3D print files - U87i capsule carrier plastic parts (early 70s version)

[VanguardAudioLabs]

If any of you follow me on IG, you've seen this project starting to take shape. I got a shattered vintage U87i from Cesar Mejia (Herbie Hancock's engineer). It had been a school mic at the school he teaches at, and…well, audio students ¯\_(ツ)_/¯ The capsule carrier head's plastic bits were beyond salvage. Even if I'd been able to plastic-weld them back together, there were too many missing pieces. I resolved to just measure, draft, and 3D print new plastic for it.

Hopefully you never need these files…but if you do, here they are for posterity.

NOTES:

• I used PLA, 100% infill, and it's quite strong. Supports are a must, so make sure your support settings are dialed in well. Alternatively, you could use a site like Xometry and sinter them from Nylon.
• PLA may not be the best choice here for the Hi-Z area of the circuit. I may yet just send it to Xometry and have them do SLS Nylon, which I believe has a higher insulation resistance than PLA (along with better tensile strength). I'm not 100% sure what material Neumann was using, I imagine Klaus could tell us.
• The capsule pin holes I put at 2.4mm, even though original spec is a 1.5mm hole molded around the old pins. A few reasons: I'm going to use the original pins and JB-weld them in place. Also, my 8-year-old old FDM 3D printer kind of sucks, and I intentionally oversized the holes because my printer tends to print through holes about 10% small. The pin retention flares measured 2.2mm on my calipers for this mic.
  Alternatives include shaping some brass socket head cap M1.6 or M2 screws, or brass pin nails, but that would require a different hole size. I've included the STEP files as well so you can modify them.
• I'm doing something similar for the riveted-in switches. I had to drill out the old rivets. Instead of tube riveting and cracking the old plastic on the switches, I'm going to use the old rivets and JB weld those in as well. I also thought about tapping in new plastic-threaded screws, but I'm worried I'd break the old plastic on the switches, which already is showing some fatigue.
• This is for the opaque plastic of the early 70s mics. The clear plastic ones are a bit different and I suspect one day I'll need to model those.
• I made the capsule rubber slot slightly thicker. Neumann molded in a piece of what appears to be aluminum at first glance for this part. I made it a bit thicker to compensate for the strength difference, but left the set screw slot to hold the capsule in place. I'll probably tap in an extra set screw or two to hold the capsule rubber in place using self-tapping plastic screws, M2 x 3mm.

Since I can't attach STEP or STL files, I've hosted them on our site and you can grab them here: https://www.vanguardaudiolabs.com/free-mic-repair-files-u87i-capsule-carrier-replacement-plastic/

 

[VanguardAudioLabs]

The final prints. Fairly happy with them, although i did have remove what i assume are little wire-holding nubs on the top part. They did not adhere very well and were snapping off. (NOTE: the red color was a customer request!)

 

[VanguardAudioLabs]

Hoping for a bit of advice from people smarter than me (of which there are plenty!)

Given that I printed these from PLA, I'm considering adding a light coat of conformal coating. PLA, even after printing, can pull a bit of moisture from the air, and this is a Hi-Z area of the circuit. Would proably also add the conformal coating once i put the components on over the standoffs/top of pins. This would theoretically hermetically seal the PLA from hygroscopic behavior as well as raise the resistivity of printed PLA which is between 1 x 10^8 and 1 x 10^12Ω, somewhat similar to standard FR4.

I'm looking at different aerosol conformal coatings and Acrylic seems to have the highest resistivity of the ones I'm looking at - about 450 TeraOhms (4.5 x 10^14). That said, all the conformal coatings are running at least 1 Teraohm which seems like plenty.

That said, Teflon (the definitive hi-resistivity insulator) is measured in PetaΩ (10^18). We use Teflon standoffs with copper rivets in Vanguard mics for that reason (and Teflon-insulated wire from the capsules), as opposed to soldering directly to the FR4 which runs around 10GΩ surface resistivity (10^10).

Any particular advice on conformal coating brands and/or types and materials to coat over the PLA?

 

[OneRoomStudio]

Could you just have the parts printed from another material by another company? Ensinger TECAPEEK (a specific type of PEEK material) has a surface resistivity of 10^15 Ω according to the datasheet, and seems to be fairly widely available. I'm sure there are other options too.

 

[VanguardAudioLabs]

Could you just have the parts printed from another material by another company? Ensinger TECAPEEK (a specific type of PEEK material) has a surface resistivity of 10^15 Ω according to the datasheet, and seems to be fairly widely available. I'm sure there are other options too.

I thought about using SLS Nylon-12 from Xometry which has a resistivity similar to the TECAPEEK. Not sure how the red dye would affect resistivity.

I've already got them printed and they printed pretty nicely, especially on my cheap Wanhao i3 that's like 8 years old (one day I'll replace it with a Prusa Mini!). I'm not hyper-commited to using those prints, but I do really enjoy going the full DIY route.

I am curious what the original Neumann material was for these early 87 capsule carriers - polycarbonate, ABS, or something else. I'm not a materials science guy by any means so I'm just guessing.

 

[OneRoomStudio]

Considering how brittle it became over time, my guess would be ABS. Plus - I believe ABS was more widely used in manufacturing in the '70's. Just a guess though...

 

[VanguardAudioLabs]

Considering how brittle it became over time, my guess would be ABS. Plus - I believe ABS was more widely used in manufacturing in the '70's. Just a guess though...

i think this one was less "brittle" and more "abused by audio students" unfortunately! Even the intact pieces I have are fairly tough to break.