Neumann KMS 85

Hey folks,

I bought a Neumann KMS 85 for quite cheap from Ebay a few weeks ago.
There's not alot of info about it, but it seems to be somekind of a "stage" mic version of the KM series from the 70's.
Also I read, that the KM85 had a bass roll off build into the capsule.
But after disassembling the mic there is what seems to be a normal KM84 body and capsule. There is "84" and a cardiod symbol engraved into the capsule and the capsule has holes on the "underside" which indicates it is a normal K84 capsule, right?

https://drive.google.com/file/d/1lNFG_DPg_4xZBaNbF9jUrMNRetDLRwnS/view?usp=sharing

Looking at the circuit, the only thing that I can think of is R2 (470 M).
This resistor value is lower than usual, but I doubt if this would give a noticable LF rolloff.
(Although the capsule capacity is only 36 pF)
470 K // 1 G.ohm = 320 M.ohm, this would give a -3 dB point with 36 pF at 14 Hz, so this can't be the only reason...

Hey Ruud,

thanks alot for your answer!
That's strange. Maybe the roll off is built into the transformer?
Also, what exactly are R18 and R19 for?
Because I also noticed that it has a real low output... I have to crank my preamp up to get a signal and it also is quite noisy...
The problem is, that the bottom end bell is missing. So it has fliyng leads. I wired an XLR connector and tested it. So maybe that is also the reason for it? Is that possible?
And do you know what C2 is for? It has double the value for "p" designation.

Remove C2 and test it again.
Maybe it's faulty.
It's feedback capacitor. It lowers output.
R18/R19 same as R15/R16 is for phantom voltage.
Just two independent sections.

Murdock said:

Also, what exactly are R18 and R19 for?

Click to expand...

The preamp presumably draws some current from the 2x6.8K junction, so you form a different junction that comes right from the phantom power to polarize the capsule. This way the capsule's polarity is as high as possible, and constant and doesn't vary based on what the FET and BJTs are doing.

If something you read actually says "built into the capsule," then I would take that literally. Capsules can be tuned to have less bass or less treble or different peaks.

ln76d said:

Remove C2 and test it again.
Maybe it's faulty.
It's feedback capacitor. It lowers output.
R18/R19 same as R15/R16 is for phantom voltage.
Just two independent sections.

Click to expand...

Sorry for reviving this old thread but I just got interested in this mic again.
Meanwhile I got a fitting connector and switched the 10pF cap (C2) for a 3pF cap. It raised the output level a little bit but it was still kinda low and also noisy...
So I just tried In76d tip and removed C2 completely. Now I get no signal at all...
Then I soldered a 2.2pF cap in but still got no signal.
Could there anything be damaged by completely removing C2?

Could there anything be damaged by completely removing C2?

Click to expand...

One end is soldered on the PCB, the other is passing through a teflon tube before a point to point connection, you may have unsoldered a resistor or the capacitor connecting the capsule to the Fet. Removing C2 will make the mic sounds harsh. You can stick with the 10pF value. Replace those old tantalum capacitors, the PCB is not too busy to work on so you can't do wrong replacing those 5 to 8 probably bad 50 years old components.

It raised the output level a little bit but it was still kinda low and also noisy...

Click to expand...

Try to clean the capsule connection and thread with Q-tip and isopropyl alcohol, watch for any residue, fibers from the Q-tip. If there's some, remove that with a super-clean soft paint brush. Don't forget to control the metal pin of the capsule makes good contact between the two spring-leads.

vetter said:

One end is soldered on the PCB, the other is passing through a teflon tube before a point to point connection, you may have unsoldered a resistor or the capacitor connecting the capsule to the Fet. Removing C2 will make the mic sounds harsh. You can stick with the 10pF value. Replace those old tantalum capacitors, the PCB is not too busy to work on so you can't do wrong replacing those 5 to 8 probably bad 50 years old components.

Try to clean the capsule connection and thread with Q-tip and isopropyl alcohol, watch for any residue, fibers from the Q-tip. If there's some, remove that with a super-clean soft paint brush. Don't forget to control the metal pin of the capsule makes good contact between the two spring-leads.

Click to expand...

Hey, thanks for the suggestions!
I allready cleaned all high z stuff with isopropyl alcohol and I also checked the connection of the cap and resistor. 
But something weird happend yesterday...
I changed the two high ohm resistors (1G and 470M) to modern 1G resistors, because I thought that couldn't hurt anything and would also improve the noise floor. I also resoldered the cables to the bottom connector because I noticed before, while the mic didn't work, that  I could wiggle the connector and get weird sounds...

So after all this I plugged it in and still got no sound  ??? Thought it must be the FET. But I left it on for a while and suddenly after a a few minutes I get signal. Clean and healthy. No problems and much quiter then before.
Plugged it in today to see if it still works. And yes, works right away.
Anybody a clue? Maybe moisture?

Ageing capacitor, moisture, bad contact, hidden relay, intermittent short, mysterious self-repairing microphone... who knows? ... bad phantom power too, maybe the problem was out of the mic.