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[David Bock]

Are you saying the U67 High cut is achieved by a HF shunt to ground?

[klaus]

C17 taps off plate, goes via 530 Ohms to ground. That's how I read it.

[David Bock]

C17 connects to  resistor string *R18 (470r) R19 (10K)  R20 (60r)* Rt=10530r then to ground, so, an RF shunt to ground not an audio band shunt (80-160pF + 10.5k series R= almost invisible to audio). C17 also AC couples through the capsule backplate to form the  HF cut feedback network with the active element (the tube). Like the positive feedback network that forms the LF cut, this HF cut network has it's values calculated from the gain of the tube.

[klaus]

You are right. I overlooked the 10KΩ, which, without the feedback, would shift the HF cut way to the limits of hearing. Feedback therefore trumps ground shunt.
Thanks.

[Uwe]

The U67 circuit would be easier to understand, if the HF-feedback were redrawn with C17 as it effectively functions, namely connected between the tube's anode to the junction of R2/C2 and the capsule back electrode.

[David Bock]

Agreed it can be a confusing schematic. I had to re-draw it to simplify and see the multiple feedback operations.
Along those same likes, there was a mod by SPA which inverted the HF cut feedback circuit to create a HF boost on that mic.

[J.J. Blair]

I bet that boost sounded great, when combined with a 1µ capsule.   

[klaus]

You are kidding, right?

[David Bock]

not me.

[J.J. Blair]

I was certainly kidding.

[Jim Williams]

Quite a few forgetable pop recordings have been done with those SPA 1 um capsule modified mics. Those solved some of the "please add more top end" problems of 1990 era recordings. I suspect they were selected to overcome Dolby SR losses, but maybe not.

[GThomp]

Hey Klaus and gang,

Thanks for the overwhelming amount of help and information provided on this thread. I am happy to report that the mic has been restored to it's original condition (minus the S-2 jumper as per your recommendation Klaus), and I'm happy to report it is sounding as good as ever!

Thanks again for all the help, and mind blowing knowledge!

Greg

[klaus]

You are welcome!

[David Bock]

Quite a few forgetable pop recordings have been done with those SPA 1 um capsule modified mics. Those solved some of the "please add more top end" problems of 1990 era recordings. I suspect they were selected to overcome Dolby SR losses, but maybe not.

To be fair there were a lot more 3 micron mods than one micron mods. My theorum on the selection of "enhanced brightness" mics, which started in the mid 1980's, is that the combination of thousands of passes of the slave tape to record and comp the vocal (loss of high end for those of you who came up in the post-tape era)  combined with the omnipresent cocaine and the infinite record budget led to selection of said mics, especially if it was "going to be a hit" (as well perhaps then you would be willing to give up a few points on the record for the privilege of renting an aphex aural exciter- remember that!?) . Possibly a good dose of browbeating salesmanship as well.  I mean, who wouldn't want the newest advancements in technology in their old mics? IIR tube gear, U47's (let alone all the broken ribbons left unused in the mic locker) etc had fallen out of favor during that time.

[Je55]

Missing C17 replacement is easy: get a 80-100pf polystyrene with a 160VDC rating and install it between its original solder points (see schematic).

Restoring C9 to its original type will be harder: This was a dry polarized electrolytic 1.0mfd. and a rather good sounding one. Hunt around for an old Siemens/AEG metal jacket type. But measure its capacitance before installation.
I would not use one of the tiny Japanese electrolytics like Nichicon.
The original cap was installed in the space underneath the top circuit board that contains C3, C5, R3, etc.

Are you sure that you can use a 160v Styroflex for c17? The Neumann data sheet calls for 500v