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Author Topic: Problem with u67  (Read 9855 times)

method1

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Problem with u67
« on: July 21, 2012, 03:40:55 pm »

Hi, this is my first post, I was referred here after posting this issue on another forum.

I think there's something wrong with my u67, it's sounding a bit dull as well as distorting even with moderate levels. I'm recording at low levels, the distortion is definitely not coming from the signal being too hot.

Here's a sample recording showing the problem. http://f.cl.ly/items/2p3w1M1W1S0r1i410I00/u67.mp3

The recording was done with the mic about 10" away from the singer.

Any way to diagnose what's going on from this clip? Any help / advice much appreciated!
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klaus

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Re: Problem with u67
« Reply #1 on: July 21, 2012, 06:46:49 pm »

Again, as I mentioned in previous posts: always start with establishing that the supply voltages produced by the power supply that go to the mic are correct:

210V B+, 6.1- 6.3VDC heater.

Then let's talk again.
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Klaus Heyne
German Masterworks®
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method1

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Re: Problem with u67
« Reply #2 on: July 21, 2012, 08:06:06 pm »

Thanks for the reply Klaus.
Could you please give me an indication of how the pins are numbered so that I can take measurements?
I have an original u67 cable and a Telefunken replacement PSU.

Regards

Joel

edit:

Not sure If I've got the pin numbers correct, but it looks like I'm getting 222v / 7.2dc which is higher than the figures you quoted. Could this be the problem?

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klaus

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Re: Problem with u67
« Reply #3 on: July 22, 2012, 04:09:46 am »

Pin 4 is heater, pin 5 is B+.
If your tech is really measuring such high voltages (with the microphone connected) you are in trouble.
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Klaus Heyne
German Masterworks®
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klaus

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Re: Problem with u67
« Reply #4 on: July 22, 2012, 11:18:59 pm »

You will find the relevant voltages at the Tuchel connector, inside the power supply, teh same pins you already measured, but this time, from the inside, after connecting the cabe and mic.

Because there are dangerously high voltages present inside the power supply, DO NOT make any voltage tests if you are not a trained technician who is familiar with the circuit layout of the supply.
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Klaus Heyne
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method1

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Re: Problem with u67
« Reply #5 on: July 24, 2012, 11:38:40 am »

Thanks Klaus

Measured with the mic connected:

195 / 6.1v

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klaus

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Re: Problem with u67
« Reply #6 on: July 24, 2012, 12:26:15 pm »

The B+ is about 15 volts too low. Unfortunately, there is no trim-out potentiometer for B+ voltage adjustments. You will have to swap out bridging or shunt resistors, or both, to arrive at 210VDC.
Once you are there, I believe, the level of distortion will likely still not go away.

The tonality of this mic sounds weird to me, unless the sound sample was eq'ed. If so, I will strike my last paragraph from the record.
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Klaus Heyne
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method1

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Re: Problem with u67
« Reply #7 on: July 24, 2012, 01:12:19 pm »

Thanks for your insights Klaus, very much appreciated.
I'll post some pictures of the microphone innards. To my untrained eye it looks unaltered, but perhaps you can shed some further light on the situation.
The recording was not eq'd at all, the microphone was placed about 10" away from the singer and recorded with no processing applied.
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method1

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Re: Problem with u67
« Reply #8 on: July 24, 2012, 02:24:39 pm »

Here are some pictures of the mic. The capsule looks a lot worse in the pictures than it does to the naked eye, but perhaps the eye deceives.

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method1

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Re: Problem with u67
« Reply #9 on: July 24, 2012, 02:25:31 pm »

More pictures:
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klaus

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Re: Problem with u67
« Reply #10 on: July 24, 2012, 02:42:18 pm »

This looks like a very nice specimen of an early U67. Both, capsule and amp look stock-original.

The next candidate to look for the origin of distortion may be diaphragm tension. Early brass K67 suffered more than later versions from loose tension. Unfortunately, it is not easy for a lay-person to see or test; for you to verify that the capsule is not the source of (mechanical diaphragm) distortion would take installing a different KK67 head to verify.

P.S.: You don't show the tube. I hope it is not a Russian EF86- another source for non-linearities in U67.
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Klaus Heyne
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method1

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Re: Problem with u67
« Reply #11 on: July 24, 2012, 03:28:44 pm »

The mic came from the NDR and has the NDR logo engraved on the front above the Neumann badge.

I just fitted a NOS Philips EF86, the tube used in the test I posted was a Haltron EF86.
I will do another test with the new tube, but at the moment it appears that the power supply is the culprit.
I will have to try and either modify the psu or get another one to test. If the problem persists then looking at the capsule would be the next in the process of elimination.

Thanks again for your time.
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method1

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Re: Problem with u67
« Reply #12 on: July 24, 2012, 09:37:10 pm »

Using a rather convoluted method, I can now get a reading of 206v / 6.1

I used a step down transformer to convert the local power to 110v and switched the PSU to 110v input instead of 230.

Is 206 acceptable or still too low? If so I will try and modify the PSU.

Thanks
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klaus

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Re: Problem with u67
« Reply #13 on: July 25, 2012, 01:01:24 am »

206 VDC is acceptable.
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Klaus Heyne
German Masterworks®
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radardoug

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Re: Problem with u67
« Reply #14 on: July 30, 2012, 04:01:10 am »

Do any of the capacitors in the power supply look like they have been leaking? Although the caps in these supplies have proven to last pretty well,  caps do still go dry. This could then give you more ripple and low h.t.
Regards,
            Doug Jane.
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