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.

 

[skyy7]

Ah I understand... So is the 30v acting like ground?

And I do seem to be getting out put when injecting a signal into R1 and R2... just a very low output.

working on drawing out the preamp portion of the mic atm

 

[Khron]

Ah I understand... So is the 30v acting like ground?

As "mid-supply", yes. Although i would suspect it's closer to 60v, as described above.

 

[skyy7]

When injecting a 1khz 0.1v ac signal at R1 or R2 at the capsule, I get a very low but high distortion signal. When I inject it at the gate of the J305, I get a good signal but with very high distortion.

I am getting 2.5V at the gate of J305, 23.5V at the drain, and 0 at the source. Does that mean its just a biasing issue?

If so, I do not understand how to select and bias a "replacement" JFET for circuits. I thought it was just the Idss I had to match and ensure all the max voltage ranges were within spec. Also, to properly bias this circuit, would I have to replace R10 or R9 with a pot?

 

[Khron]

Also, to properly bias this circuit, would I have to replace R10 or R9 with a pot?

No, you'll want to replace R12 & R13 with a pot.

But this does indeed look 95% like the CAD GXL3000 schematic i linked above, only with what looks like a fixed bias for the JFET gate (as opposed to self-bias in the GXL). That is, of course, assuming the schematic you reverse-engineered is indeed accurate...

 

[skyy7]

No, you'll want to replace R12 & R13 with a pot.

But this does indeed look 95% like the CAD GXL3000 schematic i linked above, only with what looks like a fixed bias for the JFET gate (as opposed to self-bias in the GXL). That is, of course, assuming the schematic you reverse-engineered is indeed accurate...

Ah, I see... I'm unfamiliar with anything other than a self-bias (I've only messed around with Schoeps circuits)


It seems like R13 is marked as Brown, Yellow, Brown, Brown (140R) OR Brown, Brown, Yellow, Brown (110K)... Not sure which. Would it be safe to assume I'd need something like a 2Meg pot?

 

[Khron]

It seems like R13 is marked as Brown, Yellow, Brown, Brown (140R) OR Brown, Brown, Yellow, Brown (110K)... Not sure which. Would it be safe to assume I'd need something like a 2Meg pot?

110k is much, much more likely than 140R. At least because the bias voltage is meant to be in the 1-2V region.

Even a 100k trimpot should be plenty, the actual value isn't all that important, as long as it's not SO low that it loads down the phantom power / JFET supply rail.

 

[Voyager10]

I am getting 2.5V at the gate of J305, 23.5V at the drain, and 0 at the source. Does that mean its just a biasing issue?

No, that sounds very wrong. The gate should be more negative that the source in normal operation, and (unless your DMM is very special) you wouldn't be able to measure any voltage at all because of the 1G resistor in the way.

My bet is that the source and gate are interchanged. Many TO92 FETs have the gate as the middle leg, in the J305 it's at the end.

 

[Voyager10]

A couple of other comments on the schematic as drawn -

• The C6/C7 junction isn't really connected to VL, is it? I would expect the bass roll-off switch to be connected between this junction and point H, so it bridges across C7 when closed.
• R8, the FET drain resistor, ought to be more than 1K. In most variants of this circuit it's 20K.

If it helps, there's a schematic for a similar FET configuration here: https://groupdiy.com/threads/mxl.84887/ which has measured DC voltages. If you're seeing anything wildly different it's probably a clue.

 

[micolas]

Hi friends!
Now I also arrived at the table, quite late.
I have an old Sterling ST77 Made in Germany, of course only cardioid, but the topology is similar. Yes, the circuit is classic, @Khron and @Voyager10 , you are right, but there are also small interesting details.
I can say that it is my favorite for female voices such as soul, blues,gospel, etc. It has a great sound (even on shrill voices), thick, full, velvety, smooth, sweet.
I disassembled it many years ago with the intention of increasing the polarizarion voltage of the capsule with a dc/dc converter, being a bit bothered by the 18dB self-noise (I got used to the Rode specifications).
But I gave up, I didn't want to change its tonal balance, the headroom, the finesse of the saturation, the magic.
I dont remember If circuit has deemphasis but the transformer tames and warms the sound. The transformer is not at all linear in the high range, on the contrary, it achieves a pronounced native,intrinsic attenuation, as I have never seen with other output transformers. In addition, the capsule seemed to me to be larger than 34mm, perhaps another reason for the sound to be ample and darker.
This mic is a gem.
Unfortunately, only after the New Year's holidays I will open it, I will make photos,
measurements, possibly draw the diagram.
As a side note: I was thinking of reproducing the transformer by reverse engineering, (decades ago I made all my transformers myself, I even built electric guitar pickups with good results) Or ask @igs (Igor-UTM) to reproduce it.
I found it very tempting to drop the deemphasis circuit for k67/87 capsules with lots of treble, and to obtain a beautiful sound so simply!

 

[skyy7]

No, that sounds very wrong. The gate should be more negative that the source in normal operation, and (unless your DMM is very special) you wouldn't be able to measure any voltage at all because of the 1G resistor in the way.

My bet is that the source and gate are interchanged. Many TO92 FETs have the gate as the middle leg, in the J305 it's at the end.

Ok.... I have pin 3 connected to the 1Gohm resistor, pin 2 at C7, and pin 1 at R9. I think that's correct but I've made this mistake before so I wanna make sure. I was measuring 23.5V before C7, 0V before R9, and 2.5V before the 1Gohm resistor.

 

[skyy7]

A couple of other comments on the schematic as drawn -

• The C6/C7 junction isn't really connected to VL, is it? I would expect the bass roll-off switch to be connected between this junction and point H, so it bridges across C7 when closed.
• R8, the FET drain resistor, ought to be more than 1K. In most variants of this circuit it's 20K.

If it helps, there's a schematic for a similar FET configuration here: https://groupdiy.com/threads/mxl.84887/ which has measured DC voltages. If you're seeing anything wildly different it's probably a clue.

Sorry I misread the label on the PCB... It is connected to a bass roll off switch on the switch board.

Also, sorry about R8... the brown and reds are super similar. You're most likely right as it is probably Red, Black, Black, Red, Brown (20K). I initially read it as Brown, Black, Black, Brown, Brown (1K).

Also cross checking the voltages right now.