Oktava MK-012 repair - loud pops and bangs

[12AX7]

Hi everyone!

I have a bunch of Oktava MK012s in my live mics case. 12 of them, to be more exact. 8 regular and 4 Oktamics. They're all pretty old and have done a lot of gigs but have been well taken care of.
On a few recent gigs I have noticed some of them starting to have some odd symptoms. They produce random loud pops and bangs. Not with vibration or being moved, regardless of signal level. They can be perfectly still and in a quiet room and then BANG. First time it happened I tried a different cable, still did it. When I got back I took the mic apart, re-soldered everything, tightened all screws, cleaned XLR and capsule contacts. Still does it. And now some others have developed this as well. Can't take them to gigs anymore, been using AKG C451EBs instead but I do love my Oktavas, that's why I have so many.

Anyone had this issue before? I would suspect either the FETs are going or coupling caps? What would be a good replacement FET? I have seen modders use ye olde 2SK170BL. Is it a drop in replacement when the rest of the circuit is stock? Or is there some other common issue with these? I have heard of others having this issue but can't find anything online. I'm not really looking to mod the mics, to me they sound great as-is.

 

[Khron]

Dirty capsules?

 

[OneRoomStudio]

I agree with @Khron - I would try to isolate it to the capsule(s) first to see if that's the problem. If not, it might be worth picking up a few replacement PCBs as the original material can become damaged and even hydrophilic/conductive over time. I have used these with good results: Russell Technologies

 

[Voyager10]

I've had a couple of mics (not Oktavas) where the capacitors filtering the capsule voltage were on the way out. Replacing them was the easy fix.

First thing is to check the capsules though. If there's any possibility they've been exposed to moisture it's worth packing one in a ziplock bag with a few silica gel sachets and leaving it for a week.

 

[cyrano]

I've had a couple of mics (not Oktavas) where the capacitors filtering the capsule voltage were on the way out. Replacing them was the easy fix.

I've had that with my AKG C451EBs. Not always loud clicks or bangs, but sometimes motorboating or other noises. They were fine again after replacement of the filtering caps.

 

[k brown]

I've read online that Oktava capsules are prone to developing noises from moisture trapped inside; here's a vid on fixing that:


I don't know if it's a fairly recent problem, or I've just been lucky. I bought mine from The Sound Room in the early 2000s and have never had this issue.

 

[Icantthinkofaname]

I've read online that Oktava capsules are prone to developing noises from moisture trapped inside; here's a vid on fixing that:


I don't know if it's a fairly recent problem, or I've just been lucky. I bought mine from The Sound Room in the early 2000s and have never had this issue.

I've had an issue after cleaning a couple with rubbing alcohol. Not the diaphragm or anything, but just cleaning the PCBs and body sleeve. Left them powered on for a couple days in a bag with silica gel and they worked fine.

It's hard to say what's happening because they're supposed to be somewhat moisture resistant compared to things like a Schoeps MK41, at least from the anecdotes I've heard.