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.