Does anyone have gut shots of MCA SP1 wiring?

[777funk]

I had a wire come loose and the mic quit on me. I have a guess where it goes but would like to know for sure that it's going back where it belongs. Thanks in advance if anyone happens to have pictures of the boards.

 

[Khron]

How about posting photos of yours? Some of us might be able to figure it out, y'know...

http://cdn.recordinghacks.com/images//mic_extras/mxl/SP1-PCB.jpg

There's this publicly online, but not sure the wire you mean is visible there..?

 

[777funk]

Good idea! It's been so long, I can't remember what exactly has been done in mods, but it seems that I did something like the first picture as far as how the capsule is connected (second pic shows capsule connections).

Third picture, the top clear wire was the one that broke loose. I'm thinking it's the capsule polarizing voltage supply. I guessed soldered the clear wire to the capsule face wire terminal (connects to the red wire which runs to the capsule center). The mic still doesn't work. I'm guessing since it was dropped, maybe the capsule is dead.

 

[Khron]

I guessed soldered the clear wire to the capsule face wire terminal (connects to the red wire which runs to the capsule center)

If there's 40-60V coming through that wire, you may well have zapped the JFET.

In the sense that, assuming the clear wire is indeed the capsule bias voltage, that should have gone to the pin where the BLUE capsule wire goes.

You could still try to do that, but there's NO guarantee the JFET survived this last mod / test...

Third picture, the top clear wire was the one that broke loose.

Could you have taken that photo even more from the side, so the board is even less visible?

Snarkiness aside, maybe snap a more "head-on" photo, so some of the copper tracks are visible too?

 

[777funk]

I measured the wire after I'd done some soldering the first point that looked likely and measured 37VDC when it's open circuit. So I would assume that it's the capsule supply. It comes from what appears to match up with the circuit in the lower left corner of that schematic above.

I will look closer at the transistor. If that is the gate, I bet you're right. It may not like 37VDC. It reads zero volts when in circuit with the transistor.

How the capsule is wired currently worked well before the clear wire broke loose. I need to trace what I have in relation to that JFET. My eyes are getting to where I can't see it.

 

[Khron]

How the capsule is wired currently worked well before the clear wire broke loose. I need to trace what I have in relation to that JFET. My eyes are getting to where I can't see it.

If that Zapnspark mod was done before, the capacitor between those two isolated posts is also missing, so the only way the mic could've worked before the wire broke off, is if the wire was soldered to the (top of the) isolated post that's NOT connected to the JFET.

 

[777funk]

That is correct. The cap between the posts is missing. I am pretty far off from that schematic. I did whatever I did back in 2014. Unfortunately most of the part numbers do not cross with that schematic.

But fortunately, the JFET is still alive or at least passes sound. I moved the wire to the other side of the capsule and it lives. Now to figure out what I have for the wiring and get it closer to what's above.

The two outer legs of the JFET measure 0.04V each way on a diode test. I figured it was dead based on that but I'm hearing normal sound I believe.

 

[Khron]

You got lucky, then.

But as far as i can tell, the mod may have already been completed - there's still that second (now-redundant) 1G resistor there, but leaving it in does (and hurts) nothing.

 

[777funk]

Thanks and I had thought about disconnecting the 1G resistor, glad to know it will have no effect on the sound. I don't know much about condenser mics other than glad to have it working again. Thanks for the help Khron! Nice blog by the way!

 

[Khron]

Thanks for the help Khron! Nice blog by the way!

Thanks for the kind words, and glad we managed to sort this out, and that the JFET survived

And some years back, i got tired of posting about my various projects across subsections of several other forums, and decided to "centralize" them. It's been semi-dormant for a while now, but there's a few other things in the pipeline...