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U47FET audio fades to black

 

Hi everyone, I posted a question on the groupDIY build thread for the D47FET from but have not received any response so I'm casting a wider net here.
I built this mic from the above PCBs into the relatively new body described in this with a K47 capsule from .
When I power up the mic with P48, I get audio then it fades to silence in a mere second or two. I replaced the FET, a 2N3819 with one I tested prior to soldering in, but same result.
I always use a heat sink when soldering semiconductors, styros, etc.? I inspected all joints under magnification and they look ok.
All semiconductors were purchased from Mouser, if that means anything regarding the counterfeit semiconductor market we are enjoying these days.
The schematic Dan posted on his site is .
My hope is this symptom is common enough in FET circuits that someone out there will have experienced it and can suggest likely culprits. Naturally getting the correct wording into a Google search is difficult so results are not helpful at all.
 

I'd suggest replacing the K47 with a 72pF cap so you can then inject audio signal at the capsule position and trace it through the circuit. Also check the voltages that are referenced in the schematic.

I'd also suspect a cap has failed. The styro's can be sensitive to excessive heat when soldering.

Cheers,
johann

On Mon, Aug 12, 2024 at 8:51?AM Mark Day via <neowalla=gmail.com@groups.io> wrote:
Hi everyone, I posted a question on the groupDIY build thread for the D47FET from but have not received any response so I'm casting a wider net here.
I built this mic from the above PCBs into the relatively new body described in this with a K47 capsule from .
When I power up the mic with P48, I get audio then it fades to silence in a mere second or two. I replaced the FET, a 2N3819 with one I tested prior to soldering in, but same result.
I always use a heat sink when soldering semiconductors, styros, etc.? I inspected all joints under magnification and they look ok.
All semiconductors were purchased from Mouser, if that means anything regarding the counterfeit semiconductor market we are enjoying these days.
The schematic Dan posted on his site is .
My hope is this symptom is common enough in FET circuits that someone out there will have experienced it and can suggest likely culprits. Naturally getting the correct wording into a Google search is difficult so results are not helpful at all.
 

If you used tantalum caps, be sure you placed them in the right direction. At least on SMT versions, what looks like the negative terminal with the stripe, is actually the + side.
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Did you clean the board with IPA or flux remover? Flux remanents can become conductive in the Hi-Z area and affect DC bias. On the PCB pictures, I see no provisions like teflon turrets or other means for Hi-Z insulation, but maybe they are there. If not, be sure to clean the board thoroughly but don't spill the cleaning fluid on the styroflex caps as they are sensitive to that as well.? Other film caps could be sensitive to cleaning agents too. Check out the datasheets. Caps suitable for cleaning have little standoffs, so the fluids don't get trapped underneath.
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Good luck fixing your mic!
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Jan
 

The schematic Dan posted on his site is?.
Sheesh!? This is MUCH more complicated than the original U47.? Dun unnerstan it at all
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Can any gurus explain this to an ex-mike designer who is down to his single remaining brain cell?
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Are you sure the FET is 2n3819?? ... though in those days (1984), it was one of the few LN FETs available.
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Can you check all the voltages shown on the schematic?? Both at startup and also after the sound fades away ... and also the voltage at emitter T3 / drain T1 which isn't specified.
 

For what it's worth, I built the D87, from vintagemicrophonepcbkit.com. I remember having the fade to black phenomenon. In my case it was me overheating the styroflex caps, they become resistors. I ended up replacing the damaged cap with other wima film caps and then it worked fine. Those polystyrene caps are so fragile. In my experience it's always my fault, and typically soldering related... not whether I bought the best components. I'd love it if a guru could comment on the u47fet schematic...it's one I've never understood!
 

On Wed, Aug 14, 2024 at 06:31 AM, Jonathan Dent wrote:
In my case it was me overheating the styroflex caps, they become resistors. I ended up replacing the damaged cap with other wima film caps and then it worked fine.
Small styroflex caps should be replaced by NPO/CGO ceramics.? Much more reliable, better caps and no microphony.

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