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

[egroys]

Continuity beeps, between what and what?

Capsule lead at the front of the capsule to the tube junction and every lead that's bridged there as well as the lead connected to the switch. Meaning I believe every point seems to have a solid connection. I'm super confused!!

 

[Khron]

But i'd start considering replacing R2 60meg. If no voltage ends up on the backplate, you'll get nothing in cardioid OR omni. Really sorry it took me so long to realize...

 

[egroys]

But i'd start considering replacing R2 60meg. If no voltage ends up on the backplate, you'll get nothing in cardioid OR omni. Really sorry it took me so long to realize...

No need to apologize. I can’t begin to thank you enough and the issue isn’t even solved! I pulled the resistor but my multimeter won’t measure that high so I need to get a replacement to see if that’s what’s going on!

 

[Khron]

At least temporarily, you can replace it with something even as low as 1meg or whatever you have in your stash. It's only acting as part of an RC-filter for the DC voltage applied to the backplate.

Especially with that whole "deadbug" 3D construction, it's not unreasonable that, in years of use, vibration led to material fatigue, and one of the legs of that 60meg became fractured and disconnected (perhaps even inside the glazing).

... But then again, you still measured a consistent 150mV even "downstream" of that R2... Yeah, this is making less and less sense, by the minute...

... But even with a 1meg input impedance meter (more usual is 10meg), you still should have been getting a 0.9v reading or thereabouts, so i'd still suspect that R2. Replace it with a lower value that you have (value isn't that important), and see if anything changes.

 

[egroys]

So I took off and put R2 back and then reheated some joints including C1 and I'm back in business!
Not exactly sure what was wrong but I am so, so, grateful for all the help. This community is second to none.

@Khron, how can I buy you a beer?

 

[Khron]

So I took off and put R2 back and then reheated some joints including C1 and I'm back in business!
Not exactly sure what was wrong but I am so, so, grateful for all the help. This community is second to none.

@Khron, how can I buy you a beer?

There you go then, "just" a wonky solder joint - glad we managed to suss it out!

Regarding that last bit, well... I've got a whole bunch of things "in the oven", as it were. Can't say when they'll make it to sale (maybe/hopefully this year?), but stay tuned

 

[Biula]

Boy, what a mess... fiddling with high impedance, hi voltages circuits is always a no- no for a non expert... measuring a 10 Megaohms impedance circuit with a 20.000 ohm per volt tester is nonsense!

 

[soapF]

Finished this build up yesterday, getting signal, all patterns/pads/cuts work great, h- and b+ voltages correct etc etc, however there’s zero low end (seems to cut off around 200hz after the fundamental) which led me to believe there is either an issue with the capsule, tube (EF86 EH), or transformer wiring/hi-z switch wiring.

Today I rebuilt the switching mechanisms + capsule connections with Teflon wire and ensured their integrity (side note; what a godsend Teflon wire is, first time using). However the issue persists. I followed the attached diagram to connect the transformer, this will likely be the first thing I check after I finish transplanting the PSU to its new chassis from collective cases. I’m using what I believe is an AYM k67 (backplates have continuity) and it sounds like I potentially have phase cancellation going on in the low end maybe from the tertiaries being swapped.

Anyone who can offer any input/share advice is greatly appreciated. Thank you! And thank you Dany/community for providing the materials & info you all do. The last 7 months of learning about this stuff has been a joy.

Edit: more info

 

[youngshu]

Can someone help me figure out why my psu h- is -2.3 with no mic plugged in? First psu build so I’m a little lost

 

[Murdock]

Can someone help me figure out why my psu h- is -2.3 with no mic plugged in? First psu build so I’m a little lost

Which power transformer did you use?
And which power supply circuit? The one posted on the first page?

 

[youngshu]

Which power transformer did you use?
And which power supply circuit? The one posted on the first page?

I used everything in the BOM on the vintagepcb website. My B+ is 250v which is correct according to what iv read on here. The H- is supposed to read -16 from what iv read on this thread. I have not tested with mic plugged in as im waiting for my mic body to be delivered(i dont know if that will make a difference). I Have triple checked my wiring and it seems correct to me.

 

[Murdock]

I used everything in the BOM on the vintagepcb website. My B+ is 250v which is correct according to what iv read on here. The H- is supposed to read -16 from what iv read on this thread. I have not tested with mic plugged in as im waiting for my mic body to be delivered(i dont know if that will make a difference). I Have triple checked my wiring and it seems correct to me.

Measure the voltage right after the rectifier diodes and at different points along the heater circuit to see where the drop occurs.
Maybe also measure the AC voltage right before the rectifier.

 

[pasarski]

And be careful doing that to not electricute yourself.

 

[youngshu]

ok so ac voltage of transformer is reading -1 any idea why that would be?

 

[youngshu]

ok i tuned it on and off a couple times and now its ready -12ish which is much closer any idea why that would have made a difference? i didnt do anything other than cycle it a few times im glad its working but worried about what else might be going on here.

 

[youngshu]

ok i tuned it on and off a couple times and now its ready -12ish which is much closer any idea why that would have made a difference? i didnt do anything other than cycle it a few times im glad its working but worried about what else might be going on here.

not getting 20v reading at ac on pcb. 120ac is getting to the transformer hen nothing is getting to pcb. Bad Transformer?

 

[Murdock]

not getting 20v reading at ac on pcb. 120ac is getting to the transformer hen nothing is getting to pcb. Bad Transformer?

Sounds like it.
You can desolder the transformer and measure the winding resistance. Maybe it’s open.
Can you post a picture of the transformer you have?

 

[youngshu]

Thanks for the help with the transformer Murdock i figured out i ordered the wrong one initially got everything working now. I Have no idea where to even start with this trouble shooting so i recorded a video so you guys can hear the issue and help mne figure out a plan to fix it. Thank you!