil Donovan
 
Yes the mic did work at first but not for long. It was used on vocals a few times then gave up the ghost. The diaphragm looks clean. But, it measures 2 ohms. I think I came across that thread on Gearspace.

thanks for responding!

Phil D.
 
The mic worked one day, was put away for some weeks in its case, and did not make any output whatsoever upon the next use.

That's what he told me.

Thank you,

Phil Donovan
 
A working capsule should show a very high high resistance. It's a capacitor in essence. I think you have a shorted capsule.

Did you inspect it visually under magnification?
 
Last edited:
I did inspect the capsule under magnification, not quite a microscope but, maybe 3-5x mag. The diaphragm looked fairly clean in that I didn't notice much of spots or debris. But what the low resistance seems to be saying is that either the diaphram and backplate are physically touching do to some physical happening or, some kind of moisture got in there and either corroded the material or invited a buildup of conductive debris. If it was the latter, I might entertain the thought of it being restorable but, I just don't know enough about the nature of what can occur between a diaphragm and a backplate. Still researching possibilities.

Thank you all,
Best,
Phil D.
 

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