I've read the whole thread again and up to now, none of the information seems to make any sense, is contradictory, or is physically impossible. Before summarizing the things that confused me (and others), some questions about the circuit and how you built it:
- Nodes g1...g5 represent ground. I assume you tied them all together, but if you follow this schematic exactly to design a PCB layout in, e.g., Kicad or Altium or any other ECAD package for that matter, all these nodes would not connect to each other. Anyway, I find this a very weird way of drawing a schematic and confusing, at the very least.
- T5 connects to the capsule, but if I were to take the diagram very literally and follow it blindfolded to build, the ground terminal of the capsule would be floating in mid-air as there is no such connection explicitly drawn. Did you connect the capsule ground to either the ground net on the PCBA or to the chassis? And if it is connected to the chassis, is there a connection between XLR pin 1 and the mic chassis, as depicted to the left of nodes T1/g4? Without this capsule ground connection, it would explain why there is no sound from it (assuming the circuit is properly built and biased, etc.).
OK, so let's assume for now the ground connections are all as they should be. And let's assume you built the circuit
exactly as depicted in the schematic, then let me summarize what I see as confusing or contradictory:
- If the Source voltage is at 0V, it would mean there is no bias current running. You said audio is coming through, assumingly applied to node T5 (Gate), and assumingly through a 30-100pF cap to simulate a capsule and to avoid the generator output from disrupting the JFET bias. But without drain-source current flowing, there should be no signal at the output, or it should be heavily distorted. Did you just listen to the output signal or watch it on a 'scope to see if there is any distortion?
- In your first post, you said there's 28V on the Drain, and 29V on the R1-C3-R7 net. That would indicate there is a current flowing through the JFET, but it's just way too low. In post #24, you write there's ~29V at the Drain. This is contradicting information. But still, with 29V on the Drain, there should be current running through R7 (and hence, through R1, Q1, R2), because assumingly there is 30V on the Zener Z1. Orrrrr, this assumption is just plain wrong and the current through Z1 is too low to establish the nominal 30V. Or the zener has a negative tolerance and is on 29V. To find out if there's really a bias current flowing or not, I would suggest you measure the zener voltage and all voltages across R7, R1, drain-to-source of Q1 and across R2. The sum of these voltages should equal the zener voltage. Or maybe just note down the voltages on all of the circuit nodes and write them on the schematic. Then post this schematic. And write down all the voltages exactly as displayed on the DVM, without rounding them off.
- As already pointed out before, the voltages you measured on the Gate do not make sense. You said you measured 2.4V on the Gate, but this cannot be measured with a normal DVM with e.g. 10M input impedance. But even if you would shunt the 1G resistor with a low resistance just for the voltage measurement on the Gate, it is physically impossible to measure 2.4V on the Gate if the Source is at 0V. Or C4 must be leaking (cracking?), but then the Drain voltage would not be at 28-29V.
Next to writing down all the node voltages, could you also upload pictures of the top and bottom sides of your PCBA? If possible, revealing the direction of the diodes and elcaps, the color codes on the resistors etc. or any other detail that could help us diagnose the fault.
Finally, a last word on C1: depending on your transformer and personal preference, you may want to increase the value of C1 to a much larger value than 1uF as depicted in the schematic or the 470nF that you use. You can read more about this here:
https://sound-au.com/articles/audio-xfmrs.htm. I recently built some KM84 variants and used Aliexpress ASTDS T8 7:1 transformers. I did some frequency response measurements with various coupling caps, of which you'll find the REW graphs attached in a zip file. You can see a resonant bump with smaller cap values and increasing distortion. But having said that, this could be something you'd actually want. For a "transparent" low THD mic however, you would want larger capacitor values. I understand this is probably not your first concern right now, but I just wanted to mention it.
Good luck, and I hope you'll get the issue fixed.
Jan
