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 on: February 16, 2018, 11:25:54 pm 
Started by bicarbone - Last post by klaus
(...) 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.

 on: February 16, 2018, 06:58:50 pm 
Started by bicarbone - Last post by Kai
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.

 on: February 16, 2018, 06:53:48 pm 
Started by bicarbone - Last post by bicarbone
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!

 on: February 16, 2018, 06:43:00 pm 
Started by bicarbone - Last post by Kai
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.)

 on: February 16, 2018, 04:11:11 pm 
Started by bicarbone - Last post by bicarbone

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!

 on: February 16, 2018, 09:07:05 am 
Started by musicalnyc - Last post by musicalnyc
That's great to know.  Thanks, Klaus!

 on: February 16, 2018, 01:44:24 am 
Started by musicalnyc - Last post by klaus
Hello Peter,
The advantage of the AR-47 Nuvistor plug-in: no additional changes inside the mic are necessary to re-convert to VF14 once the plug-in is removed. The mic will be 100% stock again.

The required higher Nuvistor voltage of 110VDC was obtained by replacing one of the four 1KΩ dropping resistors  in the original NG power supplies with a wire bridge, and placing that resistor in parallel with an existing one.

Once the wire bridge has been removed and the resistor soldered back in its place again (by a qualified technician, of course!) the NG is also back to stock, ready to power a VF14-equipped U47/48.

When it's done, it will look like this:

 on: February 15, 2018, 11:43:54 pm 
Started by musicalnyc - Last post by musicalnyc
I have a Neumann U48 that was converted to Nuvistor long ago via the AR47 plug-in.  I would like to restore it to VF14 specs and I am aware that I need to modify the power supply. 

Is there anything within the microphone itself that needs to be adjusted or modified (besides swapping the AR47 with a VF14) in order to reverse it to stock?

Thanks for your help!

 on: February 15, 2018, 12:43:08 pm 
Started by aremos - Last post by richbreen
I, for one, would like to hear more about the rational for higher freq. response mics, above 20kHz.

I've read of experiments showing positive benefits but many of the test methods seemed imprecise.

Talk to film SFX editors/designers who need to move recordings down several octaves while maintaining fidelity, and without moving aliasing artifacts into the audible band.

 on: February 14, 2018, 12:50:51 pm 
Started by aremos - Last post by klaus
I've read of experiments showing positive benefits but many of the test methods seemed imprecise.

Even without knowing the methodology of these experiments, I would be surprised if anything conclusive or "precise" resulted. Personal, inherently subjective, interpretation of sound continues to be the guideline for our preferences.

(Unless you call the original experiment how we ended up with the definition of one decibel "precise"...)

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