Condenser mic repair

[Yamin]

Hi, I just got a SE2200a condenser mic to repair/have a look at - as it stopped working one fine day. Anything audio is something I'm very interested in and I like to sink my teeth into it. Surprisingly there is very little that I could find on the web about microphone design/repair/servicing etc. So firstly I'll be very grateful if you could direct me to any good sources.

Anyway back to the mic I have - the soldiering work looks very bad and there seems to be some white residue around the pcb.
1. How should I clean this off, I read that using IPA (99% alcohol) is also not good to use as it might leave some residue and cause noise issues.

Please refer to Photo 1

I checked the individual components (in circuit) and everything seems to be in order, obviously I didn't remove any components wanted to make sure how to clean it off after soldering job first.


2. I checked this capacitor and it didn't show any resistance it reads O.L. It looks like there is some blackening on it as well. Could this be the culprit? The capacitor marking is 2A102J (whats the value?). The capacitor is placed between the drain and source of the FET.

Please refer to Photo 3

3. There is a mysterious component - the marking on the PCB is G2 I don't think its a transistor. What could this be.

Please refer to Photo 4

4.I supplied phantom power from my sound card - I checked the voltage between pin2 and ground, pin 3 and ground - it show the correct 48V. But when I measured at these points from the mic it shows 38V.

Please refer to Photo 5

I tried speaking into and I was able to get an output, but the output clips the moment it gets even slightly loud. I could only describe it as when DC passes to an audio amplifier.

5. How could I check the capsule if it ok?

The person who asked me to have a look also did mention that when the mic died it gave off a burning smell

Thanks in advance

[Yamin]

Photo 2 and Photo 3

[Yamin]

Photo 3,4,5 - sorry the pictures are too big 

[wraper]

2. I checked this capacitor and it didn't show any resistance it reads O.L. It looks like there is some blackening on it as well. Could this be the culprit? The capacitor marking is 2A102J (whats the value?). The capacitor is placed between the drain and source of the FET.

1 nanofarad. It would have resistance only if it was faulty.

[Yamin]

I meant with other capacitors measuring it in the ohms range shows it charging up. Ahh perhaps since its 1nF it doesn't show charging up? maybe

[wraper]

I meant with other capacitors measuring it in the ohms range shows it charging up. Ahh perhaps since its 1nF it doesn't show charging up? maybe

You mean 10-47uF electrolytic capacitors? They have 10000 - 47000 times higher capacitance. Of course you would not notice it's charging time.

[Messtechniker]

There is on the internet a schematic for the SE 2000 which
looks quite similar to what you have got. No output transformer though.
After cleaning I'd start by checking continuity of all pcb traces
especially those with vias since build quality is pretty low.

G2 would be Q2 i.e. the output transistor. Also worth a check possibly with
your nose first, since some burning is being reported.
G1 is apparently the FET. Designated as Q1 in the SE 2000 schematic.

[Yamin]

Thanks I just downloaded the schematic, I' ve been sniffing around but no luck. On another note how can I find the part number of these surface mount transistors.

[Armadillo]

The power is from phantom power supply 48V. As it name suggest, its high impedance power source [IEC 61938].

From the board, it's obvious leaky electrolytic capacitors. Quickly take those out and wash the board before it corrode in.
Replace the caps. NOT the transistors.

[Yamin]

 

From the board, it's obvious leaky electrolytic capacitors. Quickly take those out and wash the board before it corrode in.
Replace the caps. NOT the transistors.

Hey Armadillo, thanks for the heads up. Can I ask how its obvious? - is it due to the sort of residue next to it? I thought it might have been the flux left over, and is it possible to check whether its leaky without an ESR meter?

How do you suggest on washing the board? Can I use IPA?

Thanks so much for the help

[DaJMasta]

For one, I'm not sure I've ever seen one of those little plastic stabilizers for SMD caps that was brown instead of black, from some shots it does look like some liquid residue around the caps.  Even without an ESR meter, especially out of circuit, you can probably measure them at a lower value than they are rated if indeed enough dielectric has leaked out to change the color of that base piece.  IPA is probably fine for cleaning, and if it doesn't work, using it should be no harm.

For the transistors, look at the code etched into the top, it can sometimes be hard to see, then search around for that SMD code.  There are a few databases that keep track of part codes, but some datasheets will also mention them and search engines can find it in PDF datasheets sometimes.

[Armadillo]

Phantom Power means any leakage current from the caps would quickly distort your signal into clipping.

You wash with baking soda, arm & hammer brand or whatever. Put the powder onto the board, drip clean water onto it and spread the thick paste onto the board, components, thru-holes and under IC in particulars etc...
Wait for 15 minutes, then wash away. Then blow dry the board.

IPA is not for the purpose of neutralizing.

From the board, it's obvious leaky electrolytic capacitors. Quickly take those out and wash the board before it corrode in.
Replace the caps. NOT the transistors.

Hey Armadillo, thanks for the heads up. Can I ask how its obvious? - is it due to the sort of residue next to it? I thought it might have been the flux left over, and is it possible to check whether its leaky without an ESR meter?

How do you suggest on washing the board? Can I use IPA?

Thanks so much for the help

[Armadillo]

Leaky electrolytic cap means the cap has turned itself into a "Resistor Like Device".
Applying cap rated voltage across it example "50V", there will be appreciable current flowing continuously through it like a resistor.
Sometimes a 9V type DMM in ohms mode, though the voltage is about 6V only [NOT with those cheap digital DMM using 1.5V battery], can shows ohms reading.
The above applies to uF electrolytic or MLCC caps.

So general purpose multi meter covers generally a region of reliable measurements and not all measurements [Uncertainty].

For this circuit, when average current ceased to flow, the phantom 48v voltage is acting on the poor capacitors. I don't think its even rated near that voltage.

[Yamin]

Yep turns out it was the capacitors after all, I only changed the marked capacitors as they were the only one available, the others I would have to order in, and also the 10uF cap I marked - I couldn't get an SMD electrolytic cap so to check it out I used a normal electrolytic capacitor.
One thing I noticed was there is a slight noise issue. I'm hoping it because I have to not closed up the mic yet and its not caused by the IPA used to clean off the board.
The other important question is why do I still measure 38V between pin2 - gnd and pin3 - gnd. Why is it not 48V coming off the mixer. Is it because of the transformer. I couldn't[t be right? Which leads me to my next question - what effect does DC have through a transformer winding?

Thanks so much for the help

[Armadillo]

Change ALL the caps to improve on sound quality. Use proper and good quality LOW ESR capacitor and NOT ANY CAPACITOR even though it's electrolytic.

Voltage drop is normal for Phantom power supply. It is the clipping and sound quality you are concerned with.

[Edit: It is not the point of your measurement 38V that was the concerned. It was the points after few resistors drop away that actually affected the quality and caused the clipping of the ac signal. The spec design on usage of 48V must be to cater for extra LONG lines as if performing on the Stage].

[Yamin]

Where does the 10V drop? There is nothing else between the mixer output and the point I'm measuring from. I'm using a very short mic cable here.
Its a bit strange is it not? The mixer provides 48V spot on.

[Armadillo]

That could means, there are still some unnecessary current leakages. So you should change all the failing cap first unless you have the capability to test and verify the cap dielectric performance.

Also the PCB could be conductive because of the cap leakage. Have you done what you should have done, example neutralizing it?

Also, ultimately  the DC will be filtered away and the resultant small signal AC signal is amplified.

[Richard Crowley]

Where does the 10V drop? There is nothing else between the mixer output and the point I'm measuring from. I'm using a very short mic cable here.
Its a bit strange is it not? The mixer provides 48V spot on.

That is completely normal.  The "P48" power from the mixer is sourced from a rather high impedance, 6K81 on each side (XLR pin 2 and pin 3)  So significant voltage drop is expected and designed into the standard.  The high impedance protects the mixer from short circuit faults in the microphone line. And it prevents the phantom power supply from loading the AC audio signal. The circuit in the microphone needs very little power, so the voltage drop is of no significance.
Ref: https://en.wikipedia.org/wiki/Phantom_power

Remember that the circuit node coming from the microphone capsule is EXTREMELY HIGH IMPEDANCE. It is VERY sensitive to any leakage path. That is why you see those little white Teflon standoff terminals on the PC board at the top.  The photos show that the board is very dirty for a condenser microphone.  Through cleaning is recommended. If you could use some Freon circuit cleaner, that would be ideal. But not available anymore. In the absence of Freon, we would use IPA to clean circuit boards, but as you correctly observe, some forms of IPA will leave residue which is exactly what you are trying to eliminate.  Although "lower down" on the board, there are no high-impedance nodes, so IPA cleaning would be OK if you avoid the high-impedance Teflon standoff area at the top.

As @Armadillo suggests, many people do a "shotgun" replacement of ALL the electrolytic capacitors. And if you found one or more faulty capacitors, it may be a very good indication that the other caps may be of questionable performance.  For microphones without transformers, they sometimes replace the electrolytics with high-quality plastic film capacitors. But those are typically VERY much larger and won't fit into the body of smaller microphones.  But that is not as much a factor with transformer-output circuits like your mic, so I wouldn't recommend it for your situation.  Replacing the surface-mount electrolytic capacitors would be the first-order remedy.

[Yamin]

Hi Richard, how about using a little bit of distilled water at the high impedance node as there is a lot of nasty gunk there.

Thanks

[Richard Crowley]

Unless you are hearing symptoms of leakage in the high-impedance node, I would not touch it.
Symptoms would include random noise and hiss, perhaps made worse by gently exhaling on the area (to temporarily increase the humidity).  That would indicate that the "nasty gunk" is causing leakage.

If you do want to try cleaning it with distilled water I would use a new, clean, soft toothbrush and dip the ends of the brush into 1-2mm depth of water.  This will keep the amount of liquid to the absolute minimum.  I would repeat several times and rinse the brush in distilled water between each brushing.

Remember that until the water is completely evaporated, the microphone may be very noisy, low-output, or maybe even not work at all. If you have a heat gun or hair dryer, you could accelerate the evaporation, but this is VERY RISKY because excessive heat could damage many parts of the microphone including the microphone capsule itself. I certainly wouldn't approach the microphone any closer than 500cm with a heat gun or hair dryer.  Better to just put it out in the sunlight to dry out.  Of course you wouldn't want to subject it to salt spray from the ocean, etc.  No matter where you are in the Maldives, you can't be very far from the ocean.

[Yamin]

Thanks Richard, hehehe you are right about the ocean being close by, just couple of meters outside my workshop.
This project is turning into an interesting one. I have just sketched the circuit down trying to figure out what does what. Any suggestion on reading material about mic design etc...

Thanks and regards

[Richard Crowley]

I have just sketched the circuit down trying to figure out what does what. Any suggestion on reading material about mic design etc...

There are many videos on YouTube about microphone teardowns and even design discussions.
But right here in EEVblog-land there is a great series hosted by Dave with guest engineer Doug Ford.
Here is the first of the series..

https://youtu.be/ihAG6cMpUlY

On the internet there are MANY schematic diagrams for all kinds of microphones including MANY condenser mics.  Taking some time to examine several dozen will reveal many similarities and some differences.

Note that there are online forums which specialize in audio technology.  One of the prominent audio forums is www.gearslutz.com  And particularly the "GeekSlutz" Forum https://www.gearslutz.com/board/geekslutz-forum/

We have had several discussions of particular pieces of gear and their schematic diagrams and Questions and Answers on how the circuit works.  If you want to pick two or three condenser mic schematic diagrams, you can post them here in EEVblog Forum or in GearSlutz/GeekSlutz Forum and there should be many people who can discuss the different parts, how it works and why it was designed that way.