foxy



RT-219 - Remodel of the Oktava MK-219 PCB

The RT-219 is a pcb which can be used to repair or modify the Oktava MK-219
microphones. It is in itself an improvement on the original PCBs, better
quality and with a solder mask on both sides and a partial ground plane.
It offers higher electrical insulation properties and greater resistance to humidity.


The pcb is designed so that it can be used to build a copy of the original circuit
using high quality modern components. Also in the schematic I have suggested some
component value changes to improve the performance. It will also be useful if you want to try some of the circuit modifications suggested by others.


RT-219 pcb image
Price £7.50 with free p&p in UK.
£3.50 p&p for ROW.
Rave reviews from famous philosophers.
Schematic diagram, click image for pdf, or right-click > view image for photo.


You can repair or rebuild the original Oktava circuit using better quality components, or use the modified component values in my schematic, which I think make a small but worthwhile improvement. Also there are a number of mods suggested by others, Scott Dorsey for example has published his recommendations for this circuit which seem to be very popular.


My mods are fairly simple. I have increased the input resistors to 1G from 500M, recommended by almost everybody and not likely to cause problems with modern components and PCBs.


I have made a slight reduction in the gain of the FET circuit, giving a small improvement in linearity and output impedance.


I have used aluminium electrolytics for all the polarised capacitors. The properties of good quality electrolytics available now means there is nothing to be gained by using other types of capacitors in any of these positions. I have increased the value of the three 1 microfarad capacitors to 22 microfarads to take advantage of them to give a stiffer and quieter rail for the FET and better drive to the transformer at low frequencies. This also seems to make the low-cut switch setting sound a lot cleaner when used. I haven't checked out why, but I suspect there are complicated interactions between the transformer with the original 1 microfarad capacitor and the filter circuit.


For the small capacitors, I usually use various plastic film types for all of them. The COG ceramics that are often recommended also seem to work well.


About the FET


There is just one active device. There is no reason not to use the original Russian FET if it is working well. If it appears to be noisy or have a problem I usually use a J201 or a 2N3819 or something of that sort. Others, Dorsey for example, use the 2SK170BL. It is a low noise device, and can be made to work in this circuit, but I prefer something with a bit less gate to channel capacitance. Noise isn't going to be much of a problem in this circuit unless there is a fault of some sort.


If you reuse the original Russian FET, some of them have a fourth wire connected to the metal case, which can be snipped off close to the case. This will reduce the capacitance around the FET channel by a picofarad or two, and also reduce the chances of any creep, very tiny advantages but they cost nothing. As always with FETs, it is a good idea to clean between the legs with some alcohol and a small brush before you solder it into place.


If you need to adjust the operating point for the FET which you are using, the best way is to change R4. If you use an FET with higher IDSS such as the J111, R8 could be increased to 4k7. If you are inclined to run the FET a bit hotter, that's not a bad idea up to a point, but there is not much way to go before recalculating the rest of the circuit is necessary, and every extra milliamp of drain current reduces the capsule polarising voltage by about 3.5V. If using the J201 FET which I recommend, the gate is pin 3 of the package so it is rotated accordingly on the pcb pattern.


Scott Dorsey's item in RecordingMag about his work on the 219.
Stewart Tavener at Xaudia has done some very cool stuff on a pair of MK18's using these boards.
Some photos of my work on a 219


To see full-size image, right-click > view image, then click on image.
Oktava MK-219
Oktava MK-219
This version has magnet-operated reed switches.
RT-219 assembled for testing. Note that T4 and T5 are connected by a short wire during test.
RT-219 bottom view.
High impedance area. This part in particular must be very clean, both sides.
Finished electronics, ready for mods to damp body resonance.
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