Mk012 with low signal.

[gutimacuco]

Hello,

I have two Oktava Mk012 modded by Michael Joly.

One of them started to present low sensitivity, a very low signal compared to the other one. I opened them and the faulty one had much messier pcb.

I don't know a lot about eletronics and started to do continuity tests comparing the good and the bad one. Found some points that had continuity on the faulty that didn't on the good. The red lines on the third photo show these spots. I tried cleaning the pcb with isopropilic alcohol but it didn't change anything. This messiness made me think it was dirty/soldering problem.

Any thoughts?

 

[gutimacuco]

Posted one the wrong forum? Sorry, just reposted on The Lab

 

[Icantthinkofaname]

I don't know if this is related to your problem but those burgundy capacitors look like they've seen better days.

 

[rock soderstrom]

I don't know if this is related to your problem but those burgundy capacitors look like they've seen better days.

They look really bad, like dried plums.

First, did you change the capsules, the problem remains with one body?

The solder joints don't look very good overall and especially on the left board. Re-solder?

The one resistor that is mounted once on the bottom and the other time on the top seems to be different.

 

[Tubetec]

The red kidney shaped caps are silvered mica , it looks like they are covered in silicone rubber , these are extremely stable ,close tolerance components that dont often cause problems from my experience .
Dried plums are called prunes by the way .

 

[rock soderstrom]

The red kidney shaped caps are silvered mica , it looks like they are covered in silicone rubber , these are extremely stable ,close tolerance components that dont often cause problems from my experience .
Dried plums are called prunes by the way .

Thanks a lot! They look really similiar.


Back to topic:
When you have checked the solder joints, measure the voltages at the transistors.

 

[gutimacuco]

They look really bad, like dried plums.

First, did you change the capsules, the problem remains with one body?

The solder joints don't look very good overall and especially on the left board. Re-solder?

The one resistor that is mounted once on the bottom and the other time on the top seems to be different.

Problem remains on the body, tested the 6 capsules i have.

Before resoldering I want to check if I can detect other problems, thanks!

I found it weird that they had different positions for the resistors. But the quantity are the same with both, so is just a layout difference.

The red kidney shaped caps are silvered mica , it looks like they are covered in silicone rubber , these are extremely stable ,close tolerance components that dont often cause problems from my experience .
Dried plums are called prunes by the way .

Yeah, this is it, they are covered in silicone!

Thanks a lot! They look really similiar.
View attachment 102594

Back to topic:
When you have checked the solder joints, measure the voltages at the transistors.

I will, I'm just studying a bit before doing it. I have to check the supposed voltage on the schematics and measure to see if they match, is it like this?

 

[Tubetec]

I just struck a deal on a non working MJ Oktava Mk012 ,
The guy selling it said it died onstage on him ,
he is able to verify the capsule works on another mic body ,
I might just turn it into an Oktava minitube ,which uses our friend the 6S6B in a CF .
Still be interesting to find out why they stopped working ,

 

[rock soderstrom]

I will, I'm just studying a bit before doing it. I have to check the supposed voltage on the schematics and measure to see if they match, is it like this?

If you have a schematic with voltages, that's perfect for comparison. Even without a schematic, you can compare the faulty microphone with the working one at neuralgic points voltage-wise.

The next step would be to temporarily replace the capsule with a capacitor and feed in a small test signal there and then trace through the circuit with an oscilloscope and see where the signal is attenuated.

 

[gutimacuco]

I just struck a deal on a non working MJ Oktava Mk012 ,
The guy selling it said it died onstage on him ,
he is able to verify the capsule works on another mic body ,
I might just turn it into an Oktava minitube ,which uses our friend the 6S6B in a CF .
Still be interesting to find out why they stopped working ,

Hmm, interesting. Will research about this oktava minitube, never heard of it.

If you have a schematic with voltages, that's perfect for comparison. Even without a schematic, you can compare the faulty microphone with the working one at neuralgic points voltage-wise.

The next step would be to temporarily replace the capsule with a capacitor and feed in a small test signal there and then trace through the circuit with an oscilloscope and see where the signal is attenuated.

Yeah! Comparing each other voltage is the way to go. Will do and report back with the results. I don't have a oscilloscope, so hopefully I can trace the problem with the voltage comparison. Thanks a lot!

 

[Gus]

Do a search in this forum and the web. The 012 circuit has been discussed in the past.

 

[k brown]

Hmm, interesting. Will research about this oktava minitube, never heard of it.

Yeah! Comparing each other voltage is the way to go. Will do and report back with the results. I don't have a oscilloscope, so hopefully I can trace the problem with the voltage comparison. Thanks a lot!

This what I put into a pair of '012s:

 

[Tubetec]

Heres a guy who does a replacement pcb for the 012 ,
http://russelltechnologies.co.uk/microphones.html

 

[Gus]

Found one of the older threads
https://groupdiy.com/threads/diy-home-made-p2p-oktava-mk012-circuit-questions-observations.41986/

 

[Tubetec]

I had a good look around about the MK012 , both here and on a few other forums ,

Emrr Doug seems to have owned a few pairs , modded and stock ,

Michael Joly had special capsules made for an LDC headbasket called the MJEk47h ,
another company called RED also made a nice LDC for the MK012 ,
Theres a great selection of Oktava capsules to choose from also , just about every config is covered M/S X-Y , LDC front address ,side address, fig8 ,cardioid hyp cardioid

As far as what goes wrong in them ,several reports of mics with low output , people often have a pair so it easy for them to determine if the cause is capsule or pre-amp related . It seems sometimes the transistor fails but the fet survives , in any case its a very simple circuit to troubleshoot.

Theres been a few revisions of the circuit , one early variety came with a module at the front end ,
The earlier PCB's are very delicate , its no wonder Joly was able make his bread and butter fixing up peoples botched DIY attempts ,
Theres a nice mirror image symetry to the original layout. Below is the schematic with the voltages shown , from the Scott Dorsey article.



https://groupdiy.com/threads/slightly-different-2-oktava-mc-mk-012-circuit-elegance.27049/page-2

 

[Tubetec]

This guy seems to know what he's doing ,he has another video about fixing the body ,

 

[Gus]

I have repaired a failed 012 that had a bad PNP.
I am guessing it was from the screw that was turned in and was rattling around the inside of the microphone

 

[Tubetec]

Thanks Gus ,
I was reading back some of the posts on the Mk-012 over the years here,
Its an interesting circuit , the Russian mic gets the job done with two transistors , the German uses three and they say it was obsessive overengineering lost Germany the war .

 

[Tubetec]

Mic arrived this morning ,
Its an MK-012-01 ,
I did a few cold checks and the lower value resistors are all good .
The transistor measures good also ,
I suspect the fet is the problem ,

 

[Tubetec]

As far as I can see the later versions of these mics incorporated many of the upgrades people were offering as kits , swapping out tants that oozed green gunge for lytics ,etc
The Joly mod in mine incorporated a 1uF foil in place of a 10uF Tant bead and the Kidney shaped mica cap in place of a tiny ceramic ,thats it ,
The Bill Sitler mod was mentioned ,RadarDougs and others take was all the bass was all gone on the modded mics compared to stock and they sat in the cupboard .
Would the LSK389 have any relevance in replacing the LSK170BL in my mic ? can parrallel devices get me -6db noise ?
In any case I have a friend who almost certainly has stocks of K170BL to get me going ,but Im interested in other possible fet impedence convertors followed by op amps , P48's power limitatation might be done away with by using a centre tapped transformer at the end of the chain and a dedicated 48v supply rail,


Elegant an all as it is the circuit only achieves a capsule bias of 26 volts with 48V supply and 300mV max out,
That maybe doesnt take best advantage of a capsule which might withstand 100v and as long as the electronics can handle it very high SPL's without an attenuator .

Build quality , finish and the precission of the metalwork is first class in these mics ,
the silver version is bead blasted nickel plating on the case ,it has a wonderful grainy quality to the touch .
The user list of artists who used MK012 is unbelievable , its a testament to its design synergy ,

I also have an Oktava MKL-2500 cathode follower LDC , it gives a wonderfull neutral sound , in fact in this music clip it was used at about 18 inches from the acoustic guitar player/ vocalist , the bass player was more interested in chuffing gods herb and fluffing his parts ,
Its a live take remixed from multi track , its one of the best sounding recordings I've made , the solo is absolutely sweet as a nut .
the girl from Ipanema style Ohhhs and Ahhhhs are the incredible Rachel , theres a magic acoustic in her old kitchen where it was recorded .
Song 4 , Amber .
https://soundcloud.com/thecommontouch