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Author Topic: Problem with Polarity responses in Neumann U87  (Read 582 times)

onthego1

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Problem with Polarity responses in Neumann U87
« on: December 22, 2016, 10:18:00 pm »

Hey guys

I have a problem with my vintage U87, the polarities don't respond as they should! Figure-8 performs as Figure-8, however Cardioid performs as Figure-8 and Omni performs as Cardioid. And what's strange is on occasions all polarities work as they should, until i change polarity mode, then we go back to the problem. This leads me to think that there's maybe a loose connection somewhere?? Is this a simple fix?

Thanks
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klaus

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Re: Problem with Polarity responses in Neumann U87
« Reply #1 on: December 23, 2016, 02:18:11 am »

There are two often encountered scenarios that correspond to this problem:

1. The two switch contacts of the polarity selector are not closing/opening properly, due to mechanical distortion or misalignment.

2. If this is a pre-1986 U87, sometimes the two backplate halves, which should be insulated from each other through a 6µ washer, are screwed together too tightly and when they make contact, they mess up the patterns.

There is also the odd situation where someone screwed around with the mic's wiring and mis-connected the lead-outs between capsule and switch assembly.

In sum, this is quite easy to troubleshoot: with an ohm meter and the proper schematic you can trace continuity from each head pin to the proper destination of each wire.


Edit: your follow-up below, where you write that swapping heads does not change the problem makes my troubleshooting tips unlikely, at least for this mic.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

onthego1

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Re: Problem with Polarity responses in Neumann U87
« Reply #2 on: December 23, 2016, 06:14:52 am »

Thanks Klaus, appreciate the quick response.

I forgot to mention one other important observation, when the head unit is fitted to another vintage U87 (pre 1986), polarities consistently work fine, no problems. So this leads me to think maybe fault lies in the PCB.
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uwe ret

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Re: Problem with Polarity responses in Neumann U87
« Reply #3 on: December 23, 2016, 11:34:51 am »

If the head works perfectly on another U87 body, then your problem has to be traced to the bias resistors in the other body. Without the head attached you can measure the voltages on the various pins, but beware the input resistance of your voltmeter, which will form a voltage divider with the high value bias resistors! Assuming your DVM has jt's internal resistance as 10 MΩ, the you will measure against pin 7 (ground) as follows:
pin 1: ~0.5 V (1069 MΩ in series);
pin 2: ~45 V (1 MΩ in series);
pin 3: ~5.7 V (69 MΩ in series);
pin 4: 0 V (1000 MΩ to ground);
pin 5: ~5.7 V (69 MΩ in series);
pin 6: ~45 V (inconsequential for the pattern problem)
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uwe ret

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Re: Problem with Polarity responses in Neumann U87
« Reply #4 on: December 29, 2016, 08:03:20 pm »

Quote
"Thank you for your response to my U87 problem.

The readings I managed to get are as follows -

PIN1 0.4V
PIN2 38.4V
PIN3 5.8V
PIN4 0V
PIN5 0V
PIN6 41.6V

Readings are fairly close to what you stated they should be expect PIN5, which is completely off. What do you make of these recordings?"


With 0 V measured at pin 5 it appears that R4 (68 MΩ) is either open or disconnected. Your measurement should be identical to the one on pin 3.

Best regards,
Uwe Sattler
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