Neumann U67 Repair - Low End Loss - Help needed

[CJ]

most hand held meters will not be able to magnetize the core as they only use a few micro amps to measure ohms, some big bench meters use milliamps and these can temporarily magnetize the core but this will wear off over time,

to see how many ua your DMM puts out, set it to ohms and hook the leads to another DMM set to ma if you have one,

 

[RuudNL]

My multimeter puts out enough current to make a small 6 V lamp glow!
Especially in the low resistance setting, so I had my doubts.
Also somebody told me that his microphone never sounded the same after he had used a multimeter to measure the resistance of the output transformer, but this may have had a different reason. (Or it was just what he thought he was hearing...)
And yes, I know the story about the 'hard' and 'soft' core materials, but I think with a U67 it is better 'safe' than 'sorry'!

EDIT: Just measured the current: in the Ohms x 1 range my multimeter produces a current of 60 mA! 

 

[CJ]

wow, what kind of meter is that?  how long do batteries last?

 

[Moby]

My multimeter puts out enough current to make a small 6 V lamp glow!
Especially in the low resistance setting, so I had my doubts.
Also somebody told me that his microphone never sounded the same after he had used a multimeter to measure the resistance of the output transformer, but this may have had a different reason. (Or it was just what he thought he was hearing...)
And yes, I know the story about the 'hard' and 'soft' core materials, but I think with a U67 it is better 'safe' than 'sorry'!

EDIT: Just measured the current: in the Ohms x 1 range my multimeter produces a current of 60 mA! 

No worries about magnetizing the core. But again, it's possible to burn windings if u use multimeter with 60ma. Wow, change your multimeter', i never experienced such a "monster" Mumetall material doesnt handle DC, it will saturate ,that's why CJ suggested to measure C9 leak.  But right after you remove Dc it will act same as before. I experimented a lot with this material and have no doubts about it

 

[Gene Pink]

my multimeter produces a current of 60 mA! 

Yikes. An analog meter?

That current could burn out an LED, if the voltage is enough to light it.

And measuring a coil with it, while the coil may take 60mA, but when you disconnect the probe there will be a strong EMF kickback from the induction, with the voltage at the coil spiking really high. This is how an old-school ignition coil in a car works, break the 12V current on the primary when the points in the distributor open, and that primary kicks to a ~400 volt spike.

I don't know what this EMF kick spike would do to your particular transformer, but you should sell that meter to someone who works on cars. Or a fisherman that wants to bring up nightcrawlers for bait. ;-)


Any attachments to this post, DO NOT CLICK, I'm still fighting a rootkit virus, and posting this in "windows safe-mode". It's going to the shop tomorrow morning.

Gene

 

[API]

I just measured my Fluke 179 and the most i could get out of it was 1mA (in the auto mode).

 

[Conviction]

EDIT: Just measured the current: in the Ohms x 1 range my multimeter produces a current of 60 mA! 

Sounds like you should put that meter on the shelf. It must be busted?! Why would a meter require such a high DC output? Extremely inefficient resolution?

Edit: if it's digital?

My Fluke process meter is able to put out a 20mA DC source but that's for loop calibration or simulation purposes. Rarely used outside of industrial work.
While measuring resistance, it puts out less than 1mA.

 

[Whoops]

Hello,
thank you all for your help.

I got the other U67 from the studio so I had 2 to compare.

I repaired the PSU's and both have the correct output voltages now, th B+ is 210V (+-2%) and  the Heater supply is -6.37V

I measured the voltages at both microphones and they are consistent with the schematic and between both mics.

One of the microphones still has a Low end loss.
I changed the capsule of the mic with low end loss with the one of the Good mic.
And the Low end Loss follows the capsule.

So it seems it's the capsule itself that is the culprit.
When I turn the mic on ,as soon as I start to hear sound first time I speak to the bad capsule it seems that it has Low End, but then quickly disappears and becomes thin, consistently after that.

Does anyone ever experienced low end loss with an U67 capsule before?
is it repairable or the capsule is shot for good?

 

[ln76d]

Did you tried to swap the capsule sides?
How is condition?
Is it dirt?
Put some pictures here
My first idea is that the capacitance could change due to age!
You can try to gently move the screws - starting from center termination.
In worst case you can send it for reskin - i think that Thiersh is good option.
Neumann don't service that way - thay can sell new capsule.

 

[Whoops]

Did you tried to swap the capsule sides?

No, Should I try that?

How is condition?
Is it dirt?

Condition is very good in both capsules. Not dirty has other U67 capsules I've cleaned in the past that had the drop out problems.
Some light dust particles, I cleaned them after taking the pictures with some gentle blows with a Lens Cleaner.

Put some pictures here

My first idea is that the capacitance could change due to age!

Should I measure both capsules capacitance?
what values should I expect from a good working capsule

here are the capsules pictures:

 

[ln76d]

Looks pretty good.

2x50pF according U87 schematic.

Try to swap the sides - yes!

 

[radardoug]

Did you physically change the capsule, or just the whole head? Those two heads are different models. From memory, there are also parts inside the head.
Capsules can develop pinholes and fail as a result.

 

[Whoops]

Did you physically change the capsule, or just the whole head? Those two heads are different models. From memory, there are also parts inside the head.
Capsules can develop pinholes and fail as a result.

I changed the whole Capsule assembly plugin module back and forth between the mics, the low end loss always follow the same capsule assembly.

From the schematic it seems the only parts in the KK67 head assembly are the 3 switches (pattern, Pad, High pass) and one 500pc capacitor for the Hi-pass.

The strange thing is that one of the mics has a KK67 head installed while the other in a KK87s. That seems strange, Im still searching info on this but at the moment I only have information that the U67 uses the KK67 head and the U87 uses the KK87 head.
Does anyone know if there were U67 versions with the KK67 and also the KK87?

 

[ln76d]

Did you physically change the capsule, or just the whole head? Those two heads are different models. From memory, there are also parts inside the head.
Capsules can develop pinholes and fail as a result.

Good point!

From the schematic it seems the only parts in the KK67 head assembly are the 3 switches (pattern, Pad, High pass) and one 500pc capacitor for the Hi-pass.

And you want to check this all.
Also look inside is it everything corresponding to the schematic!

 

[Whoops]

From the schematic it seems the only parts in the KK67 head assembly are the 3 switches (pattern, Pad, High pass) and one 500pc capacitor for the Hi-pass.

And you want to check this all.
Also look inside is it everything corresponding to the schematic!

Will check that tomorrow

Thanks

 

[CJ]

it looks like they forgot to solder the pins on one of the units,

does the one with the limited response have the pins with no solder?

 

[radardoug]

Ahh, just caught the fact that it is a 67. You cant use an 87 head unit on a 67. The 67 has only one back plate on the capsule. Later 87's had the same capsule, but the 67 is completely different.

 

[ln76d]

Ahh, just caught the fact that it is a 67. You cant use an 87 head unit on a 67. The 67 has only one back plate on the capsule. Later 87's had the same capsule, but the 67 is completely different.

"You cant use" is little bit too  much here
It can't be historically correct but there's no problem to use it.
Backplates can be always connected together.

 

[Moby]

Yes, u can use double plate capsule shorted. Problem can be in reverse situation... Tiago, did u finally checked the capacitance? And cleaned all damned things inside?

 

[ln76d]

Agree - in reverse situation can be a problem - but in many cases there's possibility to change polarisation scheme.