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Author Topic: Neumann KM 88i stuck on fig. 8  (Read 3561 times)

bicarbone

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Neumann KM 88i stuck on fig. 8
« on: February 16, 2018, 04:11:11 pm »

Hi!

Long time lurker, have been enjoying all the precious info found here.

Here is my interrogation about a pair of Neumann KM 88i I have just purchased (consecutive serial numbers): One of them has only fig. 8 pattern working properly, doesn't switch to cardioid and emits a low end buzz when switched to omni. Swapping capsules between both amp bodies has narrowed the problem to that second amp body, as it seems.

Any idea what could that be?

I could return the faulty one to the seller, but I have been looking for a pair for ages and they are super rare to come by so I'd like to know if it could be an easy fix (outside of the unavailable spare part problem related to that very microphone).

Thanks for any clue!
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Kai

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Re: Neumann KM 88i stuck on fig. 8
« Reply #1 on: February 16, 2018, 06:43:00 pm »

With my KM56s I had problems with the switch occasinally after it haven't been moved for ages.
Careful contact cleaning remedied the problem.
As I remember (?) it was more the other way round, they didn't want to go out of cardiod, but that might be as they rested there.
In your case it migt even be a mechanical problem with the switch not working or extremly dirty ( someone tried contact cleaned and did not remove the debris.
Cardiod is switch open, so this should work.
Look at the schematic, fig. 8 is the most problematic, cardiod and omni is only the switch involved (open / front and back connected.)
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bicarbone

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Re: Neumann KM 88i stuck on fig. 8
« Reply #2 on: February 16, 2018, 06:53:48 pm »

Ok, thank you Kai for chiming in! I will definitely look into cleaning the selector contacts.

I should add that the -10 dB pad is not producing any change either, would it confirm your idea?

I'm also intrigued because Mr Heyne recently wrote that the KM 88 has a tendancy to lose the fig. 8 pattern functionality while retaining cardioid and omni capabilities, quite the opposite of what I'm experiencing :

"The biggest headache owning a KM88 is potential catastrophic loss of figure eight
pattern. The DC converter which provides rear polarization voltage in figure eight,
consists of 11 components tucked into a small metal box soldered midship to the
circuit board. Its Tantalum capacitors are starting to fail, due to aging, and cannot
be replaced without destroying all other parts in the box: they were all epoxied
together, including the fragile, hand-made transformer coil.
But loss of figure eight after converter failure need not be a disaster- omni and
cardioid will continue to work as before." Quoting http://germanmasterworks.com/galleryofgreats.html

So something else must be happening. Thanks again!
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Kai

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Re: Neumann KM 88i stuck on fig. 8
« Reply #3 on: February 16, 2018, 06:58:50 pm »

Be carefull not to spill contact cleaner into the circuit.
If possible unsolder the switch and clean it separately, and, yes, the pad is likely to be oxydated too.
Do NOT (NEVER!!!!!) spray into the circuit, use a wetted toothpick if unsoldering the switch is no option.
Stick pieces of tissue around to protect the circuit.
Wipe off as much of the cleaner as possible after, e.g. with tiny strips of paper.
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klaus

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Re: Neumann KM 88i stuck on fig. 8
« Reply #4 on: February 16, 2018, 11:25:54 pm »

(...) One of them has only fig. 8 pattern working properly, doesn't switch to cardioid and emits a low end buzz when switched to omni.

Cleaning switch contacts to revive missing patterns on KM88 usually do not work: the "switches" are hand-made oddities used only in this model; they do not resemble common switch construction: four solid-core wires are inserted perpendicularly into the switch housing. They serve as the actual contacts which a wiper connects with in one of three positions.

The contact wires, some of which are connected to the head pins via equally stiff wires, are pushed into the ivory-colored plastic body, held in place by a single gold-colored slot screw. The ends of the contact wires are bent 90║ to keep them from falling out or slipping through. With time, some of the wires may bend slightly out of contact range for the wiper, and stop conducting.

Two switches are visible in the photo. Foreground: the pattern switch; background, and out of focus: the -10dB switch.Both are the same type three-position switch, and both work on the same funky principle. In your case the -10dB switch may also have lost contact and attenuation can no longer be switched in. 

Simple remedy, if you are lucky: carefully pull, twist, and push the ends of the contact wires (their 90║ bend is visible in the picture). Some of the wires may need to be de-soldered before you can move them around and try to free their surfaces of corrosion, or straighten them enough to re-establish contact.

A more likely and unfortunately not so easy remedy: carefully remove both switches, after unsoldering ALL connections, straighten the wires, and test for good contact in all three positions before re-installing.

Both remedies can be messy if you pull the wires out too far: they are not easy to reinsert into their slots.

P.S.:   Here are more inside pictures of the KM88.

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Klaus Heyne
German Masterworks«
www.GermanMasterworks.com

bicarbone

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Re: Neumann KM 88i stuck on fig. 8
« Reply #5 on: February 18, 2018, 06:42:22 am »

Thank you Kai for the warning.

Thank you Klaus for the detailed instructions and the pic, very helpful. I thought it was going to be tricky to clean the contacts when I first opened the mic. What you write only confirms this.

I have the feeling it is actually beyond my skills to do it myself. Do you think I could ask any average to good tech to do it? Or would it be possible to send it to you?

Thanks again!
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Kai

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Re: Neumann KM 88i stuck on fig. 8
« Reply #6 on: February 18, 2018, 08:49:32 am »

...would it be possible to send it to you?
Yes.
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bicarbone

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Re: Neumann KM 88i stuck on fig. 8
« Reply #7 on: February 18, 2018, 02:12:18 pm »

Sorry, I meant sending it to Herr Klaus! I don't know, are you repairing microphones yourself? No offense, hopefully.
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Kai

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Re: Neumann KM 88i stuck on fig. 8
« Reply #8 on: February 18, 2018, 05:50:39 pm »

are you repairing microphones yourself?
Not on regular base like Klaus, just from time to time, and this wouldn't be a problem. I've seen you're located in Swiss, so - I'm in Gemany.
But maybe we're hurting the forum rules, for going on you should PM me, or Klaus.
@Klaus, feel free to delete this posting if necessary.
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klaus

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Re: Neumann KM 88i stuck on fig. 8
« Reply #9 on: February 18, 2018, 05:54:41 pm »

Not at all. This mic is a rare find and requires love and affection from anyone who would dare to dig inside and sort out the pattern switch. Go ahead and make the connection! Just be sure to report back what you found.
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Klaus Heyne
German Masterworks«
www.GermanMasterworks.com
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