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Oktava MK-219 Microphone Modifications Part Two: Electronic and Wiring Mods

Now it's on to the electronic modifications for the Oktava MK-219.

I followed the suggestions for component changes in the original Scott Dorsey article, with the exception of changing the stock JFET to a Toshiba 2SK170BL. The Toshiba transistor is quieter, or so the article implies.

But after reading a lot on the interwebs about that change, I decided to do all of the other upgrades and then see if the stock JFET was noticeably noisy and warranted a change. I did procure a handful of the Toshiba transistors in case I decided to change later.

On the right you can see an image I created which identifies the (stock) components. We'll be replacing 2 resistors and most of the stock capacitors.

I changed out the original small value caps for silver mica caps with increased values. The holes in the PCB had to be opened up a bit to allow the thicker leads on the new caps to pass through.

An easy job with my trusty Dremel tool and some small bits.

Here's the PCB after changing out the selected components. You can see the input cap near the top - that was upped in value from 680pf to a 820pf silver mica cap. That was changed along with changing R1 and R2 to 1G resistors.

You read that right - one gigohms! The only place that stocked that value that I found was Digi-Key, and they cost $3.20 a pop. My Fluke DMM doesn't even read to that value.

You can also see two other silver mica caps, as well as the electrolytics. The most important change to the electrolytics is changing the output cap, C5, from 1uF to 4.7uF - and using a better quality cap. I put an Elna Silmic II 4.7uF/50v cap here.

The Dorsey article calls for changing the original aluminum electrolytics to tantalum caps (which are also a type of electrolytic capacitor). I have never read anything good about the tone of tantalum capacitors, so I decided to just go with high-quality Nichicon and Elna aluminum electrolytics as I did with my Altec mixer and preamp rebuilds.

One daring - you might say crazy - change I made that was not suggested was changing the wiring that runs from the capsule to the circuit board.

The stock wiring is super thin - maybe 32 gauge - and seems cheap. I decided to change it for known high quality wire.

I cut apart a short hunk (maybe 5cm) of Mogami quad microphone cable and used some wire from that to replace the stock wire. I also did a silly academic exercise in which I measured the capacitance of the stock wire and compared it to the same length of Mogami wire. The Mogami wire measured maybe 1pf less. Since capacitance is critical here, I felt better.

This is the stock ground from the capsule. You probably know the capsule is fragile and if it gets punctured, it will be useless.

But that didn't stop me from using a piece of index card to protect it while I desoldered the wire from the tab on the center of the capsule and soldered a new lead on.

The hot lead is on a tab at the bottom of the capsule - much less scary to unsolder and resolder.

You can see both new leads on the capsule at the right.

The screws that hold the capsule to the chassis are small. I magnetized them with my magical Wima magnetizer tool so they'd stay on my screwdriver.

Sort of like the 'peas with honey' poem, no?

And, in the interest of absorbing vibration, I put a piece of closed-cell foam (again leftover from the c900 sound deadening project) on either side of the capsule mount. You can see it in the picture on the right I took while remounting the capsule.

The stock routing for the hot lead took a relatively long path to the board, so I drilled a new pass-though hole right near the tab on the capsule and ran the lead through that. That path saves almost 15mm of lead length, not a bad thing. The less capacitance in the capsule leads, the better.

Wait until you see how long the leads are on a stock Oktava MK-319!

Here's the new capsule wiring.

When I finished the mic and tested it, it produced nothing but static. I discovered the hot lead was touching the metal capsule mounting and shorting it to ground. Be sure you have clearance at that point.

Is this worth doing? It's your call. If you're confident working around the fragile capsule, why the heck not? If in doubt, leave the stock wiring. I probably saved less than 4pf in doing this. Maybe 5.

This is the completed PCB. Note I took the plastic cover things off the capsule as well. This supposedly makes the mic less peaky in the above 10K frequency range.

Now to test and see how well it works.

I had done a test recording with acoustic guitar and the stock mic. Then I did a second track with the modded mic. It sounds cleaner, smoother and more defined and detailed to my ear. Most noticeably, the bass is a lot 'tighter' and less flabby.

The last thing I did with the PCB is to put acrylic conformal coating on the input joints to protect them from corrosion. These high-impedance connections are sensitive and it's suggested to coat them.

Here's the reassembled mic on the bench.

I really like the look of that new mesh!

Closeup of the new headshell - you can see the capsule now!

Here's the mic in action. It sounds good.

I need to do more recordings with it, but I think the stock JFET sounds just fine to my ears.

I'm planning on modding my MK-319 next and I will be changing the transistor on that one, so we'll see if it's worth it.


Post a Comment 3 comments:

  • Anonymous said...
    June 14, 2017 at 10:53 PM
    I love your blog!! Very cool. I have 2 of these oktava 219 mics and plan to do some of what you have done. I just got an altec 1611a power amp that I am planning to mod into 2 mic preamps and 1 DI with gain pots to be added on. I will certainly reference your blog and may ask for help. I'm a beginner in doing mods. Started with a shure m67.
    Well done!
  • Lakeview Drive said...
    June 14, 2017 at 11:06 PM
    I also have akai m8, roberts 770, and ampex 600 to do....actually, I got into the shure m67 mods because I read about people using those input transformers in the tube preamps, but then learned about how cool the m67s are on their own. I began my journey down the wormhole....the m67 is my go to for kick and snare!
    Thanks again for your blog.
  • Yr Fthfl Blggr said...
    June 18, 2017 at 9:59 AM
    Thanks for the kind words about the blog.

    I remember using the M67 for radio work many years ago. I'm not surprised to see its continued popularity and the mods for it. Now I want to get one!

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