MK219 headbaskets.

  • Thread starter Thread starter Steenamaroo
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Steenamaroo

Steenamaroo

...
Hey guys,
I've had threads about this before and never quite got to the bottom of it.

I have simplified it to some extent but I'm hoping someone can put this to bed once and for all.


What I did.
I modded my oktava mk219s. Removed the metal barring across the grills, removed the grills, separated them and put one layer back into the mic.

I made sure to sand down the surface that meets the grill, cleaned any glue off the grill, and used a conductive glue.
I read <3ohms from any point on the grill to pin1. It's a nice clean job. No breaks, no gaps.

The old problem.
The mics pick up what sounds like mains hum from my body which can only be completely eradicated by touching the mic shield/grill/etc.
Say i set the mic on my knee, or hold my hand close to the grill, I get this hum.
If i touch the grill, it's stone dead and everything sounds great.


The problem only occurs on laptops. My desktop with motu 828 (mains powered) is fine.

It all started on a dell vostro laptop with mbox. After a while i put it down to the laptop because i read about the floating ground of the laptop causing problems with audio/video outputs etc.


The current problem.
Now I have a macbook pro and a presonus firewire interface and have the same problem.
On the dell i made a cable that manually grounds the laptop chassis to mains earth which solved the problem, but on the macbook that's not happening.

IF i completely unplug the macbook PSU then interference isn't picked up from my body but there is still a constant faint hum that touching the grill doesn't fix.
I wanna work out why the mic is susceptible at all.

The plan.
Anyway, to cut to the chase, I'm thinking about completely re-grilling the mics with new grills but i don't see the point in making the same mistake again (if indeed i did make a mistake), so I'm hoping someone can offer advice here.
I see some guys with the mk319 mod using brass grilles, but then i see others using one layer of the stock grille.
Does it make a difference?


Things I've ruled out.
I'm not hearing the noise of something in the room, like fan or light dimmer or anything.
I've tried different rooms, different mains sockets, even different houses.
This doesn't happen with other stock condenser mics.
It can't really be a ground loop because even manaully tieing everything to mains ground doesn't help.

I've read everything i can about this mod and the only people who had problems hadn't grounded the grill to the body properly, but I've even reglued the grills recently and made certain they were continuous with the mic body.

IDK, the only reason i bought all this crap is so i record mobile. Like, just take my bag and go. anywhere kinda thing.
Now if I wanna use my oktavas I have to bring some mains powered interfaces or whatever just to get around this hum.
It kinda defeats the purpose.

Thanks in advance all.
 
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Weird. Have you tried something like an Ebtech hum eliminator to transformer isolate the connection from the mic to the interface just to see. If that's the case it must be some weird stray voltage. Or, is it RF?
 
I haven't tried an eliminator.
I've sorta been stubborn that way cos It's only these oktavas that have the problem.


I don't know a huge amount about this kinda thing, but as I understand it there's a difference between mains ground and the laptop's ground.

The difference in potential presents itself as a hum.
What I don't get is why none of my other mics do it.

M.Joly mentioned something about (only) two customers who experienced this and AFAIK they were exceptional circumstances;
Badly wired house or something, I can't remember.
 
I'm shooting in the dark here: but how about trying a standalone phantom power adapter if you have one and turn off phantom on the interface. Also, have you tried other cables? Perhaps you're getting a ground loop between pin 1 and the shield cable. Another blind shot.
 
The layer of mess you put back isn't conductive enough! Gotta go with something else Paul.
 
Thanks blue.
I've tried different cables. Ones with pin1 attached the the cable body, and ones without.
I don't have a separate 48 supply though.


Henry, What makes ya say that?

You could be right i guess. Any ideas what would be better?
You'd think any factory mesh would have 0 resistance, even if there was two layers originally.
 
What is your guess to the type of metal that is in there now for the mesh? Do you have the other layer still hanging around?
 
Tbh i don't know the original mesh is, but I could look it up.
I'll google that :)


I do not have the old mesh anymore. The mics worked fine at my main setup so I dumped them then discovered the problem later.

I could try replacing with something that's lying around or even buy some brass mesh I guess.


IDK, if I'm stuck I'll go back to dual layer and see if that changes anything.
 
Well, i did a test run with some mesh i had lying around here.
Cut it all neat, dremeled the mk219 where it meets.
Couldn't be cleaner.
O ohms anywhich way i test.

Still buzzing.
 
It can't be though.
Well technically it can obviously, but it's soooo unlikely.

Two mics, working perfectly. Both grille modded, no electronic mods or anything like that.
Both suffer from susceptibility to AC hum.

There's a good chance they'll hit the bin tomorrow and I'll have a long chat with myself the next time i think about doing something.


Thanks for your help though, It is appreciated.
 
OK what is different now from when they were unmodded?
 
Nothing other than what I listed.

I opened them up and set the circuit boards/capsules aside.

Removed the twin layer mesh, clipped the hard metal grille out and filed down the stubs.

Used a dremel to clean the surfaces that would meet the mesh,
separated the two layers of mesh, cleaned off all the old glue, refitted one layer only, then set it in place with conductive glue.

Let everything dry then reassembled the mics.

Irritatingly, at the time I did one mic only to make sure it was fine, then did the other after.

That's all.
 
And it's a constant hum ..... even if you touch it?

Did you remove the resonating disc?

And filled in the body cavity?
 
Sorry, yeah, I did both of those things.

I removed the resonator discs. I didn't unsolder wires like most. I very carefully snipped a spoke out and lifted the disc away.
I put the screws back in as they are needed.

I also put a light layer of silicone in the top and bottom parts of the casing.

Say I set the mic on my knee, there's a constant hum, but if I touch the mic grill, xlr metal shield or laptop body, the buzz completely goes away.
If i set it on the bench it's picking up hum (maybe from nearby wiring?) but touching the laptop/grill doesn't help unless I wrap my hand right around the grill.
 
May be the connection between the two body half need to be tighter?
 
Lol, I don't wanna sound like a know-it-all but I've been there too.
Originally they had a coating on them so i don't think it's important,
but i even sanded them down on a flat surface to make sure there was perfect contact just incase.


Sorry Henry! :p
This is all great advice.
 
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