how do you test a microphone capsule?

dramadisease

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ive got a few capsules i need to do some testing on (one of them a ck12 from a dead 414!) and though some searching told me how to test the preamp circuit, is there anyway to test the capsule itself without putting it on a mic circuit?
thanks for the help!
-bryan sours
 

dale116dot7

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You need two items to test a capsule to see if it at least has a chance of working:

1. Capacitance meter, needs to read from around 15 to 150 pF.
2. Adjustable DC power supply, capable of generating about 120 volts but almost no current.
3. A 100 to 1000 Megohm resistor to feed power to the capsule from the power supply.
4. A .01uF capacitor to isolate the DC power from the capacitance meter.

Connect test rig to capsule with zero volts. Measure capacitance. You need to know the typical capacitance of the capsule. This checks for at least contact between the elements of the mic. Neumann schematics actually specify this.

Now, slowly ramp up the voltage, and watch the capacitance. You should hit around 65 volts with a nice slow rise in capacitance value, and the diaphragm should not get sucked into the backplate. This checks for diaphragm tension.

While you are doing this, you can watch the gold in the light, and watch for even deflection. If this is uneven, the diaphragm tension is uneven and this can result in a funny sound. This is very difficult to judge, in my opinion.

You can continue increasing the voltage on a normal 6u mylar diaphragm until it sticks to the backplate. This voltage is another indication of diaphragm stretch. I don't know about the thin diaphragms and whether this is a good idea.

Keep in mind that this will not check if the capsule actually produces a decent sound, or for that matter, any sound at all. But it does check that the diaphragm is attached electrically, and there are no shorts, and that there is enough diaphragm tension left to not have the diaphragm firmly sucked into the backplate while you are trying to get it to capture noises.

I have a cheap Chinese LDC that I use to test capsules for sound - some kind of Apex mic that goes for maybe $80.

-Dale
 

dramadisease

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portland - or
cool thanks for the reply! it sounds like just having a cheap mic around to throw a capsule in is alot easier though i will be doing the basic tests just to see how a capsule is reacting under test.
thanks much!
-bryan
 

dale116dot7

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Is it a teflon capsule C414? I just rediaphragmed a teflon C414, and it's a bit tough, though it can be done with some patience and a few spare diaphragms. I'm not so sure it's worth it considering you can get a new one for around $300. You should be able to tell the rough condition (go/no-go) of the capsule by eye. If the gold is flaking off, or the diaphragm is torn or stretched, you really should be able to see it - if there's a no-sound condition it's likely the electronics.
 

SSLtech

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Bryan, -just a quick note on most 414's that I see for repair:

The "PAD" switch can get pushed past the endstop: -People often press to chack that the pad is fully off, then the switch actually shorts out the capsule. People then send the mic to me marked "Dead: no output -swapped mics and everything works, so this mic is definately dead"

All I usually do if I find the switch all the way to the end is click the pad switch back to '0dB' then test and return the mic.

This may or not be your problem with the 414, but it's worth a try... You of course know the ham-handed nature of the people that handle the gear round here, but it might just help you...?

Keith
 

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