Dead capsule or something else?

[shot]

Hi!

I got SMPro MC01 - an exact clone of Aurycle A460 - that is misbehaving.
Whenever there's luder signal in it mic just shuts down itself and it takes around 10 seconds to recover.
That's basically every time I say "P" or "B" into it.

Here's the schematic of this mic - http://aurycle.com/manual/a460-diy-schematic.pdf
I've measured that source voltage on the main FET (D1 - K170) is almost where it's supposed to be - 1V.
Aurycle says it's supposed to be 0.9V but I guess due to component tolerances this 1V is okay.

Whenever I start talking louder into the capsule this voltage rises to around 3.8V and the mic goes silent.  After that, source voltage slowly drops to 0V and stays there for about 15 seconds before going back to 1V (and the mic turns back on)

Could it be that the capsule is broken?
Is it maybe the FET?
Or should I search for the fault elsewhere?

Unfortunately I don't have spare capsule or FET to replace with and then observe what would happen...



Luka

 

[ln76d]

Dirty capsule!

Check the diaphragm, check polarisation voltage.

 

[midwayfair]

Extremely loud signals can freak out a mic, but that would be much, much louder than something that comes out of your mouth.

The fact that you're getting noise at all makes it unlikely to be a broken capsule but it could be dirt ... or, if you've been talking into it without a pop screen, moisture; could even be that it's getting stuck to the backplate when you pop it.

Measure the voltages on ALL pins, not just the gate. The gate is a poor indicator of where a fault could lie. It's just the biasing voltage for the FET. You should have around 12-18V (yes, it varies that widely) on the drain, and the emitter of Q2 will probably sit around 2-3V. You can measure the supply voltage at the junction of R11 and R12. The chances of a FET or transistor spontaneously dying in a microphone are extremely remote.

If something's wrong with C2 (just as unlikely), then when the voltage rises on gate of the FET it would misbias the transistor and take a moment to recover.

 

[shot]

Oh yes, the capsule is dirty as indeed!
See the attached picture  - after I've blown some dust off there is still some (I'd say a lot!) left.

I've measured drain from the FET and it's good.  Sits at 19.2V. Should be 18V according to Aurycle paper but it's in the ballrange.
Transistor D2 has correct voltages also - base 2.2V, collector on ground and emitter 2.9V

So we shall say it's 99% chance it's the capsule...

Is there any way to clean it?
I'm hesitant to clean it with cotton wool on stick (how do you say that on english?)
Actually, I'm not keen to touch it with anything but if it's almost dead anyway, I might as well try...



Luka

 

[Khron]

I've had quite good success with some "loose" cotton wool - a small dab of that, gripped in and tightly twisted around the tip of a pair of (small-ish) tweezers. That way, you can get a smaller and "sharper" tip than a q-tip. That, a bit of isopropyl alcohol (but not enough to drip off the cotton ), a delicate touch, some patience and a good, strong desk-lamp is all it should take

 

[shot]

I'll try!
Huh... makes me nervous!

 

[ln76d]

DON'T USE ALCOHOL!!!!!!!!!!
Even a tiny drop!

Clear distilled water only!

If the capsule is so dirty, with alcohol you will take off the gold layer.
No cotton wool also! It's easier to destroy the diaphragm and also it can left fibres which are hard to remove.
Buy cheap, very soft eyeshadow brush - works like a charm!
Prepare two containers of distilled water.
One with water to use for cleaning, second to wash the brush.
Gently moves, don't be afraid to put more water on the surface.
You will need repeat cleaning at least three times.
FIrst put more water on the diaphragm using brush. Wait 2 minutes and try gently to take off the dirt.
Round moves
Usual am using one brush for 5 capsules - then go to trash or using for something else.

 

[Khron]

I've cleaned several chinese K67 capsules that are in some of my mics, and i haven't managed to flake off any of the gold yet, so.... I kinda don't think i'm (THAT) "special"

 

[ln76d]

And you still have "mirror" gold surface?

It depends on what you specify as "special".
I knew one "special kid" years ago

 

[Khron]

I'll admit, it's not exactly a "mirror finish", but at the very least, the end result has a lot less gunk on it than at the beginning ;D

PS: This kind of "special kid"? https://www.youtube.com/watch?v=pfFRv_1XdDM

 

[ln76d]

It's because isopropyl reacting with the surface  - clean with it really dirty capsule  or 30-50 years old capsule, you will see how the gold will go away.

Could you put some picture of cleaned capsule that way?

With distilled water you will get  diaphragm exactly looking like new.
There's no need to use any alcohol - because it's like lottery!
For the first two capsules i used mixed isopropyl (few drops) with distilled water - diaphragms were looking awfull.
It was test on demaged caspules.
With my method i cleaned, only last year, at least 30 capsules - every was looking like new!
Only one didn't looked good and didn't work because it should go to thrash at the first place.


No the other kind