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Author Topic: U67 Capsule - Low End Loss  (Read 2320 times)

ilcaccillo

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U67 Capsule - Low End Loss
« on: May 17, 2016, 03:59:22 pm »

Hello,
first of all thank you so much for this forum, I've learned a lot with all the usefulI information gathered here.

I'm inspecting and hopefully repairing 2x Neumann U67 microphones.
The PSU's were restored and adjusted for the correct output voltages.
All the voltages at the PSU's and at both mics are spot on with the Neumann schematics.

One of the microphones has a noticeable low end loss (no mods were made to the mics circuitry it's stock). When I change the capsule KK87 asembly module from one mic to the other the low end loss follows the capsule.
So it seems the problem is not in the microphone circuitry but at the capsule itself.

Is it normal for a Neumann U67 capsule to develop low end loss over the years?
If I measure the K67 capsules capacitances what would be the values to expect in a good working capsule?

Visually both the capsules look in great condition, a not dirty like other U67 capsules I've seen in the past. The screw of the KK87 assembly that has some sort of wax that acts as a violation seal was intact so I guess the capsules were not touched in the past and are still the original ones.

here are some pics, the capsule on the left it's the one that developed the Low End Loss:












Thank you so much for your help
Best Regards

Tiago

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Piedpiper

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Re: U67 capsule - Low End Loss
« Reply #1 on: May 17, 2016, 08:12:59 pm »

It looks like the one on the right has one of the pins broken off, though this is the "better" one.
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mbrebes

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Re: U67 capsule - Low End Loss
« Reply #2 on: May 18, 2016, 11:02:40 am »

Per the schematic, pin 7 is not used.
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usattler

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Re: U67 capsule - Low End Loss
« Reply #3 on: May 18, 2016, 11:03:37 am »

If these are really U67 microphones, then they should have KK67, (in some rare cases KK60) or, if recently refurbished, KK870 capsule heads, but definitively NOT KK87 (or KK87s) heads as shown in the pictures! The biasing and pin-out are distinctly different. Since there is no pin #7 (ground) for the U67, this may account for the 'broken' (or clipped) pin of the capsule depicted on right.
Unless these KK87 heads have been extensively modified they should not work correctly for all patterns on a U67 body.
For a KK67 or KK870 the capacitance can be measured in the figure-8 position as around 50 pF for of either half: the back plate is connected directly to pins #1 and 4, the front diaphragm to pin #2. and the rear diaphragm to pin #5 via the pattern selector switch.
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ilcaccillo

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Re: U67 capsule - Low End Loss
« Reply #4 on: May 18, 2016, 03:10:30 pm »

I dont know the reasons for these U67 having KK87 capsule Heads installed.
I dont know if they were extensively modified, but I think for the KK87 to work on a U67 you just need to connect the 2 backplates together.

The mics are old, maybe the original heads were replaced at some point, I dont know. Also the mics are from different eras, one looks later than the other. Also they are badged differently one has the Neumann Badge (KK87) the other the Telefunken badge (KK87S).

Thanks for the tips on the capacitance measurement

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klaus

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Re: U67 capsule - Low End Loss
« Reply #5 on: May 18, 2016, 06:13:28 pm »

What may have happened:
Because KK67 heads are no longer available, these may be K67 capsules mounted on KK87 head assemblies which are still offered by Neumann

And it's simple enough to check: unscrew/ disconnect the red wire from the capsule's backplate on one head, and the yellow backplate wire on the other, then take an ohm meter and touch one of the two probes on one backplate,the other probe on the other. If the meter shows zero resistance, you likely have K67 capsules, and all is well, as far as the correct U67 capsule type is concerned.

IF the above is confirmed, THEN you can further troubleshoot why one of the capsules is low-end shy:

Mount that capsule on the other head assembly and observe whether the low end in cardioid is still missing. If if is, you have a defective capsule*. If that capsule on the other head assembly now performs at full frequency, you need to do some detective work regarding wiring in the head assembly.

* if it's a capsule issue, you can further isolate the problem by reversing cardioid sides: connect the front diaphragm lead out wire to the rear diaphragm, and observe its low end in cardioid (no need to connect the original rear lead out wire, just tuck it in. You may be lucky, and find a balanced rear side which you then can permanently make the new front side.
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ilcaccillo

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Re: U67 capsule - Low End Loss
« Reply #6 on: May 18, 2016, 10:24:38 pm »

thanks for the information Klaus.
So that might be the case maybe they are K67 capsules mounted on KK87 head assemblies.

Is there any way of knowing if a capsule is a K67 or K87 just by the looks or numbering of it?




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klaus

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Re: U67 capsule - Low End Loss
« Reply #7 on: May 18, 2016, 10:32:53 pm »

For the untrained eye not an easy one.

If you would post a close-up of the gap between the two backplates, you may detect that the thin spacer separating the two brass plates is silver (aluminum) rather than the K87's clear plastic version.

Other than that, no external markings differentiate the two. Hence my suggestion to measure conductivity between the plates. The K67's aluminum spacer conducts, the one in the K87 is made of plastic, and does not.

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Klaus Heyne
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ilcaccillo

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Re: U67 Capsule - Low End Loss
« Reply #8 on: May 18, 2016, 10:58:00 pm »

Thank you Klaus, I will do that.

And also thanks for the tips to troubleshoot the capsule, will try those also.



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