Post by svart on Jan 27, 2022 9:08:13 GMT -6
So I bought a few broken MD421 for CHEAP. Some have broken switches, some have the notorious "thin" sound. I intended to use the thin sounding ones to repair the ones with broken switches and such, but I was intrigued by the idea that a mic somehow started sounding "thin" over time and progressively gets worse.
There should be no mechanics that would cause this in a mic. Some claim that the voice coil rubs against the magnet internally and wears it down causing the coil to fail, but I have yet to find ANY data that supports this conclusion despite it being repeated on many forums and websites. Some simply call it "coil rub". While I've seen speakers that have done this, there is usually a telltale way to see if the coil has been compromised, and that is to test the coil impedance. A rubbed coil will show drastically lower impedance as the coils short but will eventually open completely. The MD421 states that it has 200 ohm impedance. It also doesn't have a transformer, so measuring the mic impedance should yield the coil impedance directly. I have not found anyone who has determined a MD421 to be dead from "coil rub" to have stated the coil impedance of the dead mic.
Anyway, I did a little research and the overall opinion was that the capsule assembly was not repairable and needed to be replaced. Since the older MD421 has no replacement and the MD421-II is an entirely different animal inside, the only option is to figure out what the issue is or junk the mic. Since these bodies are still in good shape, they could also be sold to folks who've broken the clip mounting tabs on theirs.
I did find a website that claimed that they disassembled an older MD421 and found "iron dust" stuck to the diaphragm directly above the voice coil. The magnetic field was pulling it and holding it on the diaphragm and it was in turn pulling the diaphragm so that it bottomed out.
Sounds like a reasonable theory to me, but where would this magnetic dust come from? There's multiple layers of fine mesh between the outside world and the diaphragm that should keep this from happening. Even if there was "coil rub", the soft copper and aluminum coil would wear out before the magnet/steel body would ever create dust. It would also obviously be under the capsule, not on top of it.
So I did what any reasonable explorer would do. I took the worst of the "thin" sounding mics and took it apart in the name of science. I also tested the coil resistance, which was around 200 ohms as expected.
I didn't take any pictures of this mic because I was working quickly and just wanted to see what I would find. I have a couple other "thin" mics I can take apart and document if I can find the source of the problem.
Once down to the capsule assembly, the biggest issue is that the dust protection netting is generally attached to the basket that holds the humbucking coil, which itself is glued onto the edge of the main capsule assembly.
Luckily (or unluckily for the mic) the dust protection screen was more than half loose around the edge (entrance for dust?). The humbucking coil basket also seemed to be slightly loose on one side. I gently pried around until I found an area that would pull up slightly. Light pressure continued to lift the basket edge until I saw strings of elastic adhesive starting to pull away. I was able to cut those and the basket continued to easily pull away as I continued to cut the adhesive with a small knife blade. I unsoldered the humbucking coil from the posts and the capsule was exposed.
Now, the most obvious thing was that the capsule was covered in a lot of dust particles. Gently blowing on it did not dislodge the particles at all. I touched the diaphragm gently with my finger and it was sticky, as if the capsule was randomly coated in a sticky substance which had trapped a large amount of dust particles. This sticky area was heavier around the edges of the capsule, got thinner over the ridges and almost absent up to the voice coil. There was none over the middle dome. It did not seem evenly distributed either. I do not believe this was intentionally applied for any reason.
My theory is that the glue that was used for the diaphragm-to-base and humbucking basket to the diaphragm/base was some kind of organic adhesive, such as rubber glue. Over time it's possible that the solvent in it had outgassed and deposited a thin layer of adhesive on the diaphragm. This would make some sense as it would be heavier around the edge and lighter towards the center. I also posit that the dust particles added a lot of weight to the diaphragm and made it harder to move which resulted in a thinner sound. Some might say that a heavier diaphragm would lower the frequency of resonance, but if you take a speaker and start pushing on the edges of the cone, the sound becomes weaker and thinner. A dynamic mic is just a small speaker used backwards.
I saw particles over the voice coil area but a magnetized screw driver blade did not really pick many of them up. I did pull up a few, but no more than maybe 10% of what was there. Maybe other mics have other issues with metallic particles, but I do not believe this one did. It's also entirely possible that it only takes extremely small amounts of magnetic dust to affect the tone but I don't believe it to be a major contributor.
Anyway, I poured clean 99% alcohol and opened brand new cleaning swabs in an attempt to remove the sticky residue and dust particles. Alcohol may not be the best choice for this, but I currently don't know what material the diaphragm is made from. I'm assuming the diaphragm is mylar at this point. Naptha might be a better choice in the future but I'll need to see how this goes first.
I used quite a bit of alcohol at first to soak the sticky areas. I swabbed around gently until I noticed the sticky film starting to disappear. With it, the dust was coming up as well. It took a few rounds with a clean swab and more alcohol but all of the sticky film and dust were removed.
While I don't really have any kind of way to test this theory, it looks and feels like the diaphragm moves more freely than before. It could be wishful thinking on my part though. We'll see when I put it back together and test.
There should be no mechanics that would cause this in a mic. Some claim that the voice coil rubs against the magnet internally and wears it down causing the coil to fail, but I have yet to find ANY data that supports this conclusion despite it being repeated on many forums and websites. Some simply call it "coil rub". While I've seen speakers that have done this, there is usually a telltale way to see if the coil has been compromised, and that is to test the coil impedance. A rubbed coil will show drastically lower impedance as the coils short but will eventually open completely. The MD421 states that it has 200 ohm impedance. It also doesn't have a transformer, so measuring the mic impedance should yield the coil impedance directly. I have not found anyone who has determined a MD421 to be dead from "coil rub" to have stated the coil impedance of the dead mic.
Anyway, I did a little research and the overall opinion was that the capsule assembly was not repairable and needed to be replaced. Since the older MD421 has no replacement and the MD421-II is an entirely different animal inside, the only option is to figure out what the issue is or junk the mic. Since these bodies are still in good shape, they could also be sold to folks who've broken the clip mounting tabs on theirs.
I did find a website that claimed that they disassembled an older MD421 and found "iron dust" stuck to the diaphragm directly above the voice coil. The magnetic field was pulling it and holding it on the diaphragm and it was in turn pulling the diaphragm so that it bottomed out.
Sounds like a reasonable theory to me, but where would this magnetic dust come from? There's multiple layers of fine mesh between the outside world and the diaphragm that should keep this from happening. Even if there was "coil rub", the soft copper and aluminum coil would wear out before the magnet/steel body would ever create dust. It would also obviously be under the capsule, not on top of it.
So I did what any reasonable explorer would do. I took the worst of the "thin" sounding mics and took it apart in the name of science. I also tested the coil resistance, which was around 200 ohms as expected.
I didn't take any pictures of this mic because I was working quickly and just wanted to see what I would find. I have a couple other "thin" mics I can take apart and document if I can find the source of the problem.
Once down to the capsule assembly, the biggest issue is that the dust protection netting is generally attached to the basket that holds the humbucking coil, which itself is glued onto the edge of the main capsule assembly.
Luckily (or unluckily for the mic) the dust protection screen was more than half loose around the edge (entrance for dust?). The humbucking coil basket also seemed to be slightly loose on one side. I gently pried around until I found an area that would pull up slightly. Light pressure continued to lift the basket edge until I saw strings of elastic adhesive starting to pull away. I was able to cut those and the basket continued to easily pull away as I continued to cut the adhesive with a small knife blade. I unsoldered the humbucking coil from the posts and the capsule was exposed.
Now, the most obvious thing was that the capsule was covered in a lot of dust particles. Gently blowing on it did not dislodge the particles at all. I touched the diaphragm gently with my finger and it was sticky, as if the capsule was randomly coated in a sticky substance which had trapped a large amount of dust particles. This sticky area was heavier around the edges of the capsule, got thinner over the ridges and almost absent up to the voice coil. There was none over the middle dome. It did not seem evenly distributed either. I do not believe this was intentionally applied for any reason.
My theory is that the glue that was used for the diaphragm-to-base and humbucking basket to the diaphragm/base was some kind of organic adhesive, such as rubber glue. Over time it's possible that the solvent in it had outgassed and deposited a thin layer of adhesive on the diaphragm. This would make some sense as it would be heavier around the edge and lighter towards the center. I also posit that the dust particles added a lot of weight to the diaphragm and made it harder to move which resulted in a thinner sound. Some might say that a heavier diaphragm would lower the frequency of resonance, but if you take a speaker and start pushing on the edges of the cone, the sound becomes weaker and thinner. A dynamic mic is just a small speaker used backwards.
I saw particles over the voice coil area but a magnetized screw driver blade did not really pick many of them up. I did pull up a few, but no more than maybe 10% of what was there. Maybe other mics have other issues with metallic particles, but I do not believe this one did. It's also entirely possible that it only takes extremely small amounts of magnetic dust to affect the tone but I don't believe it to be a major contributor.
Anyway, I poured clean 99% alcohol and opened brand new cleaning swabs in an attempt to remove the sticky residue and dust particles. Alcohol may not be the best choice for this, but I currently don't know what material the diaphragm is made from. I'm assuming the diaphragm is mylar at this point. Naptha might be a better choice in the future but I'll need to see how this goes first.
I used quite a bit of alcohol at first to soak the sticky areas. I swabbed around gently until I noticed the sticky film starting to disappear. With it, the dust was coming up as well. It took a few rounds with a clean swab and more alcohol but all of the sticky film and dust were removed.
While I don't really have any kind of way to test this theory, it looks and feels like the diaphragm moves more freely than before. It could be wishful thinking on my part though. We'll see when I put it back together and test.
