AKG P220 microphone failure repair.

[kostix]

Hello to all!

Recently I have been given for repair an AKG P220 microphone and it has a peculiar failure, my client told me that it didnt not work.. i ran some tests, apparently it works but if I say something very loudly right into the capsule screen, the output sound goes totally off and then very slowly recovers back, sometimes with what sounds like a very random low-passed noise. I am wondering if the capsule is damaged somehow and goes out of spec of the rest of the circuit after receiving a stronger acoustic shock (a loud shout right into the mic) or the failure is part of the circuits.
I have an AKG P420 mic at hand and it does not behave like this, the output never goes dead no matter the level of sound close to the screen.
I tested the P220 with a cheap Phonic console I have here (which measures around 51V on the phantom lines unconneted to the mic) and it behaved like this when strong levels of sound are produced right at the capsule. I left it laying on a table without touching it recording  the room ambience for 4 hours and it did not fail once.
After testing I gave it back to my client who tested it with a Tascam computer interface, as well as with using an external phantom power supply and he reported back that the sound goes out/off periodicaly, kinda randomly and back on, so no stable correct operation.

The P220 mic is the black model with entirely surface mount components on the boards. The mic is out of warranty so mostly its up to me to figure out how to repair it.

I have a Neumann mic at hand and I was thinking of swapping the capsule with the AKG to see if that will indicate if the problem resides in the capsule or the circuit, but still have to try it.

Any advice would be greatly appreciated.

Best regards,
Konstantin

 

[Khron]

Kinda sounds like the diaphragm sticks to the backplate, when too much airflow hits it. How dirty is it?

It might be interesting to stick the mic (or at least the capsule) in a sealed bag with as much silica gel (ie. desiccant) as you can get.

 

[ln76d]

Didn't stick to the backplate, it's just dirty - i cleaned many perceptions capsules. 200,220,400, 420.
It's also typical for NT1a with k67. Both mikes works with high polarisation voltage (in nt1a it's just insane!!!) and the dirt glued by saliva is enough. Design of the circuit partially sucks, can be improved

 

[kostix]

Hey guys, thanks for the point outs. From your personal experience, what is an appropriate, doable procedure for cleaning such a surface? The capsule is indeed kinda dirty from what I recall.

Best regards,
Konstantin

 

[ln76d]

Distilled water and delicate brush - don't even try for the first time on your customer capsules Find some cheap capsule and learn first.

 

[Anna]

Hello dear all! Me and my friend are currently trying to run diagnostics of one p 220 with an ambition to fix it and modify it ourselves. In fact it's a dead body of p 220... Currently It doesn't send any signal at all, the only thing we get from it is clipping when the xlr cable moves. Strange, no idea what can be the reason. Nobody dropped it, nothing happened to it, it's surely old but stood on the shelf most of it's life. We opened it, everything looks clean and attached properly, visually there's no defects... Any ideas, what to start with? Maybe someone has circuit schematics of this mic, that would help us a lot!
Thank you for any clues.

 

[micolas]

@Anna
First check the microphone cable, with another condenser microphone, with Phantom activated. In the absence of another microphone, check at the XLR connector if you have Phantom voltages 48v (+/- 4v), pins 2 and 3, with a DMM.
Also, the pins of the connectors (mother/father) of the cable, microphone and preamp had to be checked, if they are not oxidized or if there are no broken, unsoldered wires, etc.
Edit: It must first be determined if the problem is with the microphone or after the microphone, in the audio chain.

 

[micolas]

Maybe someone has circuit schematics of this mic, that would help us a lot!

An older schematic drawn by reverse engineering
Henry Spragens / @Khron

https://groupdiy.com/threads/akg-pe...min-mod-p200-p100-p400-p420.67473/post-908876

 

[Anna]

@Anna
First check the microphone cable, with another condenser microphone, with Phantom activated. In the absence of another microphone, check at the XLR connector if you have Phantom voltages 48v (+/- 4v), pins 2 and 3, with a DMM.
Also, the pins of the connectors (mother/father) of the cable, microphone and preamp had to be checked, if they are not oxidized or if there are no broken, unsoldered wires, etc.
Edit: It must first be determined if the problem is with the microphone or after the microphone, in the audio chain.

the phantom power is fine, the cable is superb, wires are clean and attached, from pins to the board resistance is 0, that's why I'm here)

 

[micolas]

the phantom power is fine, the cable is superb, wires are clean and attached, from pins to the board resistance is 0, that's why I'm here)

So you have to dismantle the microphone and check the connections to the xlr, where the phantom voltage reaches, if the DC/DC converter works and provides voltage polarization to the capsule, if the audio circuit is powered, etc.
First,
*When the microphone is connected and the gain of the preamplifier is increased to the maximum, you can hear the self-noise of the microphone, if you knock on the body of the microphone, do you hear a resonant, microphonic sound in the monitors?