R/E/P > Klaus Heyne's Mic Lab

U87 pad switch permanently 'on'

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A friend of mine has an 'old' Neumann U87 microphone.
He complained about a high self noise level and asked me to have a look at it.
I noticed that the -10 dB 'pad' switch seemed to be permanently active, no matter what the position of the slider is.
I carefully opened the switch assembly and saw that it is a version with reed contacts.
For some reason, the reed contact was always closed, even if the small magnet in the slider was in the 'off' position.
I replaced the reed contact, but this did not change the situation.
Of course I can't lower the reed contact (to create more distance between the slider magnet and the reed contact) or move the reed contact to a different position.
The small magnet in the slider seems to be glued in.
Did somebody ever had this problem before?
Personally I would disable the pad switch alltogether, but my friend want to keep all functionality.


--- Quote ---I replaced the reed contact, but this did not change the situation
--- End quote ---
That part of your post seems odd.
Yes, the glass-embedded reed switches can, in rare cases, get permanently stuck in the "on" position, i.e. the two reeds fail to separate, even after the magnetic force is removed, but would that happen twice in a row? It's highly unlikely, unless the reeds in your replacement fried from over-voltage.

Choosing the right replacement reed switch rated for the proper voltage (Neumann does not sell it, so it will be aftermarket) and paying attention to precise installation of the fragile glass enclosure is crucial, for the reed/magnet combo to properly work.

Here is what I would do:

1. remove the head from the mic
2. remove the magnet switch
2. measure continuity of the reed element (it should be open, but if both reeds look stuck together under a magnifying glass, examine whether you chose the correct reed switch for this application)
3. re-install the reed element to the precise height of 1.0 mm off the base, and make sure it's soldered in perfectly level with the base.

The precise distance between reed and magnet is crucial to make the contacts work properly when sliding the switch in and out of range.

Good luck!

Of course I have tested if the contact is open without the magnet in place!
For some reason it seems the magnet in the slider is just 'too strong', because it closes the reed contact even when the magnet is 'far' away.
Apart from the size, there isn't very much choice in reed contacts.
As far as I know, no such thing as 'sensitive' or 'not so sentive' reed contacts does exist...
The problem is that I can't place the reed contact any lower (it is already flat on the PCB) and I can't change the distance from the magnet, because it is glued in the slider.
I had hoped that it was a case of an 'extremely sensitive' reed contact, but the new one behaves exactly the same.
Strange situation. I still love the 'old' construction, with the physical contacts!

OK, now we know it's not a case of permanently stuck contacts, but of a mismatch between the magnetic force of the switch and the sensitivity of your replacement reed.

The stock magnet is likely not too strong, otherwise this problem would have shown up in more than 100,000 switches Neumann has installed so far.
I bet, if you interchanged the -10dB (aftermarket) and the low-cut (stock-Neumann) reeds, it will solve the -10dB switch problem, as this issue is not known to happen with the stock-original Neumann switch and reed combo.

• When the original reed contact is unsoldered (= distant from the magnet), does it open?
• Is the magnet in the slider still in correct position, maybe the glue has failed?


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