MK-219 Dorsey Mod Grounding problem

[phatmateo]

Hey guys, I've searched the forum to see if anyone had any similar problems, but couldn't find anything.  I have a low cycle hum on my mk-219 w/Dorsey mod that I can't get rid of.  It has to be a grounding problem, but I'm having problems tracing it down.  When I touch the grill the hum gets super loud.  Would lining the casing with silicone have anything to do with this?  As far as I know, the circuit just grounds to the casing near the xlr.  Any help is appreciated!  I can post pics if you need them

 

[rodabod]

Check there is good continuity of earth through the whole case. I assume you have a multimeter.

 

[rascalseven]

Somehow it the ground from your pcb is not making good contact with the metal from the mic's case, it seems.  Check to make sure that pcb's ground plane makes contact with the screw hole on the XLR and make sure you haven't painted over the corresponding hole in the mic chassis.  This is a poor design, IMO, for ensuring good ground contact, so care must be taken to preserve this critical connection if your mic is going to be quiet.

I did this mod on an MK319 that I got when Guitar Center cleared out the Oktava line a few years back.  Best $55 I ever spent (the mic, not the mod).  I use use it as a poor man's FET47 outside the kick drum constantly.  Beautiful for that, really.  Also nice on picked acoustic (a BIG sound) and also amps.  Occasionally for vocals, but a tad dark for that IMO, and I have many other much nicer models around for vocals.

JC

 

[Rossi]

Yes, definitely check the ground connection to the mic housing. Also, if you removed the outside grill, make sure the mesh has good electrical contact to the housing. The the mic case metal is not easy to solder to.

 

[phatmateo]

Thanks for all the replies   The grill was not properly grounded to one side of the casing, which was causing the problems.  It took a while to scratch away at the metal so I could get some solder to actually stick to the grill and the case(Rossi was right!).  Why does the grill need to be grounded?  Can someone explain?  I'm new to this and want to understand.  I get how grounding works, but I do not understand why only one un-grounded grill was causing all the hum.  Thanks guys

 

[PRR]

> Why does the grill need to be grounded?

A condenser microphone has a fairly large (~~1") electrode connected to a sensitive and VERY high-impedance amplifier.

The world is full of HUM and buzzz.

The electrode will pick up all the hum/buzz in the room and contaminate your audio.

The best answer is to completely seal the capsule in a solid metal box, tied to the amplifier common. But then it does not "hear" very well.

It works out that metal mesh works well enough, IF it is all firmly connected to common.

But any poor connection leaves some part of the mesh "floating" in the hum/buzz fields, so the hum/buzz sneak in.

It is like the fence in my yard. Most of it is very solid and tight. But if there is ANY gap or loose spot, my Corgi-dogs get out of the yard.

 

[phatmateo]

thanks for clarifying.  And here I thought grills were just to protect the capsule.  Interesting stuff

 

[phatmateo]

not sure if you guys can help or not, but do you know the best place to buy new screening for the microphones?  I might eventually replace the stock screens because they are misaligned and I'd like only one layer.  I'm in the states.  Thanks!

 

[aronaut]

wire mesh for screening:  mcmaster carr

 

[phatmateo]

thanks, I figured, but don't I have to buy in large amounts there?  I'll check it out

 

[aronaut]

you can order whatever, mcmaster's the best

 

[phatmateo]

Whats the best wire mesh dimensions for a microphone?  I could order a sample pack, but I figured I'd just ask since you seem like you have ordered it before.  Thanks aronaut!

 

[Commander Fluffypants]

Hello old thread and GDIY folks...

I modded my 219 and have a buzz... but I also lost one of the brass shims. Could that be a source of my grounding issues? Where can I get another one? Can I solder a wire or put something else in there?

Thanks!