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Author Topic: U87i Oddities  (Read 5201 times)

Michael O.

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U87i Oddities
« on: October 26, 2015, 09:11:46 pm »

This is my first post to the forum and I'd to begin by saying greetings from New Orleans, and my appreciation goes out to the members of the forum whose collective knowledge has been extremely helpful in my dealings with microphones.

Anyway, I recently acquired my first U87i. It's a great sounding example with engravings that would lead me to believe it spent some time at RCA in Nashville ("RCA 4" in a few places on the interior and exterior of the body), and it has a few seemingly anomalous attributes that have raised a few questions in my mind. I'll try to be as clear and concise as possible while being adequately descriptive.

1. The Capsule Mount: The first attached photo shows the capsule mount which is seemingly a partial SM69 capsule mount containing a later plastic surround replacement K87. I've read on this forum that some early iterations of the U87 had these mounts, but, in addition to confirming my suspicions about the SM69 mount, Neumann's mic dating service e-mail confirmed that my 87 is from 1974 (with a serial number of 227xx). The body and head assembly are both of similarly-aged clear perspex, so I have no reason to assume they're from different eras. The only thing I know for certain about the provenance is that the capsule was replaced by Gotham in the early 80's, so my question is why in this mic's service history would someone have installed such an odd and non-standard part?

2. Blue Dot: I thought it was commonly accepted that Neumann switched to blue dots by the serial numbers to indicate the U.S. version around the early 80's, however this 1974 u87 body has a blue dot. Did the blue and red dots simply indicate different impedance-strappings and pad settings, or is there something weird here?

3. The Body/Shell: The mic initially came with a very old looking and quite beat up body that, oddly enough, lacked the battery meter slit. I sourced an old body that's in much better shape (the second attachment), and this body has an odd hole in it that allows access to the two test points. Any backstory on these, and how it may have ended up in circulation (seems like an in-workshop sort of test thing)?

Anyway, it may just be that this is a typical franken-87, that over the years has been modified and bastardized with all kinds of odd mismatched parts, but I look forward to any insight you guys may be able to provide.

Thank you,
Michael

klaus

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Re: U87i Oddities
« Reply #1 on: October 27, 2015, 03:31:12 am »

Hello Michael, and welcome!

Item #1, capsule mount: I have not seen the SM69 capsule mount being used later than 1970. It may have been an outlier in this case: You may not know this, but the dating Neumann provides are not manufacturing, but delivery dates of first sales. To pinpoint the year when the mic was made, rather than delivered, would require a couple of close-ups of the amp compartment, and de-soldering of one of the two parallel-mounted 1 mfd. electrolytic filter caps: their manufacturing date is imprinted as xx.xx (month+ last two of year).
This is a longwinded way of saying: before you assume a visit from Mr. Frankenstein, make sure that this is not a late delivery of a mic actually manufactured a few years earlier. (Side note: a portion of the mount was removed on the left side, to accommodate the revised wire connection to the front backplate, introduced in K87 capsules in the 1980s).

Item #2, blue dot to indicate low-output version: There was a switch from red to blue in the 1970s. I have never seen blue dot mics with SM69-capsule mounts, as the old mounts had been used up long before the blue dot came to be. I have seen mics that predate blue dots getting service at Neumann in Berlin, and repainted with a blue dot at that time.

By the way, I always thought both colors indicate the same thing: a mic with 6dB attenuated output + 50 Ohms strapping, intended for the U.S. market, but I just found this in my records:

The models without build-out resistors and with 50 ohm
transformers display a red dot; 50 ohm models with built-in pads display
a blue dot, usually next to the serial number.
All this comes from Engineering Bulletin ENG125/ENG2/p12, from
Gotham, dated 9/29/82.)


My experience with color-dotted U87 and KM8x has been different: regardless whether the pad resistors were bypassed or not, any factory strapped 50Ω mic had a color dot, and there was a clear transition in the early to mid 1970s, afte which red dots were no longer used.

Item #3, cut-out in housing tube: That looks like a nicely executed job by a studio maintenance engineer with extra time on his hands ....(A post, further down, corrected my assumption. I was wrong, this was indeed a test accessory made by Neumann.)


Again, a close-up of the amplifier section could be helpful in pinpointing the approximate date of manufacture of this mic.
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Klaus Heyne
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soapfoot

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Re: U87i Oddities
« Reply #2 on: October 28, 2015, 09:22:33 am »

before you assume a visit from Mr. Frankenstein

That's Dr. Frankenstein, to you!

In all seriousness, that's excellent and interesting info, Klaus.
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klaus

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Re: U87i Oddities
« Reply #3 on: October 28, 2015, 02:07:10 pm »

Yikes! An inexcusable cultural faux pas, Dr. F. Won't happen again.
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Klaus Heyne
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Vertigorecording

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Re: U87i Oddities
« Reply #4 on: October 28, 2015, 02:54:07 pm »

Hello Everybody,
This is also my first post. I am a long-time lurker and a great admirer of Klaus's knowledge and his work. His generosity has helped me in so many ways in my own work. I can finally help bring some clarity to an issue, quite by accident. I was browsing through an old, circa 1982 U87 owners manual and came upon this picture of the MA87 test set. It clearly shows the body tube in question that mates with a capacitor head to allow amp testing in the U87. I hope the picture uploads.
All the best,
Charlie Bolois
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klaus

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Re: U87i Oddities
« Reply #5 on: October 28, 2015, 03:00:42 pm »

Excellent! I stand corrected and have amended my earlier post.
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Michael O.

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Re: U87i Oddities
« Reply #6 on: October 29, 2015, 09:09:21 pm »

Thank you everybody for the replies. Right when I thought I'd read it all about U87's you guys supply me with some genuinely new and interesting info.

I'd always made the incorrect assumption that Neumann serial numbers referred to a date/sequence of manufacture, and it does make more sense in context that this mic could've been relatively old stock (as the SM69 mount would suggest) that was shipped in '74 for whatever reason. If that's the case, and it really is a '68/'69 model, it would explain why it came with a U67-type shell (i.e., one that has the older finish compared with the newer and shinier Ai shells, and that's, of course, sans-battery meter porthole; I'll include a photo of it next to the replacement shell incase that helps elucidate anything) that the previous owner assumed was a newer replacement, but could just be one of the shells they machined prior to including the battery porthole.

Also, I'd never thought of dating the mic itself by dating its components. That's a great idea (it's like microphone archaeology, hah), and I'm going to check that cap date next time my steady-handed tech and his soldering station are around the studio. I'll include the closeup I have of the component side of the PCB so you can get a fuller story on the mic. There's no circuit diagram label sticker, and, to my not so expert eye, it looks relatively unmolested other than those two resistors near the bottom, that I believe are part of the 6db pad that was probably deactivated at some point in this mic's career (unfortunately I don't have photos of beneath the battery holder thing, I need to check that out).

Thanks again you guys, and that image of the test accessories is a perfect example of why I love the internet; it's just amazing that there are still things to be learned about a microphone model that's been superlatively popular for nearly fifty years.

klaus

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Re: U87i Oddities
« Reply #7 on: October 30, 2015, 01:31:52 am »

Your mic's component layout reveals that it was manufactured before May 1971:
Starting with that date and schematic 930-016, the 820pf capacitor (position C6) was changed to 220pf.
(I do not own schematics #014 and #015 (Uwe?), so assume your layout is schematic 013 or earlier.

Lifting one leg of one of the two parallel-mounted, gold "70V" capacitors will show their manufacturing date, which usually is about 3-9 months prior to the manufacture of the mic.
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Klaus Heyne
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Uwe

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Re: U87i Oddities
« Reply #8 on: October 30, 2015, 11:51:11 am »

The pcb depicted corresponds with the 1968 schematic version U87-930-13.

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J.J. Blair

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Re: U87i Oddities
« Reply #9 on: October 31, 2015, 02:00:17 am »

Hi, Charlie!  Nice to see you around these parts!

BTW, is that Michael O'Keefe, of Los Angeles, songwriter and actor Michael O'Keefe?
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Vertigorecording

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Re: U87i Oddities
« Reply #10 on: November 02, 2015, 08:55:31 pm »

Thanks JJ…..
Good to be seen!
-Charlie
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Michael O.

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Re: U87i Oddities
« Reply #11 on: November 03, 2015, 02:36:22 am »

BTW, is that Michael O'Keefe, of Los Angeles, songwriter and actor Michael O'Keefe?

Nope, I'm a significantly younger, significantly less famous New Orleanian Michael O'Keefe. I write songs and recently opened my own recording studio, amongst other stuff.

Thank you for the schematic and the info, Uwe. It's indispensable to know which to use when there were so many circuit revisions in such a short span of time. I'm no EE and not familiar with this circuit in general; would that replaced capacitor (C6) have any appreciable effect on the sound? Thanks again for all the info, guys.



klaus

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Re: U87i Oddities
« Reply #12 on: November 03, 2015, 04:29:41 am »

C6 attenuates the highs. With an 820pf cap, the -6dB @16kHz is achieved with ease. A 220pf styrene cap in that position makes the mic sound a tiny bit more open in the highs, by a dB or two.
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Klaus Heyne
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Uwe

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Re: U87i Oddities
« Reply #13 on: November 04, 2015, 12:41:14 pm »

C3 is equally responsible for shaping the frequency response and gain of the amplifier section! It is a mere 3.3 pF in the versions where C6 is 820 pF and 10 pF where C6 is 220 pF! These combinations result in virtually identical overall results.
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klaus

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Re: U87i Oddities
« Reply #14 on: November 04, 2015, 03:14:35 pm »

Interesting take, Uwe. I always regarded C3 contributing global, not frequency-discriminating negativen feedback? Do you agree that, if only C6 is lowered, and C3 is not changed, only the high-frequency response is affected, as I wrote?
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Klaus Heyne
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