Sennheiser MKH RF mic tuning info?

A resource for technical documentation. Datasheets, application notes, instruction manuals, books and links to resources are found in the Document Library.
Post Reply
emrr
Posts: 468
Joined: Sun Mar 02, 2008 11:27 pm
Location: NC, USA
Contact:

Sennheiser MKH RF mic tuning info?

Post by emrr » Fri Apr 21, 2017 9:16 am

Curious if anyone knows the RF coil adjustment procedure for older MKH series mics such as MKH105, MKH406, or MKH435? I find various 'twiddle the adjustment and look at transient symmetry' comments, but clearly there's an official procedure. I've just sent Sennheiser a request for quote, no idea if they still adjust these older models or not.
Best,

Doug Williams
Electromagnetic Radiation Recorders

User avatar
mediatechnology
Posts: 2974
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Sennheiser MKH RF mic tuning info?

Post by mediatechnology » Tue Apr 25, 2017 4:26 pm

I did find this MKH-105 schematic.

Image
Sennheiser MKH-105 Schematic

Your question made me curious.
It looks like L1 peaks the oscillator (and oscillator coupling) into the discriminator and L2 tunes the discriminator.
If I were to guess - which I know is not the kind of answer you were looking for - L1 should be peaked and L2 adjusted for minimum even-order distortion.
It would seem like the goal is maximum output with the least distortion.

EDIT: Check the MK415 "L2" alignment using a "galvanometer" and 25 pF reference capacitor here: http://www.coutant.org/mkh805/mkh415.pdf (pdf page 9)
See also: http://www.coutant.org/mkh805/mkh405.pdf

Test point "A" is the junction of D1, D2 which is the discriminator out.
Point "B" is the top end of R5.
With the capsule's reference capacitance installed (25 pF for the '415? YMMV), the DC voltage is minimized between A and B by adjusting L2.
The limit they give on one model is ±50mV DC.
This sets the DC reference for the FM demodulator based on the no-signal capacitance of the capsule.

They make no reference to adjusting L1 as far as I can see.
"We have grown to love our captors and demonize those that try to lead us to freedom."

emrr
Posts: 468
Joined: Sun Mar 02, 2008 11:27 pm
Location: NC, USA
Contact:

Re: Sennheiser MKH RF mic tuning info?

Post by emrr » Wed Apr 26, 2017 8:21 am

Ah, hadn't caught that 415 document. Thanks! Seems simple enough to try myself.

I received this response from Sennheiser:
The MKH105 we no longer work on as we do not have the parts for them. The MKH406 and the MKH435 can be sent in to be looked at. The technician will be able to send you an estimate once this arrives here. Thank you.
Best,

Doug Williams
Electromagnetic Radiation Recorders

User avatar
mediatechnology
Posts: 2974
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Sennheiser MKH RF mic tuning info?

Post by mediatechnology » Wed Apr 26, 2017 8:31 am

My neighbor Pete at http://www.416tupgrade.com offered this by email:
Looks like they have about as much info as I do. I've never worked on a 105, but on the MKH 416, 816, 415, 815, 405, 435, and 406 mics, (pre SMT) its the same routine- sub a 27pf capacitor in place of the capsule and adjust the transformer closest to the capsule for 0mv, +/- 50mv, at points A and B, which are easily located by the protruding test pins soldered on the foil side. I say "closest" because Sennheiser interchanged the designators L1 and L2 at some point in the development of the MKH series. I just know its the transformer closest to the capsule. I leave the center coil alone. If there is a 3rd coil, it is the 8mHz frequency adjustment, and I set it for 8mHz using a receiver. The early models use a crystal; the most recent MKH versions use an 8mHz ceramic resonator. Then a drop of candle wax over each slug.

I believe all the adjustments are pretty broad. Its tough to make these mics fail. Typical repairs include open RF blocking inductors at pins 2 and 3, failed crystals or cracked/loose ferrite transformer enclosures, failed chassis-to -circuit board solder points from a previous repair, and failed tantalums. Even a capsule that presents shorting can be resurrected with a couple of good raps on a table. Also the XLR pin assembly on all models is held only by a compression ring, and is somewhat wobbly. Enough years go by and one of the internal leads will break free.
"We have grown to love our captors and demonize those that try to lead us to freedom."

Post Reply

Who is online

Users browsing this forum: Bing [Bot] and 4 guests