[bockaudio]

The point of using Mylar, over other plastic films, is it's hygroscopic.

[Tim Campbell]

David not only does PET film not absorb water, it is very mechanically stable with temperature and unaffected by most solvents.
It is aso readily available in desired thicknesses, cheap and easy to metalise.

[GeorgeToledo]

I ran across an interesting white paper awhile back that had tested the effects of water and eventual hydrolysis on mylar, and the tables had basically outlined how it loses tension.

Searching for a few minutes isn’t turning it up, but I will post it back here if I run across it. But, iirc, the general theme is that Mylar does absorb moisture, and if there is too much it becomes brittle and loses tension. I may have misunderstood.

[Gus]

Good links
http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/Mylar_Physical_Properties.pdf

https://europe.dupontteijinfilms.com/media/2431/info-mylar-a-electrical-properties.pdf

A search for "PET humidity ageing" or "Mylar humidity ageing" will find more links

[GeorgeToledo]

Hostaphan brand is sometimes used as well, not able to sit down and look through their specs at the moment.

Anyway, I suppose this is possibly getting orthogonal to the original point of how the tension of a k67 is possibly being tweaked after the fact. Moisture is probably fairly unlikely to come into play.

Maybe it is more about some kind of burn in happening and it being deemed to be better after the fact.

[GeorgeToledo]

The original test and info in this thread is very interesting, just wanted to say thanks for the post.

[Tim Campbell]

Well no one should expect a microphone capsule to work for an indefinite period of time. The fact that many of them work flawlessly for decades is to their credit but they are manufactured within practical restrictions to serve their purpose reasonably. Very few capsules except for measurement purposes are manufactured to an ideal.
That's easy to see just from their varied frequency response.
Let's celebrate their flaws and fragility.

[TillM]

Hi Tim,
I have in mind, that you also made a K67 capsule.
Is this still current ?

[GeorgeToledo]

kingkorg,

It comes to mind that Stephen Paul used to complain about the machining of the Neumann backplates after they moved to more automated production.

He says that he would ream out the holes in the backplate more. Scratches all over the backplate are also visible on his modded capsules.

I thought it was interesting, in that he was essentially going the opposite direction from the mod experiment in the thread starter post. As far as I understand it.

Some of his old posts about this may still be searchable in google groups, rec pro audio mailing list. I remember reading some there a year or so ago.

[kingkorg]

kingkorg,

Scratches all over the backplate are also visible on his modded capsules.

This is kinda known "mod" to prevent membrane from sticking to the backplate. Could be he also raised the voltage, so this might have been the cure.

I think it was a contemporary trend. Most 90's and 00's mic mods were about making mics brighter.  Adding more "detail". Most u87 mods were about removing hf attenuation.

And then trends changed, probably due to digital recording, everybody wanting flatter, "warmer" mics.

[GeorgeToledo]

For sure, the scratching is pretty well known, very visible in pictures.

I think he wrote that he would look at the holes in the backplate under a microscope, and after Neumann moved to CNC, that he thought the holes appeared rougher than previously, or sometimes with burrs around the edge (?), not adequately done according to him. So for one of his mods he wrote that he would touch up every hole...I can’t remember if he wrote he would re-drill, or what. But each hole is going to get somewhat bigger in that case.

Just came to mind given the idea of *filling* the holes more. I’m not mentioning it to ascribe any particular level of validity (or invalidity) to any of his approaches.

[bockaudio]

I should have mentioned isotropic as well.

[bockaudio]

Arf!

Could not this be it's own topic? (the lists of parallel electronic devices that miraculously do not exhibit time delay between them)

[bockaudio]

Well no one should expect a microphone capsule to work for an indefinite period of time.

Priceless.

[bockaudio]

kingkorg,

It comes to mind that Stephen Paul used to complain about the machining of the Neumann backplates after they moved to more automated production.

He says that he would ream out the holes in the backplate more. Scratches all over the backplate are also visible on his modded capsules.

I thought it was interesting, in that he was essentially going the opposite direction from the mod experiment in the thread starter post. As far as I understand it.

Some of his old posts about this may still be searchable in google groups, rec pro audio mailing list. I remember reading some there a year or so ago.

Remember he destroyed all those Neumann made capsules in order to use thinner membranes. Never made his own backplates.
If you re-membrane one of his destroyed backplates with the old, original thickness you don't get the original response. The worst of both worlds.

[bockaudio]

Maybe it is more about some kind of burn in happening and it being deemed to be better after the fact.

In terms of vintage mics? Of course, the plastic, and all the plastic parts ARE different and aged. Does burn in matter on a new mic is the harder to answer question, and then, is the capsule separate from the electronics in that regard?

[GeorgeToledo]

Remember he destroyed all those Neumann made capsules in order to use thinner membranes. Never made his own backplates.
If you re-membrane one of his destroyed backplates with the old, original thickness you don't get the original response. The worst of both worlds.

Yeah, absolutely.

I always found it a little bizarre that he was listed by Neumann under their authorized repair, at least for a good long while (I don’t know if that was the case until the later days or what). I guess he supposedly did do regular repair/non-mod.

[bockaudio]

This is kinda known "mod" to prevent membrane from sticking to the backplate. Could be he also raised the voltage, so this might have been the cure.

Their lapping process. K67s had plastic stops in the backplate to prevent membrane stick.

I think it was a contemporary trend. Most 90's and 00's mic mods were about making mics brighter.  Adding more "detail". Most u87 mods were about removing hf attenuation.

Add the 67 to that, and this was more of an '80's thing. Because tape and hundreds or thousands of passes, HF deterioration every time. Also studio "mood enhancers" excited people but did no good for hearing.

And then trends changed, probably due to digital recording, everybody wanting flatter, "warmer" mics.

True.

[bockaudio]

I guess he supposedly did do regular repair/non-mod.

I have come across a few which they stuck to the script and just repaired the mic. Mostly I think the upsell was the deal.
Regarding the detail of their U67 mod, besides the capsule, they reversed the plate NFB to ADD, not shunt, HF.

[kingkorg]

Totally off topic, but since you are here. I had a Bock iFet for repair not so long ago. A dead switch, easily sorted out.

I just have to mention and praise you for wonderful work on that thing, start to finish. Wonderful idea, wonderfully accomplished, manufactured like a dream, phenomenal capsule (although i would love to know the origin i'm not going to ask). Measures and sounds killer. All one could wish from a mic. And that hefty transformer! A true thing of beauty. These days one of rare mics that truly have appropriate price/quality ratio.

P.S i am no kiss ass, if anything i have a tendency to find flaws in almost everything and point them out. Especially high end stuff. I can't say anything bad about that mic!

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