Neumann U67 Clone : D-U67 Tube Microphone Build Thread.
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egroys
Well-known member
I'll give it a go tonight. The 60meg is floating so there are no pads or traces involved.
Both ends? Can't imagine why...
egroys
Well-known member
Last night I disassembled the mic **almost** entirely. I replaced the thick wires with nimble ones, I reheated **almost** every joint and bridge.
With high hopes, I was deflated to find that this changed nothing. I know that it has to be one small thing but cannot seem to find it.
What else could have fig8 working and cardioid and omni not!? While in the cardioid and omni position, I still get audio from the mic when I am poking around with a chopstick.
Here is R2 from the build guide
Does the same behaviour happen with the grill and body tube off? Mic will hum like crazy but you should still be able to check front/rear audio.
Be careful.
Be careful.
Obviously (?) the fixed voltage applied to the backplate is missing. Front diaphragm is (more or less) at ground potential; without ~60V on the backplate, you'll get no signal in cardioid or omni.
In fig8, the rear diaphragm gets 120v, so even with the backplate floating, you might get SOME signal. May or may not be closer to a cardioid pattern though - I can't quite say which side the "front" of it might be, though. Or some wonky omni pattern, even? How's the side-rejection in fig8, by the way?
egroys
Well-known member
Yes, same thing is happening with both grill and tube off.
egroys
Well-known member
I do get some signal in cardioid, but like 3% level of fig8. Omni is even less.
Fig8 sounds more like 'reverse' cardioid but is loud and clear. So the rear capsule is like 100% and the front sounds like 85%. The side rejection is pretty solid, so it's definitely not omni.
Any other measurements I could take? Everywhere I do continuity checks, I am getting solid beeps.
What voltage are you getting at the 2meg/3meg divider? "Before" that 60meg resistor going to the backplate...
egroys
Well-known member
103V at the R13/15 junction and 47V on the C4 side of R13 and R12
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egroys
Well-known member
49V
So far, so good. Can you solder a piece of wire in parallel with R2 (in essence, bypassing it), and see what voltage you can measure ON the capsule's backplate? What capsule were you using, by the way?
egroys
Well-known member
Beesneez K67. So to be clear, the wire goes on each side of R2?
So the two backplates are indeed in electrical contact, and only one wires is coming from them to the circuit, right?
Yes.
egroys
Well-known member
And is that with R2 bypassed with the wire, or not?
Come to think of it, it might be easier to just move where the backplate wire is soldered in the circuit, for this test... Ie. moving it from where it is now (one end of R2) to the R12/R13 node.
egroys
Well-known member
With the wire bypassing R2, yes. I will solder it to the R12/13 node shortly.
egroys
Well-known member
3.3V with the backplate connected directly to the R12/13 junction
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