Neumann U67 Clone : D-U67 Tube Microphone Build Thread.

[jakearoo]

I had my bv12 ami transformer wired really wrong on two mics. Figured out what I wired wrong but the tubes won’t light up. How can I know if I fried the transformer or just need new tubes? Thank you a billion. I’m learning.

 

[egroys]

So my 67s have been workin wonderfully for a few years but recently I lost almost all signal in one.

Then I noticed is fig8 works fine but cardioid and omni have extremely low signal levels.

I fiddled around on the inside and nothing stands out as wrong. Continuity checks all appear fine and the capsule connections are solid.
I'm going to reflow the solder joints on the pattern switch.

Can anyone suggest what else I might have missed?

 

[OneRoomStudio]

So my 67s have been workin wonderfully for a few years but recently I lost almost all signal in one.

Then I noticed is fig8 works fine but cardioid and omni have extremely low signal levels.

I fiddled around on the inside and nothing stands out as wrong. Continuity checks all appear fine and the capsule connections are solid.
I'm going to reflow the solder joints on the pattern switch.

Can anyone suggest what else I might have missed?

Try checking voltages at the capsule connections. Maybe there’s something wrong with the voltage going to the front diaphragm or backplate.

 

[rock soderstrom]

I'm going to reflow the solder joints on the pattern switch.

Perhaps the switching contacts on this switch are also slightly corroded?

 

[egroys]

This does seem like the case when I test fig8. I'll do some concrete voltage tests but assuming continuity is solid on the front diaphragm and backplate, what could I do to verify if it's the switch? And if it is in fact corroded, is replacement the only option?

 

[rock soderstrom]

And if it is in fact corroded, is replacement the only option?

Switching the switch several times could improve the situation. I would not use contact spray at this point!

 

[egroys]

Ok so in cardioid I am measuring:
FD: -0.65V
RD: 3.0V (but it beings to drop as soon as I start measuring)
BCK: 238mv

B+ is 211V

Seems pretty off...

I'm an amateur so any help would be appreciated. This fella was working fine for almost 2 years...

 

[Khron]

Ok so in cardioid I am measuring:
FD: -0.65V
RD: 3.0V (but it beings to drop as soon as I start measuring)
BCK: 162mv

B+ is 211V

Seems pretty off...

I'm an amateur so any help would be appreciated. This fella was working fine for almost 2 years...

Where are you measuring those voltages?

 

[egroys]

Where are you measuring those voltages?

At the turret pin junctions on the tube PCB for FD and BCK, and for the BD on the wire connection pad on that same tube PCB

 

[Khron]

Rookie mistake - no offense

Keep in mind, your voltmeter has a certain input impedance (1meg or 10meg, whatever - check the spec sheet). The turret pin junctions have a 60meg(?) resistor in series with those.

When you apply your voltmeter there, you're basically putting a 1-10meg resistor between the turret pin and ground, and that forms a voltage divider (picture a potentiometer, if it's easier to understand). Which WILL end up showing you a far lower voltage than really is there (normally).

Rather probe between ground and just before the respective high-value resistors, for a somewhat-more-realistic reading.

 

[egroys]

Rookie mistake - no offense

Keep in mind, your voltmeter has a certain input impedance (1meg or 10meg, whatever - check the spec sheet). The turret pin junctions have a 60meg(?) resistor in series with those.

When you apply your voltmeter there, you're basically putting a 1-10meg resistor between the turret pin and ground, and that forms a voltage divider (picture a potentiometer, if it's easier to understand). Which WILL end up showing you a far lower voltage than really is there.

Rather probe between ground and just before the respective high-value resistors, for a somewhat-more-realistic reading.

Super rookie here so none taken! Does that mean before R1 and R2 in my case?

 

[Khron]

Assuming the component designators are the same as in the original Neumann schematics, you want to measure before R2 (backplate voltage, ~60v), before R16 (rear diaphragm voltage for fig-8, ~120v), and before R9 (front diaphragm / grid bias voltage, some small negative voltage with respect to ground).

 

[egroys]

Copy that. On it.
Side note: Could a faulty C8 be the culprit here? I am noticing on of my tube board pins is possibly making contact

 

[Khron]

Even if C8 was busted, that would only drag down the 120v for the fig-8 pattern. But it's most likely not, because the outside is not (or should not be) conductive.

That being said, seeing even the official build photos makes me almost physically ill

 

[egroys]

Lol ok good to know.

Here is what I have
R2 = 49V
R16 =110V
R9 = -205mv

Thank you so much for this guidance, btw!

 

[Khron]

Considering the divider for those voltages is made up of 2-3meg resistors, those readings sound about right, so it's unlikely your issues are caused by that.

Unless, as Rock pointed out, your pattern switch is on its way out... Or actually, the connection to the front diaphragm? Because if fig-8 works fine, the connection between C1, C10, R8 and the tube grid should be fine, and so should the backplate connection.

How about if you reverse where the wires from the front and rear diaphragms are connected? Does the issue follow the diaphragm, or the position of the pattern switch?

 

[egroys]

To clarify, where the front and rear diaphragms connect at the pattern switch, yes?

 

[Khron]

I'd rather swap red and orange (not red and white).

 

[egroys]

Ok, I'll have to try this later on. I will report back. Many thanks!!

 

[egroys]

I'd rather swap red and orange (not red and white).

I swapped them and now in the fig8 (right) switch position, Cardioid works beautifully.
In the center Cardioid and left Omni positions, the signal is extremely low as exhibited before.

Does that confirm that I have a bad switch?