New diaphragms and voice coils for vintage German mics?

[Whoops]

Would you mind sharing his details over here on this thread, for future reference/someone wants to get theirs fixed by him ?

Im not sure if his contact details should be shared here.
But anyone can send me a PM.
Send me a PM

Anyway anyone can google search the name, also he is a member here, his username is “panman”

 

[christopantz]

Back with an update:

Decided to hang this project up and take a break for a while (potentially indefinitely), as I’m preparing to move cities, but wanted to do a write up of findings and challenges in case anyone has ideas/wants to continue this work, or if I pick it up in the future.

I did 3 revisions to the 3D printed mold before getting something usable. I’ll attach the .step files for the positive and negative molds if you'd like to get them made for your own projects. I had it 3D printed through CraftCloud, using high temperature resin, and MSLA printing. The company that printed it is Advanced Additive Innovations, and the print quality was great. Very detailed, good surface finish, and enough resolution for the .6mm vent holes in the bottom part of the mold.

As for molding, between the .5mil and 1mil Mylar, I found the 1mil to be best. I had some trouble heating it consistently, because it seemed like the threshold between not pliable enough and decomposing was very very fine. I did get a few impressions that were good though. I used the negative mold to help press the concave features into the positive mold, as well as a cotton swab to coax the Mylar into the pull of the vacuum. I then clamped the positive and negative molds together with the stretched and molded Mylar between the molds, to get a lot of clamping pressure and make sure the impression was nice and sharp.

As for winding the coil, I continued using 40 gauge enameled copper magnet wire and a 3/4 inch chrome core, but experimented with ways to easily release the coil from the core. I ultimately settled on using marine grease on the core (only because it’s what I already had laying around), then wrapping it in a layer of the .5mil Mylar, and winding the coil over that. I then wound the coil in 2 layers, applying a fairly liberal amount of super glue to the coil after the first layer, then doing the second layer and wiping off the excess glue. I was then able to carefully pull the coil off the core.

As for attaching the diaphragm to the metal plate that it sits on, I think contact cement is probably the best option, although I wasn’t able to get exact and thin enough coverage to the plate or the diaphragm. I did try masking the plate with tape, which worked to an extent, but I really think I should have used a spray adhesive for even coverage. As for the diaphragm, I didn’t think ahead as to how I would mask the parts of the Mylar that should not have adhesive, so I ended up carefully using a little plastic applicator. Not exact work, but I made it work.

As for attaching the coil to the Mylar diaphragm, this part is where I really struggled. I think contact cement is the right adhesive for this, but again, the stuff I was using was probably much too viscous for this application. I also found that my coil seemed just a hair too big at its outer diameter, and would seize up against the edges of the diaphragm plate where it is meant to move freely. There’s really just a maybe half millimeter air gap between the diaphragm plate and the permanent magnet where the coil is meant to move, so any adhesive residue or sloppy placement ruins it.

If I were to pick this back up again, I would probably try to get the molds machined out of aluminum, likely at JLC in China since their prices are cheaper than anything here in the states. I’m also thinking the 1 mil Mylar might actually be too thin, because the Mylar itself stretches when it is formed, and it loses some structural integrity, especially around the dome, which seems to kind of collapse with any pressure instead of holding shape. Maybe 1.5 mil Mylar would do the trick? I’m also wondering if the dome shouldn’t be spherical, but maybe instead elliptical, as maybe that would have some more structural integrity? No science or evidence to back this up, just a slight gut feeling.

I would also want to get a custom coil winding core that is the exact right size for the coil. It initially seemed like the 3/4 inch / 19mm outer diameter was perfect, but with 2 layers of 40 gauge magnet wire, plus whatever superglue residue is left over, plus the lubricant and .5 mil Mylar I used to release the coil from the core, I think the core diameter needs to be ever so slightly smaller. I also believe the height of the coil needs to be right around 2.5mm so as to not bottom out against the permanent magnet,, but I could be wrong. I would want to design the core with this height in mind, so that I could easily wind 2 layers without worrying about measuring each time.

I would also want to design tools for the exact masking of the diaphragm and plate to apply adhesive, a tool to center the diaphragm perfectly on the plate, and a tool to center the coil perfectly on the diaphragm.

If anyone finds this information helpful and makes any further discoveries using what I’ve supplied here, all I ask is that you share those discoveries freely. I feel some level of defeat over not figuring this out, but from the start I figured this was the most likely outcome anyway. Sharing the knowledge and findings freely is how I know the time isn’t wasted, so this is me trying to do my part in keeping this information free.

--

P.S. I still have a nonworking RE-20, and if anyone has any suggestions as to people who can actually do this type of repair on an RE-20, I’m all ears. Otherwise, I’m considering trying to source and use a headphone driver like someone mentioned here earlier.

 

[MicMaven]

If anyone finds this information helpful and

I find it all extremely interesting and enlightening. You should be elated and enlightened, not defeated or disappointed. Life is a journey, you know. You have simply not reached the destination on this project.

Putting it aside awhile will let your subconscious mind solve the problem over time. I used to keep a notebook on the nightstand to write down good ideas after sleeping on a problem for a night or three. That way I would not forget the solution by morning!

Great thread with lots of good input all around. Let us know when you wake up at 4 AM with an epiphany that works. James

 

[Paul W]

Another process to consider is pressure forming. Instead of sucking the air out one side, or in addition to it, push air in from the other side, and considerably harder.

Vacuum forming is much more common as a DIY thing, partly because if you mess up things tend to implode with less than 1 atmosphere of pressure; if you're using several atmospheres of pressure, and it comes apart, it explodes with considerable force.

For something this size, though, it shouldn't be hard to make a little pressure-forming box that can withstand several atmospheres of pressure and not explode.

For example, you could use regular plumbing fittings.

You could even use water pressure from the municipal water supply as the pressure source. In the US, municipal water pressure is usually 3 or 4 atmospheres above ambient air pressure, and of course plumbing fittings are supposed to withstand that comfortably.

 

[christopantz]

I find it all extremely interesting and enlightening. You should be elated and enlightened, not defeated or disappointed. Life is a journey, you know. You have simply not reached the destination on this project.

Putting it aside awhile will let your subconscious mind solve the problem over time. I used to keep a notebook on the nightstand to write down good ideas after sleeping on a problem for a night or three. That way I would not forget the solution by morning!

Great thread with lots of good input all around. Let us know when you wake up at 4 AM with an epiphany that works. James

Thanks so much James, what a kind thing to say. Made my day.