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Author Topic: U87 capsule problem: Shorted Backplates  (Read 627 times)

radek_p

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U87 capsule problem: Shorted Backplates
« on: August 19, 2019, 07:13:50 pm »

Dear folks, dear Klaus,

I'd like to ask you for some generous help with my U87.

It works fine in cardio and omni (consistent sound from front, rear and sides). In 8 position however, it gives weaker signal from front and almost no signal from the rear side.

When I test my head with another body, the problem remains the same. When I connect another head to my body, it works fine. I.e. the body can be ruled out.

I measured 0 Ohm between pins 2 and 5 when the head is disconnected from body. It is 0 Ohm regardless of the pattern selected. I also removed the grill and disconnected one of the two backplate wires, still 0 Ohm between the two backplates. By this I believe I ruled out the pattern selector switch.

Is there anything I can do to renew the isolation between backplates?

U87 #32337
head KK 87 B #11201

photos attached
schematics attached for your convenience

Many thanks, Radek
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afterlifestudios

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Re: U87 fo8 problem - backplates connected
« Reply #1 on: August 19, 2019, 08:16:17 pm »

I can’t tell from the thumbnail photos (I’m on my phone) but are you certain it’s a k87 capsule and not a k67/870 capsule?  (The k67/870 backplates are connected.)  They are used in u67 and u87ai.

You don’t show what mic body you have, but your schematic is for an original u87 (non a) so you must use a k87.



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klaus

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Re: U87 fo8 problem - backplates connected
« Reply #2 on: August 19, 2019, 08:52:17 pm »

The photos show a U87, not U87Ai head.

The problem you describe-shorting of the two backplate halves-is not unusual on the original KK87 capsules: the 40µ Nylon spacer that separates the halves gets pinched sometimes and one or both slightly burred, irregular brass ridges at the edge of the back plates touch the opposing surface: the halves are no longer isolating.

Temporary solution (vs. professional solution of deburring the backplates and replacing the spacer): losen the four backplate screws (two in front, two in back) enough, so that you can move the backplate halves a bit back and forth. Now slightly tighten the four screws back up, and observe with an ohm meter that the short does not return as you tighten up. If it does, back up one or several screws enough to get full isolation.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

mikezietsman

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Re: U87 fo8 problem - backplates connected
« Reply #3 on: August 20, 2019, 09:16:38 am »

Klaus,
Does the tension between the two back plates (ie. the tightness of the four bigger connecting screws) not have an effect on the sound of the capsule?

Thanks,
Mike

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klaus

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Re: U87 capsule problem -the backplates are shorted
« Reply #4 on: August 20, 2019, 12:32:33 pm »

The four backplate mounting screws are not intended to determine diaphragm tension, or affect capsule parameters, as long as you don't crank them like mad. In that case, there may be mechanical distortion.
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Klaus Heyne
German Masterworks®
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radek_p

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Re: U87 capsule problem: Shorted Backplates
« Reply #5 on: August 21, 2019, 05:06:14 am »

I had quite a busy evening with screwdrivers. I loosened the screws till the ohmmeter showed isolation, assembled the mike, it work for a while in fo8, then the problem went back. Disassembled, loosened even more, again the problem went back. Then I removed the 4 screws entirely, removed the capsule from the holder and let it rest this way over the night. Assembled this morning but rotated the capsule 45 or 90 degrees in the holder. All screws tightened just very lightly. It is still working now, but apparently the capsule needs some professional treatment (it probably has not been touched for 40 years)

Many thanks, Klaus.
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klaus

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Re: U87 capsule problem: Shorted Backplates
« Reply #6 on: August 21, 2019, 11:22:52 am »

If the Nylon spacer has been pinched hard in the past, the next level of repair would be to take the spacer out and install it in reverse orientation, so to expose less of the pinched section to the current position on the backplate.
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Klaus Heyne
German Masterworks®
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afterlifestudios

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Re: U87 capsule problem: Shorted Backplates
« Reply #7 on: August 21, 2019, 11:50:44 am »

Is it worth double checking that the spacer itself is non conductive?  (ie. nylon.)
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klaus

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Re: U87 capsule problem: Shorted Backplates
« Reply #8 on: August 21, 2019, 06:29:56 pm »

No. The spacer's material properties do not come into play when the backplates are shorted.
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Klaus Heyne
German Masterworks®
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radek_p

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Re: U87 capsule problem: Shorted Backplates
« Reply #9 on: August 22, 2019, 10:13:21 am »

Could you please outline the procedure to separate the two capsule halves and access the spacer?

When the capsule is out of the holder and all 4 screws as well as the plastic cap (terminology?) are removed, the capsule seems still very compact.

The mic is now working, however I would like to know how difficult would be the next step if things get worse in the future.

Many thanks Klaus for invaluable information. It is interesting to start to know how things work behind the curtain (grill).
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klaus

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Re: U87 capsule problem: Shorted Backplates
« Reply #10 on: August 22, 2019, 12:27:10 pm »

I am reluctant to disclose the next step, for two reasons. The workplace needs to be very clean; and the isolation tape which, where present, acts as additional barrier for contaminants transferring to the backplate, needs to be removed, the surfaces cleaned of its adhesive, and replaced.

If you want to experiment, and don't mind suffering collateral damage in the process, the two halves will come apart as soon as screws and tape are off. Make sure that the two guide/alignment pins inserted into the backplate stay where they are. for proper realignment during reassembly.

Remove the spacer and carefully inspect the backplate edges for damage, or for kinks in  the spacer (the original cause of the shorting).
Feel around the edge of each backplate half and make sure there are no burs. Reassemble the backplate halves and use a different position for the spacer, both rotational and front/back. Test for shorts as you tighten the screws and reinstall the capsule.

Good luck!
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com
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