Help troubleshooting faulty Oktava MK-012

[maxkriza]

Hi all
Mic is faulty/dead no sound. Capsule checks ok but preamp voltages are all wrong. 41v on pins 2&3 is ok
but around 40v after the two 13k resistors instead of 20v. On the Fet source 9v instead of
20v. Pnp base 9v instead of 6v. I suspect the fet is faulty but not sure.
Help much appreciated!
thanks
Max

 

[Sredna]

Try to put in another Fet (2N3819 or similar) just to see if it passes audio.
If yes, have fun reading threads about Fet selecting! 

I recently had a faulty 012 where the PNP was dead, replaced it with a 2N5087 and voilà!

 

[julien31]

I had one dead quiet too. It was a faulty electrolytic cap. If I remember well, it was easy to check with a continuity tester, just test the two legs of each cap without even desoldering. If it beeps, the cap is dead! First thing to check IMO

 

[maxkriza]

thanks, electrolytics are fine. must be some other fault.

 

[Gus]

Check ALL the transistors voltage nodes
Drain gate and source to ground to check the JFET
Emitter base and collector to ground to check the PNP

Report the voltages and it should be easy to figure out what is wrong

Was the microphone being worked on when it failed did a body screw fall inside and cause an issue?

Search for the 012 threads at this site for more information on the 012 circuit

link to a thread
http://www.groupdiy.com/index.php?topic=41986.0

 

[maxkriza]

Thanks Gus. I found that one of the legs that came from the fet was not soldered
properly. I Had to bend the 680m resistor out of the way and broke it in half ... damn.
after soldering the Fet I get all the right voltages so that was probably the fault.
Any Idea where to get these high ohm resistors in the UK?
thanks again for all the help
Max

 

[gabygab]

Hello,

This has nothing to do with the original post other than the fact that it's an oktava mk-012.

This microphone started crapping out on me about a year after I got it. It's a shame because I got some really nice recordings out of the pair.I opened it up today and found that where Pin 3 connects was all corroded on the pcb. There is no metal there. Being the noob that I am, I was wondering if there is a fix for this. I was thinking of just soldering it to the negative end of the electrolytic cap next to it. Would that work?

Regards,

Gaby

 

[Landins]

Should work. Make sure you dont bypass anything. Copper paths are a in fact just a unwanted necessity anyways.

 

[gabygab]

It worked! Thanks a lot!!

 

[Landins]

It worked! Thanks a lot!!

Sweet! Isnt it lovely when faults are easy to find and easy to fix? I often find myself hoping for broken wires and wrong component values when something is not working.

 

[gabygab]

I've got to say it feels amazing! It encourages me to make more music, and make chili too!