Need help understanding a Chinese microphone circuit and troubleshooting

[skyy7]

Hello!

I am still a beginner in all this, but I arrogantly bought a broken ST79 and was attempting to fix it. The circuit seems to be a KM84 copy with a DC-DC external polarization board that outputs 60V.

Initially, the mic's output would increase and decrease every few seconds. I found that the JFET's drain and source were shorted together, and I replaced the 2SK170 with a J305 with a matching IDSS. When I attempted to solder everything back up, I noticed there was now no output. I am trying to diagnose it, but I do NOT understand how the capsule is polarized in this circuit. The capsule is a 3-wire 32mm K67 copy. I tried my best to trace the circuit and filled out as many values as I could.

I measure 1.28V at "R1" and "R2" when the mic is in Omni, but I believe these are the polarizing voltages with a high impedance, so I don't think I can measure it with my multimeter. 16.9V at "E" and "VL".

My question is... What is E and VL? E seems to be where the backplate wire connects to, but why is there 16.9V? It seems the capsule is polarized at the membrane via a 1G resistor while the output is taken at the backplate. At what point should the membrane and backplate connect to the circuit?

When injecting a 1kHz signal at 0.1V at the gate, I get a good output, but when injecting the same signal at "E," I get no output. Please let me know if I am missing anything. Thank you in advance!

 

[Khron]

I replaced the 2SK170 with a J305 with a matching IDSS.

I hope you kept in mind that the two have a different pinout, right?

Otherwise, I'm not sure about the "KM84-ness" of the circuit, since that's the only board NOT pictured, but the capsule bias generator & pattern switching reminds me a lot of the CAD GXL3000.

http://recordinghacks.com/images/mic_extras/cad/gxl3000-schematic.png

See if it resembles your mic at all.

 

[skyy7]

I hope you kept in mind that the two have a different pinout, right?

Otherwise, I'm not sure about the "KM84-ness" of the circuit, since that's the only board NOT pictured, but the capsule bias generator & pattern switching reminds me a lot of the CAD GXL3000.

http://recordinghacks.com/images/mic_extras/cad/gxl3000-schematic.png

See if it resembles your mic at all.

Yup, not going to make that mistake again... saw that the pinout was different and adjusted accordingly. Based on the test signals I think that section of the mic is fine.

Looking at CAD it seems to be super similar but not an exact copy. Thanks for the find! So based on the CAD circuit, E should be connected to the backplate yet when it is there is no output...

I've just tested the capsule in another circuit and there was no problems. And why would there be 17V at the backplate? Is that normal?

 

[Michael Tibes]

Can the dc voltage at the capsule be measured with normal equipment, is the source impedance not too high?

 

[micolas]

Can the dc voltage at the capsule be measured with normal equipment, is the source impedance not too high?

You are right, the impedance is very high, the bias voltages must be measured before the high value resistors. To check the proper operation of the dc/dc converter, I would measure immediately after the diodes of the voltage multiplier.
But @skyy7 knows this, he mentioned in his first post.
*More interesting to see would be the audio part and the diagram of the whole microphone should be drawn.
*It is not arrogance to buy a microphone with problems, on the contrary I always do that, I repair it, modify it or use it as a donor. I think that's the spirit of DIY.

 

[skyy7]

You are right, the impedance is very high, the bias voltages must be measured before the high value resistors. To check the proper operation of the dc/dc converter, I would measure immediately after the diodes of the voltage multiplier.
But @skyy7 knows this, he mentioned in his first post.
*More interesting to see would be the audio part and the diagram of the whole microphone should be drawn.
*It is not arrogance to buy a microphone with problems, on the contrary I always do that, I repair it, modify it or use it as a donor. I think that's the spirit of DIY.

I checked at the rectifying diodes and I was getting a nice 60v ac signal

I did notice a weird thing while testing a bit further that when I inject a sine signal at the gate of the JFET the output is actually quite a bit higher than my other mics and theres a weird clicking sound.

I'll report back with the audio portion circuit and a few voltage readings!

 

[ZHOUJUN]

Have you ruled out the capsule problem? You still need to troubleshoot each toggle switch. Good luck!

 

[skyy7]

Sorry! Been running into personal issues. Will let you guys know asap. I have confirmed it wasnt the capsule by trying it on another known working circuit.

 

[Voyager10]

My question is... What is E and VL? E seems to be where the backplate wire connects to, but why is there 16.9V? It seems the capsule is polarized at the membrane via a 1G resistor while the output is taken at the backplate. At what point should the membrane and backplate connect to the circuit?

E and VL are the backplate connection, and should be at half the DC-DC converter's output, i.e. +30V if the converter is producing +60V. It's fed via two 51M resistors so it will read low with an ordinary DMM; getting 17V isn't surprising.

R1 and R2 should be front & rear diaphragm connections. R1 gets +60V polarisation voltage; R2 gets +60V in omni, +30V in cardioid and 0V in figure-8. These are fed via 1G resistors so won't be easy to measure.

The amplifier input is taken from the junction of C51 & C52, which combines the signals from the two diaphragms. It's not taken from the backplate so injecting signal at point E won't work.

Do you get an output if you inject signal at R1 or R2? If so it's looking like a polarization voltage problem, if not maybe there's a problem with the JFET bias.