Neumann Vintage U87 Clone : Build Thread.

Arkenberg · Nov 7, 2015

xWESTWOODx said:
Thanks for the reply Pip.

'm using the peluso k87i capsule. I'll try giving the PC board a good clean again and see if that helps. I tried doing a search in the forum for other comments/documentation, but I can't seem to find anything. Any recommendations on what I should be searching for?

Thanks again for all the help,

Mat

Had same problem when I built my two Mics. I found I had made one or more bad soldier connections. It seemed to be on the large Caps on the main board. The negative side did not want to soldier very well.
Jon

Arkenberg · Nov 7, 2015

dacapitan said:
Im still having a problem with my mic and have been finding it hard to get any real troubleshooting help here, if anyone could please help me through this, I would really appreciate it, as I don't know what the problem is or how to go about solving it.

since first calibration and power on - the FET keeps blowing when I trim the drain voltage to 11.5VDC. I have replaced it a few times now.

Something that I have noticed which may be completely unrelated - is, with my working mic is when the drain voltage is 11.5V the source leg is around 2.5VDC. But, with my problematic Mic, when the drain gets to 11.5V the source leg is at over 4VDC, the drain leg stays at 11.5VDC for about half a minute and then it drops to 4v (same as source leg) and it cant be trimed higher than 7v - where the drain and source measure 7v at this point, and I have to replace the FET.

I took voltage readings according to the schematic, below are the results:

Problem Mic:
R18 - 46.1VDC (shematic: 46.6VDC)
R19 - 46.1VDC (shematic: 46.6VDC)
Zener 24 - 22.98VDC (shematic: 23.6VDC)
R12, R14, C10 - 19.28VDC (shematic: 20.62VDC)
C5 - 39.7VDC (shematic: 40VDC)
FC - 39.7VDC (shematic: 40VDC)

Working Mic:
R18 - 46.6VDC (shematic: 46.6VDC)
R19 - 46.1VDC (shematic: 46.6VDC)
Zener - 24 22.88VDC (shematic: 23.6VDC)
R12, R14, C10 - 20.96VDC shematic: 20.62VDC)
C5 - 42VDC (shematic: 40VDC)
FC - 39.7VDC (shematic: 40VDC)

Capsule and Transformer were not connected when this happened, I have Tried disconnecting the switch board too but I still get that high 4V voltage at the source, so I quickly trim it down before I have to replace it again.

Can you send pictures of the board please. It might help to see what you are working with.

Jon

Christopher L · Nov 14, 2015

Could a c12 capsule with isolated backplates be used as the capsule. I have one and thought since backplates where isolated this might work? anyone tried? thanks in advance

ln76d · Nov 14, 2015

Christopher L said:
Could a c12 capsule with isolated backplates be used as the capsule. I have one and thought since backplates where isolated this might work? anyone tried? thanks in advance

Did you mean chinese "CK12"? If so - yes. It's mostly edge terminated K67 copy. Isolated or non isolated backplate will work. With isolated, you will need to connect both backplates. BTW, you can remove your both posts in the wrong topic by "remove" button

Jazzdude · Nov 23, 2015

Hi,

I've just finished building a U87 clone into a Samson C03 body. Everything works, but the PAD switch doesn't. It almost entirely kills the signal, only leaving frequencies above a few kHz through, with a lot of hiss. There's one other symptom which I find odd but I'm not sure if it's standard behavior. Switching the pattern I notice a really strong click and then silence for a while, as if the capsules are being repolarised. Is that normal? My other multi-pattern mics don't do that.

I've checked the capsule connections, and they're all right. I've also measured the backplane isolation, it's also fine. What I've found was that the styrene cap C15 has a isolation of only ~~1.7~~ 0.7 MOhm , which doesn't seem to be a lot for the high impedance board. Also, the cap I got is labeled 560J not 560P, not sure what that means.

So, is my styrene capacitor bad? If yes, what are the recommended non-styrene alternatives? The BOM doesn't list an alternative for C15.

Thanks,

Jazz

Jazzdude · Nov 24, 2015

Jazzdude said:
I've just finished building a U87 clone into a Samson C03 body. Everything works, but the PAD switch doesn't. It almost entirely kills the signal, only leaving frequencies above a few kHz through, with a lot of hiss. There's one other symptom which I find odd but I'm not sure if it's standard behavior. Switching the pattern I notice a really strong click and then silence for a while, as if the capsules are being repolarised. Is that normal? My other multi-pattern mics don't do that.

I've found the problem was a ground connection on the switch board that I had overlooked. Now everything works as it should!

OrganizedKonfusion · Nov 28, 2015

So, I took a break from trying to troubleshoot my mic, as I've had plenty of other projects to keep me busy for awhile, but now I'd like to take another look at the issue that I outlined in the quote below. My question is; do I need to disconnect my capsule in order to troubleshoot the circuit? Or is that only if I want to trouble shoot my capsule? I neglected to check all my voltages when I finished the mic.

OrganizedKonfusion said:
Ok, so I finished my D87 and it works great in cardioid! The problem is with the the other two polar patterns... in either one, I get alot of static noise that sounds like I've got my mic out in some strong winds. I still get signal from the capsule, it just sounds faint and drowned out by all the noise. Forgive my ignorance, but since the mic works in cardioid I would assume that the front and rear diaphragm connections are good and maybe the problem is with either the front back connection or the rear back connection? Could a bad solder joint cause sound like that? Any opinions would be greatly appreciated.

OrganizedKonfusion said:
Ok, well I switched out the saddle for my mount and it hasn't paid off. I have the noise back in figure 8 pattern and its still in Omni as well. This saddle mount was slightly thicker than the last so I drilled holes to line up with the capsule and then I drilled a little well so the saddle screws would sit a little deeper and thread into the capsule a bit more. Same problem. I decided to try those patterns out with different combinations of low cut and pad engaged. It seems like the switches do everything that they should. However, when I'm in figure 8 and the low cut is engaged the noise disappears. I turned up my pre and was able to get a clean recording without any audible noise. I can only hear noise again if I really crank the pre. In Omni when I engage the low cut I still have noise its just a little less present. I think my next best guess is to check my wiring and switches for cold solder joints...

Correction: Noise disappears completely in figure 8 with low cut engaged. Definitely going to take a look at my switches.

mappyo · Nov 29, 2015

Hi, I recently finished building my d-u87 mic and wanted to share a pic. This one uses an AMI transformer and a tskguy capsule. Sounds so good...

Guitylerham · Dec 10, 2015

Hey guys, I'm nearly finished with my u87 build and being my first, I've run into a problem. I can't seem to bias my jfet above 2.5vdc! I can bias it lower than that with R11 trimmer but not above. I get 47vdc phantom through the xlr, I get 22.8vdc at the zener and 19.27vdc at the top of R14. I injected some audio signal into the drain of the jfet and heard it back through the preamp that I have connected to the mic. I also injected the signal into FD and heard a quieter signal that once I adjust my preamp gain, came back up to nominal level. It sounded clean and clear. So this leads me to believe that the mic is baaasically functioning ok, it just hasn't been biased yet and is currently unable to be biased correctly. I have continuity between the main board and ground (chassis). My IDSS of my jfet is down around 6ma. I can't think of any other pertinent information but this has me really stumped. I know I'm close! Thanks guys for all the awesome help so far. I've made it through half the pages on this thread looking for solutions and I'll keep looking in the meantime.
-Tyler

poctop · Dec 10, 2015

Guitylerham said:
Hey guys, I'm nearly finished with my u87 build and being my first, I've run into a problem. I can't seem to bias my jfet above 2.5vdc! I can bias it lower than that with R11 trimmer but not above. I get 47vdc phantom through the xlr, I get 22.8vdc at the zener and 19.27vdc at the top of R14. I injected some audio signal into the drain of the jfet and heard it back through the preamp that I have connected to the mic. I also injected the signal into FD and heard a quieter signal that once I adjust my preamp gain, came back up to nominal level. It sounded clean and clear. So this leads me to believe that the mic is baaasically functioning ok, it just hasn't been biased yet and is currently unable to be biased correctly. I have continuity between the main board and ground (chassis). My IDSS of my jfet is down around 6ma. I can't think of any other pertinent information but this has me really stumped. I know I'm close! Thanks guys for all the awesome help so far. I've made it through half the pages on this thread looking for solutions and I'll keep looking in the meantime.
-Tyler

it would seems to me that you are measuring the source voltage not the drain voltage then ,
you should be measuring the drain voltage

depending on what version of board you have R6 either installed backward for green pcb only ,
on the blue board you dont need to reverse R6 and there is an indication of Bias injection Signal Near R6 ,
you should be able to adjust the DRAIN Voltage to about 11.5V as a starter ,
BEst,
Dan,

Guitylerham · Dec 10, 2015

poctop said:
Guitylerham said:

Hey guys, I'm nearly finished with my u87 build and being my first, I've run into a problem. I can't seem to bias my jfet above 2.5vdc! I can bias it lower than that with R11 trimmer but not above. I get 47vdc phantom through the xlr, I get 22.8vdc at the zener and 19.27vdc at the top of R14. I injected some audio signal into the drain of the jfet and heard it back through the preamp that I have connected to the mic. I also injected the signal into FD and heard a quieter signal that once I adjust my preamp gain, came back up to nominal level. It sounded clean and clear. So this leads me to believe that the mic is baaasically functioning ok, it just hasn't been biased yet and is currently unable to be biased correctly. I have continuity between the main board and ground (chassis). My IDSS of my jfet is down around 6ma. I can't think of any other pertinent information but this has me really stumped. I know I'm close! Thanks guys for all the awesome help so far. I've made it through half the pages on this thread looking for solutions and I'll keep looking in the meantime.
-Tyler

Click to expand...

it would seems to me that you are measuring the source voltage not the drain voltage then ,
you should be measuring the drain voltage
depending on what version of board you have R6 either installed backward for green pcb only ,
on the blue board you dont need to reverse R6 and there is an indication of Bias injection Signal Near R6 ,
you should be able to adjust the DRAIN Voltage to about 11.5V as a starter ,
BEst,
Dan,

Thanks for responding, Dan! I believe I have the jfet oriented correctly and am measuring from the Drain. I have the green tapered boards and I installed R6 backwards as directed. In order to measure drain voltage, I connect my black probe to chassis/ground (and it does read continuity with the board) and the red probe to the Drain leg of the jfet, correct? I haven't tried injecting audio signal at R6 yet because it worked when I injected it either at the Drain directly and even at FD, the input of the capsule!

The Drain is the leg closest to us in the picture, middle leg goes to isolation pin, which connects to "g" on the other board.

Edit: regarding wiring the transformer, can I assume it's been wired correctly based on the voltage at my zener? My T13 transformer has a set of yellow wires reading about 22 ohms or so and black wires reading over 400 ohms. I wired the two yellows to the two pads closest to the xlr and the black wires to the other two pads. My assumption is that a coil is a coil so it doesn't matter which yellow goes to which of the two yellow pads and same with black.

Guitylerham · Dec 10, 2015

Tantalums are POLARIZED with tiny + sign. God, how could I have overlooked that. Stay tuned!

So I replaced those capacitors correctly. Got more voltage on the Drain. As I'd increase the voltage, it would spike up to 12-16vdc and then settle back down to 6vdc and then I'd turn the trimmer more, it'd spike up again and then settle down to something higher like 7vdc and so on until at one point I got the stabilized reading to be about 12vdc but then something happened and now it won't bias over 6.7vdc. So I replaced the Jfet thinking I blew it but I still have the same problem. What now? I was so close earlier!

Khron · Dec 10, 2015

For what it's worth, tantalums are really sensitive to being used "out of spec" (and being reverse-biased pretty much qualifies as that). I'd try, first of all, replacing the one you installed backwards.

And, on a personal note, despite the "vintage-ness" of it all, i shudder at the sight of tantalums, especially in the signal path. Not like there's not plenty of physical room in a U87 body for a few good-quality electrolytics ;D But that's just me...

Guitylerham · Dec 10, 2015

Well I'm about to throw in the towel. I replaced one of the tantalums with the correct electrolytic and tested it out. Got weak signal. Couldn't bias right. So on a hunch, I swapped black and yellow xfrmr leads and got much louder signal. Could bias the jfet to 11.5vdc. Signal was great. Hooked up the capsules, had a working mic albeit a bit noisey and stormy sounding. Figured I should clean the boards one last time with isopropyl, figured if I was careful and only scrubbed the back side that the styrene caps would be ok. Well buttoned it back up and there was nothing but horrible screechy noise and no signal from speaking into the capsule. Opened it back up looked around, cleaned up what looked like chalky crust on the trace side of the board and then boom, nothing. Checked with my meter, my xlr is sending 47vdc, but my board isn't getting it. its not even making it to the board where the xlr wires are soldered to. I checked for continuity between the pins and those pads and it reads good. So I'm pretty close to quitting and just buying all new parts or something. Thanks for the help so far, guys. I need a break.

Khron · Dec 10, 2015

I don't suppose you've tried another xlr cable, have you?

Guitylerham · Dec 10, 2015

Khron said:
I don't suppose you've tried another xlr cable, have you?

Good idea. I didn't try a different xlr but I touched up a couple solder joints and got the phantom power back but never got it to work again. Now I can't bias my the Drain on the jfet to anything but 22vdc so something blew. I can't work with that board anymore, I don't trust anything on it now. I've already compromised several pads by soldering so many times. Gonna order a new board and parts.

Khron · Dec 10, 2015

Ah, bummer then

Better luck next time, hopefully...

Guitylerham · Dec 10, 2015

Khron said:
Ah, bummer then Better luck next time, hopefully...

Thanks!

mixinmono · Dec 14, 2015

Hi everyone! I'm just about to order two of the most recent BOM for a matched pair build ;D ;D. Unfortunately, the resistors for R4 & R5 are out of stock on Mouser and I'm thinking I'll just opt to use the 68Mohm resistors instead. Does anyone have experience using the 68Mohm resistors? Is there much of a difference between them compared to the 60Mohm resistors? Thanks!

fsalter · Dec 14, 2015

There is absolutely no dicernible difference...other than the price.

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