Oktava MK-012-01 grounding hum/no signal from output

[kanurys]

Hi Guys. This will be my first post here. I have a pair of Oktava MK-012-01's and one of them is broken. It emits a loud grounding hum which changes amplitude depending on what is around it or if I'm touching the case.

I've isolated it to just the electronics in the mic and found a few things:

1. Since I have a pair I tried the mic capsule on the other mic body and it works fine.

2. I tried both mics using the same high quality XLR cable and the functional mic works well on that cable.

3. The components on each circuit board were probably assembled by different people, as all but the polarized ones are reversed and some of the wires are formed differently.

4. The working mic sounds great. I'm testing them on a Presonus Audiobox with phantom power output at a clean 46.8v without anything plugged in. Measured with my Fluke meter on the circuit board:

When the working mic is plugged in it reads
Pin1 - 0v
Pin2 - 39.32v
Pin3 - 39.33v

When the broken mic is plugged in it reads
Pin1 - 0v
Pin2 - 0.33v
Pin3 - 29.38v

5. The connections between the XLR pins and the circuit board have good continuity and none of the components seem to have poor solder connections.

My theory is that there is a leaky capacitor in there somewhere. This is based on the variance in the voltage readings. I couldn't see any sort of direct ground short between pin2 and pin1 but there is a .1uF decoupling cap between them, as there is for pin3 and pin1.

Thoughts?

 

[Gus]

Check change the PNP transistor.  Was one of the setscrews loose inside.

Pin 2 is the hint
I fixed a 012 that had a loose screw and a bad PNP.  The PNP drives pin 2

 

[buildafriend]

I would guess there's a short in there. no clue if it's a leaky cap.

Recap em. Can't hurt. Cheap and easy. Try replacing the transistors too. Cheap and easy. Maybe throw a socket in there for the transistor and see if other ones sound better. If you replace one T, might want to replace the other. Keep things even in the matched pair.

check the grounding, check the grounding, and then check the grounding.

 

[kanurys]

Thanks. I'll check it out. Screws are snug.

 

[buildafriend]

i second pin 2 as being the biggest hint

 

[kanurys]

Upon more research, I found a failed cap. C4 in this diagram reads 0.4 ohms. The same cap on the good mic reads well over 100k ohms. It also is doccumented in on this post a ways down. http://www.electricalaudio.com/phpBB3/viewtopic.php?f=5&p=1600007#p1607089

 

[kanurys]

I'm really liking this article and it's suggestions will also probably take care of my problems. Maybe I'll make a parts list and put it here when I repair my mics.
http://www.recordingmag.com/resources/resourceDetail/315.html

Some of the part numbers are outdated. Here is what I found at Digikey to replace them:

2x or 4x - R1 & R2: 1000M (1G) resistor, MOX-200001007JE
1x - C6:                  820 pF COG ceramic capacitor, 445-4390-ND OR 445-4325-ND
1x C2:                    10 uF 25V tantalum capacitor, 478-1841-ND OR 399-1418-ND
2x total C3, C4:      68 uF 63V electrolytic capacitor, 4073PHCT-ND
1x C5:                    47 uF 40V electrolytic capacitor, 4047PHCT-ND
1x Q1:                    2SK170BL FET Transisor, This one from B&D Ent. http://www.bdenterprisesinc.com/2sk170bl/ OR EBAY is a great place to get these
1x Q2:                    transistors 2N5087, This one from Mouser: http://www.mouser.com/ProductDetail/Central-Semiconductor/2N5087/?qs=Ypxpq5eNvNWYtGeWLkwqHg==

I plan on getting two set of these parts and upgrading both of my mics.

 

[Gus]

kanurys
I see you have 4 posts at the time of this post.  I suggest you take some time and search this site for threads on the 012 circuit.
The threads have good information.

You might like the circuit stock better than with the changed parts.  The stock parts in 012 can be fine

 

[Rossi]

I had a failed output cap in one of mine, too. It's not that easy to find a 68u lytic that physically fits. It must be rated 50V or better 63V. Phantom power is 48V +/-4V, so the maximum allowed voltage is 52V. If you can't find 68u, take 100u, if you can find one that fits, if not, 47u will do. It would probably be best to replace both 68u caps at the same time.

IMO it's hardly worth modding the circuit. It sounds fine as it is. As Gus says, there were a number of discussions about the 012 circuit and possible mods.

 

[kanurys]

Thanks guys. I looked at a lot of the mods out there. Most of them were not only inspired by wanting to change the sound but also because people couldn't find the stock replacement parts, or parts that fit.

I'm not opposed to keeping it stock if I can find the parts.