Oktava MK-18 Phantom Power Conversion

I recently acquired an Oktava MK-18, the multi-pattern predecessor to the MK-219. The original phantom power supply was not included, which I've read puts out +60V (I've also read +70V somewhere) instead of +48V. I'm told the mic will work on +48, but the noise floor is substantially higher. 

There's a lonely gearslutz post quoting a translated russian text suggesting it could be converted to +48 by simply changing two capacitors, though it doesn't specify which, nor what values.

If anyone's done this before, or is familiar enough with the MK-18 to suggest something, I'd appreciate any help I can get.

Never worked on one but I found this it has a 18 schematic
https://www.gearslutz.com/board/geekslutz-forum/850232-oktava-mk-220-mod-help.html

I like the compound pair. 

C6 is a filter cap, going from 60V to 48V you should be OK with the voltage rating
Are C7 and C8 values 5uf?
what caps were changed in the thread?

We can do some math or run a sim if we know what the phantom resistors are in the 60VDC supply.
Does anyone know what the 60VDC phantom supply resistor values are?

However the fast thing you can try is jumping in different parallel resistors across R12 to reduce the value and increase the voltage to the gain stage.  Don't do this unless you know how to.
R12 "drops" the phantom voltage and is part of an RC filter with C6
The gain stage operating point will be changed by changing the voltage at the R12, C6, emitter and R9 node
I would guess R10 and R11 are selected for the JFET measured IDSS, Vgsoff etc. are the resistor values pencilled on the transformer?

Have you tried the microphone with a 48VDC phantom supply?

Mic schematic

PSU Schematic

Maybe would be worth to build the PSU?

In76d
Do you know the R2 R3 values? I did not see any values in the PS schematic.

I don't have the mic in hand yet, still awaiting delivery. Once I have it I can check values.

I'm not opposed to building a PSU, but without proper values on the schematic, I'm not qualified to fill in the blanks.

Gus said:

In76d
Do you know the R2 R3 values? I did not see any values in the PS schematic.

Click to expand...

Unfortunately not - i even worked on this mic, but didn't had any need to check it.

thecr4ne said:

I don't have the mic in hand yet, still awaiting delivery. Once I have it I can check values.

I'm not opposed to building a PSU, but without proper values on the schematic, I'm not qualified to fill in the blanks.

Click to expand...

First of all, i would write to Oktava and ask for schematic - if they will refuse, then i would ask just for R2 and R3 values.
Write, that these two resistors are burn't and you want to fix PSU by yourself.
Then i would buy some decent phantom power supply  (with option of powering two microphones) and just convert it to one 60V PSU.
You don't need to build exact circuit - this is just power supply - need to be well filtered and noiseless and just deliver proper voltage. I would go for 60V, not 70V.

If you will not find R2 and R3 values, then you can always use double (stereo) potentiometer and find the values, measuring drop of voltage inside microphone.

Here's what I did to mine!

http://xaudiaelektrik.blogspot.co.uk/2017/02/oktava-mk18-vs-russel-technologies-mod.html

Cheers
Stewart

Here's what I did to mine!

http://xaudiaelektrik.blogspot.co.uk/2017/02/oktava-mk18-vs-russel-technologies-mod.html

Cheers
Stewart

Click to expand...

Thanks Stewart, I actually stumbled onto this while researching the mic. I already have a nice working pair of MK 219s. so this doesn't seem worth while to me. I'd much rather have the multi-pattern than another 219.

Before conversion, mine did work (to some extent) on +48V.  Reducing the values of R13 & R14 may help improve the noise performance.

The real problem with both of mine was that the switches were awful.

Have fun!
Stewart

Rather than building a whole new +60V PSU, how about modding an existing +48V external PSU to up the voltage. Thoughts?

thecr4ne said:

Rather than building a whole new +60V PSU, how about modding an existing +48V external PSU to up the voltage. Thoughts?

Click to expand...

I will quote myself

[quote author=me]
Then i would buy some decent phantom power supply  (with option of powering two microphones) and just convert it to one 60V PSU.
You don't need to build exact circuit - this is just power supply - need to be well filtered and noiseless and just deliver proper voltage. I would go for 60V, not 70V.
[/quote]

I will quote myself

Click to expand...

Must've been where I got the idea.  Oops.  I'll look into getting something solid enough to try, and maybe come back to get a second  opinion on the actual mod itself. Should be fairly simple right? Swap a few resistors, DC current comes down, DC voltage goes up...keep the AC clean...or am I optimistically over-simplifying?

Long overdue update here. I never did pursue the +60V psu as everyone kept telling me +48V would work fine.

zebra50 said:

Here's what I did to mine!

http://xaudiaelektrik.blogspot.co.uk/2017/02/oktava-mk18-vs-russel-technologies-mod.html
Cheers
Stewart

Click to expand...

I DID however contact russell technologies and they made some PCBs for this. There's not obvious link on their website, but there is a page here Russell Technologies RT-18 for Oktava MK-18 pics are of one of my mics in original state and the boards he made for them. I did convert them, but never got around to posting photos. I broke one of the switches in the process and miraculously found matching switches on ebay, and have a few spares still. I will say that one of the mics had low output or some issue with one of the pattern selections, which was no longer the case after the conversion, so we'll blame that on crappy old PCB's traces. I'll dig around for some pics, I'd rather not have to re-open the mics as the switches are annoying to align when closing them back up. THey both sound great...that is to say, like a multipattern MK-219 would sound.