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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.
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Post by svart on Jan 27, 2022 18:57:38 GMT -6
A quick update.
So I reassembled the mic and gave it a test.
It sounds good to me. In fact, it sounds better than the MD421 I've been using for years.
So I took a few minutes to test all the mics. I fixed one mic that had a broken switch using a switch from a dead MD421 body that I've had for years.
Out of 7 MD421-U-5 mics, 5 were clearly thin sounding. One was extremely thin and tinny sounding. The rest were various degrees of "not right".
One MD421-II sounds ultra thin. I know the guts are different than the MD421-U-5 so I'll leave that til later to troubleshoot.
The other 2 MD421-U-5 sounded "normal" ish. One has a broken grill so that's probably why it was sold with the lot. The last one sounds fairly decent but I think if the other mics are fixed with diaphragm cleanups, I'll go ahead and give it a cleaning as well.
So for the next mic, I picked the worst of the remaining MD421-U-5 mics. I disassembled it and checked the switch, which was fine. I then opened up the capsule to find an extreme amount of dust on the humbucking coil assembly and a partially unfixed dust protector. Removing the humbucking coil was as easy as the first mic, it essentially just pried up without much force. I found that the capsule was COATED in heavy dust, possibly worse than the first mic and the sticky residue is present just like the first mic.
Tomorrow I will clean the capsule under the microscope and take more pictures to post.
Right now I think the phenomenon of low-end loss in the older MD421-u-5 is purely a case of dust getting into the capsule assembly and sticking to the diaphragm due to some kind of sticky residue residing on it. Over time, the dust builds up to the point where the weight of the diaphragm is too heavy to properly move effectively dampening out the low frequency response.
I'm trying to identify a glue other than cyanoacrylate to use to reassemble the mic capsule assemblies. CA glue can outgas over time and I don't think I should use it around the diaphragm. I'm going to test a few glues to see which will work best.
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Post by EmRR on Jan 27, 2022 19:05:21 GMT -6
Great info
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Post by Blackdawg on Jan 27, 2022 19:05:44 GMT -6
Very cool to hear this. As you go you should do a Before and After recording!
The one you cleaned sounded good then?
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Post by svart on Jan 27, 2022 19:29:53 GMT -6
Very cool to hear this. As you go you should do a Before and After recording! The one you cleaned sounded good then? It did not lack bottom end anymore. It sounded pretty normal to me. I only tried it on my voice, but the rest all sounded thinner in comparison. I think I'll clean and repair the next one then if it seems to fix it, I can then say that I believe this is truly the problem. The third mic I will do before/after on something low frequency, maybe a floor tom. |
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Post by wiz on Jan 28, 2022 2:42:42 GMT -6
I have one of these laying around... if you could take pictures would be great.. if not that's cool
Cheers
Wiz
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Post by svart on Jan 28, 2022 8:41:01 GMT -6
Ok, another update. I took the mic motor into work today so I could look through the microscope and get some pictures. This capsule was far dirtier than the last one! It was easier to clean though. I did find that this one had the metallic dust around the top of the voice coil like the other person said. Perhaps it IS part of the problem on some mics. I guess I'll see as I go forth cleaning more of these. I also took apart the MD421-II just to see what was up. It's such a different mic inside. All plastic. There is some metal, but I swear it serves no purpose but to make it heavier. The mic motor and capsule are radically different and very lightweight in comparison. It probably has a very high gauss neo magnet or something to be so light yet only be 200 ohm impedance. Either way, I peeled back the dust protection and the capsule was pristine. This mic might have a completely different problem. I attempted to get into the switch assembly to see what was going on, but it's clear that it was never meant to be repaired. It was assembled and then soldered from the outside so disassembly would be hard to do. Back to the MD421-U-5 at hand. I also don't intend for this to be a HOW-TO. If folks are willing to try this, it's on their own. Getting the humbucking coil off the capsule is precarious at best and could be easily broken, rendering it un-repairable. Cleaning the capsule is also somewhat dangerous. If the debris punctures the membrane or if you wipe too hard you might tear it. If the membrane gets kinked or crinkled it's also un-repairable. First pic is of the mic motor as I removed it from the basket:  The second is the capsule as it was when I removed the humbucking coil basket:  The dust and debris, but if you look close you can see some dark dust around the top of the voice coil. It's magnetic debris.  I swept up the magnetic dust and debris with a q-tip into a small pile to show how much there was:  the q-tips after cleaning. Some of the debris I blew off with canned air once it was wet enough to come loose.  Cleaned capsule:  |
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Post by EmRR on Jan 28, 2022 13:03:54 GMT -6
man that's a lot of heavy dust
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Post by svart on Jan 28, 2022 20:06:06 GMT -6
So the second mic was much improved. It's not quite as bassy as the first, but it's no longer small and tinny sounding.
I'm not sure what to make of it. I know that a lot of mics have a bit of range even if comes to tone, so I'm not sure if there's something still wrong, or of it's just a tolerance thing.
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Post by jhamm80 on Jan 28, 2022 22:22:54 GMT -6
So the second mic was much improved. It's not quite as bassy as the first, but it's no longer small and tinny sounding. I'm not sure what to make of it. I know that a lot of mics have a bit of range even if comes to tone, so I'm not sure if there's something still wrong, or of it's just a tolerance thing. Could the sticky substance be residue from foam degrading? I've never owned a 421 so I have no idea if foam is used in the basket, but foam gets sticky and gross as it degrades and turns to dust. I stopped keeping a few of my dynamic in an old foam case when I noticed it leaving residue on them. This is a neat project and a great way to score nice mics. |
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anders
Junior Member
Posts: 94
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Post by anders on Jan 29, 2022 15:32:08 GMT -6
Super interesting! I've killed two vintage MD421s trying to save them (well one of them was already dead when it arrived at the ER room – the humbucker coil had had too many 5B sticks on the nose through the years). The third one I managed to give a new lease of life with surplus bits from one of the others. Theory: If they're old enough to have been around when people smoked everywhere, nicotine deposits can sometimes be a little sticky.
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Post by soundintheround on Jan 30, 2022 23:53:37 GMT -6
Rack equipment, synthesizers, even old mixers I'm OK with fixing....but man I hate working on Mics.
I feel like a Swiss watch maker every time I gotta open one up and try to work on it....usually screw something up.
But I do have a thin-sounding 421 (actually a friend's on somewhat permanent loan), so might have to give this a go. Good info, thanks!
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Post by svart on Jan 31, 2022 9:18:25 GMT -6
So the second mic was much improved. It's not quite as bassy as the first, but it's no longer small and tinny sounding. I'm not sure what to make of it. I know that a lot of mics have a bit of range even if comes to tone, so I'm not sure if there's something still wrong, or of it's just a tolerance thing. Could the sticky substance be residue from foam degrading? I've never owned a 421 so I have no idea if foam is used in the basket, but foam gets sticky and gross as it degrades and turns to dust. I stopped keeping a few of my dynamic in an old foam case when I noticed it leaving residue on them. This is a neat project and a great way to score nice mics. There's no foam in the capsule assembly or the grill. It just seems like it's excess glue that's crept up from where they joined the diaphragm to the base. What's strange is that there is a wild difference in how much is on the diaphragm from mic to mic. |
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Post by svart on Jan 31, 2022 9:19:36 GMT -6
Super interesting! I've killed two vintage MD421s trying to save them (well one of them was already dead when it arrived at the ER room – the humbucker coil had had too many 5B sticks on the nose through the years). The third one I managed to give a new lease of life with surplus bits from one of the others. Theory: If they're old enough to have been around when people smoked everywhere, nicotine deposits can sometimes be a little sticky. this is a clear sticky material. It's very rubber glue-like. It seems to be the same stuff that they used to glue the diaphragm to the base. |
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Post by svart on Jan 31, 2022 9:24:02 GMT -6
Rack equipment, synthesizers, even old mixers I'm OK with fixing....but man I hate working on Mics. I feel like a Swiss watch maker every time I gotta open one up and try to work on it....usually screw something up. But I do have a thin-sounding 421 (actually a friend's on somewhat permanent loan), so might have to give this a go. Good info, thanks! So far it's a working theory. I have one that was much improved by cleaning, but it's still not as bassy as the first mic I cleaned. The mic motors seem to be different as in different manufacturing methods. One mic has a copper coating on all the internals, while another seems to have a nickel/tin coating and the markings on the assembly are different. They're all MD421-U-5 mics but seem to be from different eras. Also, I've cleaned 3 more capsules, all of which had more of the magnetic material on them. I haven't tested them though, but I'd say that make very sure that you get that magnetic stuff off the capsule as well. It's very hard to see without a magnifier of some type and I almost missed it on one mic because it was in a very thin layer spread out around the whole diaphragm and not clumped up like the other mic. |
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Post by svart on Jan 31, 2022 9:46:22 GMT -6
I had a previous MD421 that had a busted capsule that I used the switch assembly to fix another mic. This left me with a dead capsule and a good body that I'll use in the future. So what would somebody like me do with something like this? Disassembly, that's what! Assembly without diaphragm. Just plastic behind the diaphragm.  The bottom of the assembly screws on/off. I see that the plastic is glued to the bottom of the actual threaded metal ring. I think they assemble the metal parts first and then glue the plastic base on after they attach the plastic hose which vents the area just under the diaphragm.  The center pole piece just pops out. It's not glued or attached in any other way. The magnetic flux holds it pretty firmly, but the screw-on base locks it in place it seems. There is also a plastic ring that goes over the center pole which looks to hold it at a certain depth based on a brass threaded nut on the pole piece.  Closeup of the center pole piece and the depth-adjusting nut. It's pretty clear that these were adjustable, probably tuned at the factory for response I think.  If you ever wondered why there was a plastic "plug" on the bottom of the MD421 capsule assembly, I believe it was for adjustment of the pole-piece depth. It looks to give access to the threaded nut/ring. I think they adjusted the depth for some kind of response and then tightened the base to lock it down. I have a capsule assembly with a crinkled diaphragm (was like that when I opened it..) that I might experiment with by adjusting the pole piece a little to see what happens. I wonder if I can adjust for more bass response or something? |
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Post by svart on Jan 31, 2022 21:14:32 GMT -6
Ok, so I recleaned the one mic that was improved but not completely fixed. I focused on making sure there was no magnetic dust around the diaphragm.
It works great now. I must have missed some dust or something.
I did another 3 capsules as well.
Two of them were totally fixed and sound great. The 3rd did improve a little but not as much. It also has a ripple in the diaphragm that was there when I opened it. There doesn't look to be a way to fix it, unfortunately. I don't know how the ripple would have gotten there and the diaphragm ring looks flat and true.
I'll do the rest of the capsules tomorrow and see where we end up.
So now I do think that the magnetic dust plays a fairly large role in the tone.
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Post by wiz on Jan 31, 2022 23:22:37 GMT -6
Thanks so much for documenting this
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Post by svart on Feb 1, 2022 9:05:22 GMT -6
I cleaned the last 3 mic capsules this morning. I focused on cleaning in two stages to test whether or not cleaning with a q-tip only is good enough. First, I cleaned as I have been doing, using the swabs to get the glue residue off the capsules as well as attempting to get the swabs down into the groove where the coil meets the diaphragm. Second, I then went back with a medium bristle brush and swept the groove to see if anything was left. There was still some magnetic dust on all 3 capsules that the q-tip swabs didn't get but was completely hidden from view even using the lab binocular microscope. Once swept up it's easy to see, but still hard to remove. Canned air won't dislodge it. I had to use a magnetized screwdriver tip to pick it up while being very careful not to let the magnetic flux pull the screwdriver into the diaphragm which could gouge it. Clearly the swab-only cleaning wasn't enough to fully clean the mic back to a factory-fresh tone if there is magnetic dust present. So while I initially dismissed the magnetic dust theory I had found on the internet, I now admit that I was wrong to assume this and now realize it's definitely part of the tone problem, at least on some mics. Cleaning the diaphragm of the gooey residue does a lot for the missing bass, but the last mile is definitely the magnetic dust. In all of the mics, the silky dust protector that's supposed to be attached to the humbucking coil basket had come loose around the edge. The mics with the worst dust and debris on the capsule predictably had the worst case of the dust protector coming loose and vice versa. So in summary, the dust protection silk coming loose allows dust and debris to get into the capsule and stick to the diaphragm as the diaphragm has some kind of sticky residue on it. I believe that the magnetic dust might be from the outer protection grill, which seems to be plain steel. The inner grill/mesh is stainless and is not magnetic. Bonus pic of the dirtiest MD421 capsule ever:  |
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dmp
Full Member
https://www.probstelectronics.net/
Posts: 18 
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Post by dmp on Feb 1, 2022 14:19:29 GMT -6
Great thread. Many vintage dynamic mics could use a diaphragm cleaning.
"So now I do think that the magnetic dust plays a fairly large role in the tone."
yes++
What kind of brush did you use to get the metal grit out of the VC groove?
I've used qtips like you, and also fuzzy wire craft project thingies (what are those called?), which work better I think. You can kind of drag it around the diaphragm.
I tried putting a metal screwdriver close once and it was a near disaster with the magnetic pull. Wouldn't recommend.
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Post by svart on Feb 1, 2022 14:24:42 GMT -6
Great thread. Many vintage dynamic mics could use a diaphragm cleaning. "So now I do think that the magnetic dust plays a fairly large role in the tone." yes++ What kind of brush did you use to get the metal grit out of the VC groove? I've used qtips like you, and also fuzzy wire craft project thingies (what are those called?), which work better I think. You can kind of drag it around the diaphragm. I tried putting a metal screwdriver close once and it was a near disaster with the magnetic pull. Wouldn't recommend. I use a trimmed down "acid brush" which is a cheap horsehair brush with a metal handle, but I think a plain toothbrush might work as well if the bristles are thin enough. |
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Post by svart on Feb 1, 2022 19:41:43 GMT -6
So of the last 3, 2 sounded great. One didn't work at all and had no continuity, so I guess I damaged it while cleaning or installing it.
So I was a little peeved at this and decided to try my luck at the md421-ii again.
I bypassed the switch and found it still sounded thin.
I then looked at the capsule again. After unsoldering the humbucking coil, I found what looked like clips, so I pried them and the humbucking coil basket started to come loose.
After getting this off, the diaphragm looked very clean, but I could see dark specks around the crease over the voice coil.
Yep, more magnetic dust!
I have no idea how it got in there, but I used a toothbrush, which wasn't as good as I had thought it might be but it worked, and swept up the dust and used a magnetized screwdriver to draw it away.
This made the mic work great.
Now I have fixed both the md421-u-5 and the md421-ii mics.
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Post by Blackdawg on Feb 2, 2022 0:09:28 GMT -6
awesome work man.
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Post by svart on Feb 2, 2022 9:35:17 GMT -6
For my next trick I took one of the dead capsules and attempted removing the diaphragm.
It came up surprisingly easy, almost as if the glue had not bonded well even though it was still quite sticky/gooey on the top side. I was surprised that the wires were still intact right up to the edge of the outer ring. I could see where the one had broken right on the edge. It looked as if the wire had curled over the edge right where the solder post was located and perhaps when pushing on the post slightly to remove the humbucking coil this squished the wire.
These wires are TINY. I took a 30ga stranded wire and stripped it to get the strands out to try to solder and lengthen the coil wires but even those strands are still larger. Comparatively, a human hair is about 2x the thickness!
While this seemed very doable, the biggest problem I've faced is that even at the lowest setting, my soldering iron and solder just burn the thin coil wire right away. I'm not sure how I'm going to attach anything to it now, but if I can, I think I can fix the mic capsule.
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Post by EmRR on Feb 2, 2022 10:14:32 GMT -6
Years ago I had some junk paging mics with broken coil leads, and managed to fix all of them. Don't know why I went there, but it was possible!
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