Avantone CK7 FET Microphone - Needed Repairs

[InTheBox]

Hello:

This is my first post on the forum. I am not an engineer, just an old guitar player with a microphone problem. I own an Avantone CK-7 microphone. Recently when I tried to use it for a recording session, there was a signal that came through the preamp into my DAW, but it was a very very weak signal. This microphone has always provided a robust signal. So, I know we have power because there is signal bleeding through, but very little amplitude. I have watched a number of You Tube videos now on repairing microphones, but none apply directly to my situation. I would appreciate some guidance from the experts on how to approach fixing this problem. I contacted Avantone, and the cost of the repair would equal the value of the microphone. I'm willing to attempt this myself. I tried to build my first guitar pedal last year. I made a mess of the soldering and never got it to work, but at least I gained some knowledge of the electronic components and what they are used for. My guess is that the problem lies with a transistor, or an electrolytic capacitor..... but I've never fixed a microphone before. Attached are some pictures I took of the Avantone. Your thoughts would be greatly appreciated.

Thanks,

Joe

 

[Khron]

What voltages are you getting between ground (metal chassis / frame), and the positive terminals of each of those orange round tantalum capacitors?

 

[InTheBox]

Thanks for your response. I'll check on it this morning and report it back on the forum post. Need to have my coffee before I start playing with electricity. LoL

 

[Khron]

Can't really do that much damage with the heavily-current-limited phantom power supply, though...

 

[InTheBox]

Hope I did this correctly., Using my multi meter on Continuity, I got 159.6 ohms for C4. C3 ranged from 1 to 20 ohms, never a solid reading. C7 did nothing, No reaction whatever.

 

[InTheBox]

I wish I had a schematic for this circuit but I don't.

 

[InTheBox]

And Thank You Khron for your help.

 

[Khron]

I'm quite sure i mentioned voltages, not resistances

And by the looks of the board, it's most likely the JFET+PNP deal, with a Schoeps-like oscillator board for the capsule bias voltages.

Not necessarily 100000% identical, but very similar to:
http://recordinghacks.com/images/mic_extras/cad/gxl3000-schematic.png

 

[InTheBox]

You are correct. Forgive my mistake. I just used the DC V setting on the multimeter. I set it at 200m and got the following readings.
C7 .3
C3 0
C4 0

I tried the other positions and they all gave a zero V reading.

Hope I did this correctly.

 

[Khron]

Oh, my apologies for not being even more clear - voltage readings are to be taken with the microphone plugged in, and phantom power on

 

[InTheBox]

I also took the voltages using the AC V setting. I don't see a bridge rectifier on the circuit board so dont know if this is AC or DC voltage.

Using the AC V setting"
C7 2.0
C3 0
C4 0

 

[InTheBox]

Thanks.

Using AC V at 200m, results were

C7 38.5
C4 36.5 to 37
C3 0 to .1

 

[Khron]

Phantom power is supposed to be DC, so AC readings are useless / meaningless.

 

[InTheBox]

My apologies again. Using DC V at 200m:

C7 47.8
C4 191
C3 189.9

 

[InTheBox]

Thank you for the schematic.

 

[InTheBox]

Decided to test everything on 20V DC

C7 1.21 V
C3 14.47V
C4 1.91V

 

[InTheBox]

Looking at the schematic, the voltages i am seeing do match match those on the schematic closely at all.

 

[InTheBox]

Just retested at 20V DC and got the same reading as noted above:
C7 1.21 V
C3 14.47V
C4 1.91V

 

[Khron]

As luck would have it, it would appear that even the component designators match.

What about the voltage between chassis and the banded end of D1, on the other circuit board?

 

[InTheBox]

I get a reading of 16.5 with DC V set for 200m