U87i give virtually no output

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Rob_K

Member
Joined
Oct 17, 2015
Messages
11
Hi there,
I wonder if someone could help me here, I have built one of these microphones and it works well, we are very pleased with it. But I come to build a second one, and there is virtually no sound coming from it at all. If you ramp up the gain and shout into it, there is a very small signal showing, so it is not nothing, however if you switch one of the settings on it, it pops a full signal through.

The mic calibrated really nicely using a function generator and oscilloscope when building it, so I am hoping that the main circuits are fine, and without leading people on the wrong path, I am assuming maybe that I have the transformer the wrong way round, as it seems logical to me that a 1:10 transformer, the wrong way round would give a x20 attenuation, i.e. losing the x10 as if it wasn't there, and then reducing again by x10 for being the wrong way round.

I have tried to test it using the logic of one of the windings giving a low resistance and the other windings giving a much greater resistance, which is the case:

One set of windings = 33 Ω
other windings = 1.36 KΩ

but then I am assuming that that there should be infinite resistance between the windings, however I am getting something similar to 4MΩ which seems a little low.

I am completely new to this, and don't really know how to troubleshoot this at all, could someone give me a few pointers. I am used to troubleshooting things that have already been built correctly.

Please point me in some direction.

Kind regards
Rob
 
Long story short, the 1.36k winding is the input (towards the FET & coupling capacitor), and the 33ohm winding goes to the XLR.
 
Thank you, I will give that a go this evening and see if that solves the problem. To test this, I desoldered the capsule just in case any wayward current went that way, then de soldered the transformer to test it. While I was back in that position, I checked the voltages throughout the circuit, and they were all within 1-2 V shown on the schematic, that said, the phantom power was only 46V not the claimed 48V. I am assuming that this is within tolerance though, as I used the same interface to test both the mics.

Kind regards
Rob
 
Well, as per the standard, the 48v is supplied through a pair of 6.8kohm resistors (inside the preamp), so with any current draw, there WILL be some voltage drop across those. 1-2V is nothing to worry about - at least in the specs of modern microphones, most say they need the phantom power to be anywhere between 44v and 52v (roughly +/-10%) :)
 
Ok, I'm  little lost now, the transformer appeared to be wired correctly, but there is still virtually no output signal. Somebody has suggested that the jfet may have failed, but I am kinda clutching at straws here. all the voltages throughout the circuit match those on the wiring diagram, and it is calibrating ok and I would expect there to be an issue here if the transistor was faulty.

Is it possible that the cap is faulty? What should I be looking at next here?

Any help on this would be really appreciated.

Kind regards
Rob
 
I don't suppose you have any other capsules you could test the mic circuitry with, do you? Or a signal generator?
 
Just use the same signal generator and oscilloscope which you  used for biasing fet and trace the signal.
There's a  possibility that you have somewhere faulty connection or broken pad on PCB.
Tracing signal should show the point.
For start you can check the signal before and after output capacitor.
Also you can start with checking all the ground connections in the circuit with simple continuity tester in multimeter (the "beeper").
Is the one side of the transformer winding which schould go to ground properly connected?
 
Thanks very much for all your input, it has been a very busy time, will be working with this this week hopefully.

One quick question, how robust is the capsule, at present I have de soldered it in case I zap it, is this a wise move or not necessary ?

Kind regards
Rob
 
You don't need capsule, even better - put instead the capsule signal from generator by 47-100pF capacitor.
 
Thanks for all your input. After chatting to a couple of people, it seems that the transistor may have failed, it is a 2N3819 JFET, but this component is no longer available, what would should be used instead?
 
Don't chat, just check :)
2n3819 you cn find on ebay.
Also you can use 2SK170BL, J305, 2N5457 and many others.
 
ln76d said:
Don't chat, just check :)
2n3819 you cn find on ebay.
Also you can use 2SK170BL, J305, 2N5457 and many others.

parts from the BOM
http://ca.mouser.com/Search/ProductDetail.aspx?R=2N3819virtualkey61000000virtualkey610-2N3819
D.
 
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