Cleaning a dynamic capsule with magnets

[OneRoomStudio]

I thought some folks here might be interested in this. I've cleaned/refurbished a number of D12e capsules in the past, but it has always been a little bit of a challenging process to remove all the magnetic material and get the diaphragm moving well. I have always thought that using a magnet would work wonders, but I was too scared to do so with such a nice (and increasingly rare) microphone.

While digging through my parts bin the other day, I came across an old dynamic capsule (I have no idea where it came from) that I always meant to try to clean up and use as a "character" mic. I wired it up as seen here:

The first test didn't go so well. I had to max out the gain on my highest-gain preamp just to get this:

View attachment Mic Tests#36DynamicTest1.wav
I was going to give up until I remembered the magnet idea. This capsule is worth nothing, so it seemed like the perfect candidate. I took a strongly magnetized screwdriver and slowly worked the tip back and forth just above (~0.5mm) the surface of the diaphragm. I immediately noticed material building up on the tip of the screwdriver. After my initial test, here's where I was (much less gain was needed from the preamp this time):

View attachment Mic Tests#37DynamicTest2.wav
I did another pass, concentrating on the ring between the outside folded edges of the diaphragm and the center dome. This is where most of the magnetic material had built up. I followed that up with some gentle air from a camera lens cleaning bulb. While the frequency response is still narrow (as I expected from this capsule), the gain is now much closer to a modern dynamic capsule:

View attachment Mic Tests#38DynamicTest3.wav
I wish I could think of a safe way to do something similar with D12e capsules...but this is far too fiddly for now (one slip of the screwdriver tip could ruin the capsule permanently). Anyway...a successful experiment none the less.

 

[panman]

The trick is to use a small tip screwdriver. The smaller it is, the weaker the pull and thus safer. The iron particles have to be picked individually. Not safe, but needs practise. Again, I need to remind, that the particles gathered around the dome have been pulled down for decades and the diaphragm has been deformed concave in most cases, often one side more than the other causing the coil rub. Removing those particles will not make the D12 sound ok, but much more is needed in most cases.

 

[frans]

I had good luck cleaning membranes (only moving coil mics) with sticky tape. There's the milky-white tape from 3M that has no smearing glue to it, but a rather clean sticky coating and i used it on many occasions to remove dust, dirt, hair and magnetic particles from membranes. With double-sided sticky tape which i fixed on half a matchstick i even got lucky on mics that have a resonator cap on top of the membrane, getting in sideways. I always use a biiiig magnifiying lens while doing this.

 

[MicMaven]

Curious - Suppose I used a very strong neodymium magnet kept some distance away from the diaphragm with a frame or spacer of some kind to keep the cartridge from clanging into the magnet. I might also use a very soft camel hair brush and Mr. OneRoomStudio's lens bulb blower to assist the magnet in removing particles from the cartridge. Or would that be too dangerous, perhaps too much magnetism? I would hate to deform the diaphragm by drawing it too far out.

Also, is there any way to clean UNDER the diaphragm? (suspecting this is asking far too much!)

Intriguing discussion. As alwyas I am merely asking - NOT recommending! / James

 

[k brown]

Curious - Suppose I used a very strong neodymium magnet kept some distance away from the diaphragm with a frame or spacer of some kind to keep the cartridge from clanging into the magnet. I might also use a very soft camel hair brush and Mr. OneRoomStudio's lens bulb blower to assist the magnet in removing particles from the cartridge. Or would that be too dangerous, perhaps too much magnetism? I would hate to deform the diaphragm by drawing it too far out.

Also, is there any way to clean UNDER the diaphragm? (suspecting this is asking far too much!)

Intriguing discussion. As alwyas I am merely asking - NOT recommending! / James

Just what I was thinking while reading the first post.

I doubt there's a danger in the magnet pulling on the diaphragm; most (if not all) mic voice coils are copper wire wound around a paper, plastic or aluminum former - so not magnetic.

 

[Paul W]

Would it make sense to use a piece of steel wire as a very narrow polepiece to narrow the magnetic field around the grain of junk in question.

Maybe put a transparent straw around that point, to keep the diaphragm from moving, and then lower the magnet/wire into the straw until it picks up the grain?

 

[Khron]

Would it make sense to use a piece of steel wire as a very narrow polepiece to narrow the magnetic field around the grain of junk in question.

Maybe put a transparent straw around that point, to keep the diaphragm from moving, and then lower the magnet/wire into the straw until it picks up the grain?

I would think some sort of non-magnetic cover going close-enough over the diaphragm, but solid enough to not buckle under the pull of a stronger magnet, would be useful..? The stronger external magnet would pull the crap off the diaphragm, but the "cover" would also make it easy to throw away the removed debris.

 

[12afael]

I have used tape to clean metal filings. I´m just too scared to put something magnetic near that will be attracted to the membrane and destroy it.

 

[MagnetoSound]

I would think some sort of non-magnetic cover going close-enough over the diaphragm, but solid enough to not buckle under the pull of a stronger magnet, would be useful..? The stronger external magnet would pull the crap off the diaphragm, but the "cover" would also make it easy to throw away the removed debris.

Perhaps some kind of plastic mesh like you see on a fly swatter?

 

[Khron]

Perhaps some kind of plastic mesh like you see on a fly swatter?

Not sure a mesh is necessary, magnetic fields go through solid plastic (too).

 

[MagnetoSound]

To protect the diaphragm from the screwdriver or whatever, but let the particles through

 

[Khron]

To protect the diaphragm from the screwdriver or whatever, but let the particles through

Why necessarily through?

 

[OneRoomStudio]

Along these lines, I had the thought to glue a magnet to the top of a plastic soda bottle cap, so that the "rim" of the cap created a spacer over the capsule. An ideal part would be the same diameter as the capsule so you could place it down on the "frame" of the capsule and have just enough space so that the inside of the cap doesn't touch the diaphragm. I agree with @Khron - no need to let the particles through to the magnet. They can just collect on the inside of the cap to be cleaned out after.

 

[MagnetoSound]

Why necessarily through?

Perhaps we are visualising different things. I’m thinking of placing a magnetised screwdriver (say), close enough to the diaphragm in order to attract loose magnetic particles to it but at risk of damaging the diaphragm, so placing something between them to prevent it from touching the diaphragm. If that thing was a) non-ferrous and b) had holes in to let the particles through, it would make a good protector, no?

EDIT: I just saw what you meant. The particles could just stick to the bottom of the plastic thing and be drawn away with it. I guess that works too.

 

[MagnetoSound]

Along these lines, I had the thought to glue a magnet to the top of a plastic soda bottle cap, so that the "rim" of the cap created a spacer over the capsule. An ideal part would be the same diameter as the capsule so you could place it down on the "frame" of the capsule and have just enough space so that the inside of the cap doesn't touch the diaphragm. I agree with @Khron - no need to let the particles through to the magnet. They can just collect on the inside of the cap to be cleaned out after.

Something like this maybe?

 

[MicMaven]

I would think some sort of non-magnetic cover going close-enough over the diaphragm, but solid enough to not buckle under the pull of a stronger magnet, would be useful..?

Yep. Of course, I am less worried about diaphragm, itself, as it may not be metallized or magnetic. I am more concerned a stronger external magnet might pull together with any magnetic material inside the cartridge, or with the cartridge case, and slam into the magnet, damaging the diaphragm in the process. Could it distort and damage the voice coil in some way?

I am also concerned about possible polarization issues between a strong external magnet and an internal magnet in the cartridge, you know, attracting versus repelling, and other issues spring to mind in addition to the relative strength of multiple interacting magnetic fields.

So, just to be super safe and transparent, I provide my Standard Disclaimer, below . . . . which means you are truly on your own, brother!

Happy trails to all - interesting topic for sure! James

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[MagnetoSound]

 

[Paul W]

Can you polarize the probe so that it repels the magnet in the capsule, rather than attracting it? I’m thinking of a steel wire attached to whichever end of a bar magnet is appropriate.

 

[Paul W]

I guess you could polarize the coil oppositely to the fixed magnet, with a little DC current, so that it pushes the diaphragm forward a bit and more or less cancels the field of the fixed magnet. That would reduce the tendency to pull the probe into the diaphragm, and give you a bit more slack in case you do push on the diaphragm a little bit.

 

[OneRoomStudio]

As long as you prevent the magnet you're using from being drawn into the capsule by using a spacer device (like the bottle cap below), then you shouldn't risk damaging anything. In fact, putting a strong magnet of the opposite polarity close to the mic magnet would theoretically make it stronger too (bonus).