ELA M251 Inspired Microphone - Build Thread

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dmp

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The polarity voltage should *only* connect to R16, not C11.
This is the picture from the 1st page. The polarity connection is connecting to R16 and C11. The schematic seems to show C11 as C5 though, but also shows the polarity connected to both.
Confusing that the schematic has different component labeling than the board.

If R14 is correctly installed as 8M I would not suspect C1

with the mic plugged in, polarity at the mic is half what's being sent from the PSU.
Points to the problem being at the polarity connection on the mic board, or R16, C5/C11 connection.
 

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ripemedia

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Definitely there is an extra DC current drain which is dragging down B+, which in turn drags down everything else. The polarity voltage should *only* connect to R16, not C11. But a mis-wiring seems unlikely given it was working before.

I would first suspect C1 / C5. You can lift them out and re-test, because they are there for noise reasons: the mike will completely function without either of them.
Thanks. I am still confused about the circuit flow. I'm attaching an image of the PCB. According to this, POL is supposed to connect to R16 and C11, so this seems counter to what you suggested. Also, I can't find C5 anywhere.
 

ripemedia

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This is the picture from the 1st page. The polarity connection is connecting to R16 and C11. The schematic seems to show C11 as C5 though, but also shows the polarity connected to both.
Confusing that the schematic has different component labeling than the board.

If R14 is correctly installed as 8M I would not suspect C1


Points to the problem being at the polarity connection on the mic board, or R16, C5/C11 connection.
Ah. Got it. This clears it up for me. Thanks!
 

Matador

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Apologies, I didn't realize that there were still builds based on those earlier versions.

Mis-wiring of the board seems unlikely given it was working properly before. The main components that can "go bad" that are consistent with what you are seeing is a cap that might have started leaking DC. I would disconnect the two filter caps (the one on the polarization, and the other where the 60V rail is generated) and see if the voltage(s) recover to normal.
 

ripemedia

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Apologies, I didn't realize that there were still builds based on those earlier versions.

Mis-wiring of the board seems unlikely given it was working properly before. The main components that can "go bad" that are consistent with what you are seeing is a cap that might have started leaking DC. I would disconnect the two filter caps (the one on the polarization, and the other where the 60V rail is generated) and see if the voltage(s) recover to normal.
Ok I lifted C1 and C11/C5 and the problem was still there — POL value is about half what it is when then mic is not connected.. I don’t understand what it could be since there seems to be a simple path from the polarity input.

I did some checking at the mic connector and POL is getting a clean connection to the PSU. And I confirmed POL isn’t shorting with any other inputs at the connector.

Let me know what else I can check to try to fix this. Thanks!
 

dmp

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Pull out R16. Check that the POL voltage doesn't sag with it removed, and then put in a new 30 M
 

ripemedia

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Well this is bizarre. I removed R16 and the voltage sag is still there. I’m really puzzled now.
 

dmp

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Just keep removing things until you find the problem. Lift the pol wire from the mic pcb?
Bad XLR, bad cable, the problem is somewhere
 

ripemedia

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Just keep removing things until you find the problem. Lift the pol wire from the mic pcb?
Bad XLR, bad cable, the problem is somewhere
Thanks. I removed B+ from the mic board and now I’m getting a proper reading at POL. Could you point me in the direction of how B+ voltage on the board could affect POL? Thanks!
 

dmp

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Sounds like your capsule is shorted from the diaphragm to backplate.
Try a different capsule? First you could re-hookup everything and remove the capsule and see if the voltages are right.
 

ripemedia

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Sounds like your capsule is shorted from the diaphragm to backplate.
Try a different capsule? First you could re-hookup everything and remove the capsule and see if the voltages are right.

Thanks. I did try a different capsule previously and had the same issue. I just now tried removing all the capsule connections completely and the issue still exists. I guess it’s good to know the capsule isn’t messed up, but I’m still at a loss as to what the issue is.
 

dmp

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With the capsule out, there shouldn't be any electrical connection between B+ and POL on the PCB (unless the schematic is wrong)
You could post some pictures?
 

ripemedia

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Here are some pics. This is with everything reattached. Thanks!
 

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ripemedia

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Hi. Still looking for a solution for this. Maybe it's time to try replacing some components? Is it most likely to be a capacitor that is the issue? Could this have been caused by a voltage spike? Last time I used the mic it was not plugged into a power conditioner. If that has blown a component, which could it be and is there a way for me to test it? Thanks.
 

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