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[Derek Samuel Reese]

I've come across what looks like a good quality U67 capsule from 1974.
It has never been re-skinned. There is a small amount of gold missing but the seller says it sounds fantastic when he compared it to his other vintage U67s.

I know from Klaus, saying looks aren't everything when it comes to a capsule, hearing is believing !!
Any thought on the pic's ?

[Derek Samuel Reese]

other side

[Derek Samuel Reese]

closer shot,
the rust I am seeing makes me concerned ? is that rust from the screw that transferred to the membrane ?
is it detrimental ?

[Derek Samuel Reese]

Sorry about the separate picture posts, they would all fit in a single post.

[klaus]

Derek,
The capsule's pictures reveal corona oxidization. There was a period, In the mid-1970s, when a layer of the conductive gold sputtered onto the diaphragms tended to oxidize and form a corona-like halo, accelerated by the the electro-chemical reaction from the applied DC voltage. When corona oxidization is bad enough, the inner section of the sputtering can lose conductivity, so make sure that you test for increased steady-state noise when you encounter these capsules.

[Derek Samuel Reese]

Good to know thank you Klaus, so would it be wise to stay away from buying a capsule in this state ?
Kind of looks like it can only get worse over time.
The seller told me it sounded great and there wasn't any sign of noise.
I fear that this oxidation will continue to spread or even get deeper through singing.
I use a pop filter but over time this may become an issue.

[klaus]

Two options, aside of abandoning the prospect altogether:

1. Get a money-back guarantee so you can test the capsule yourself and confirm that it still performs to spec

2. Buy it cheaply enough so that a potential total loss does not hurt

With corona oxidization it's not entirely clear to me whether the process is continuous or whether, once damage is done as seen on your picture, it stops at some point. I don't think I'll live long enough to be certain.

[Derek Samuel Reese]

The seller is in Spain and doesn't want to do a money back guarantee, he wants $750 for it which is a great price if it sounds great, but this oxidation thing troubles me.
And if you don't know and no one else chimes in on this capsule i think I'm gonna pass and wait for one that is in better shape and closer to NYC.
Thanks as always Klaus

[uwe ret]

RE: Corona Oxidization:

How does the gold film, pure to permit sputtering, oxidize? I can see the deterioration, but is that really oxidation?

[klaus]

The question  is not new: how could could gold corrode? I confirmed the issue with Berlin. There was some kind of substrate material used to bond the gold to the Mylar. That material over time had an electro-chemical reaction causing the corona oxidization.

[Derek Samuel Reese]

I did a little searching just on oxidation and i am sure there are many other and different factors that play in to this, but.. i asked Google What affects the rate of oxidation?
and the answer was----
A number of factors affect the oxidation rate, including temperature, pressure, crystal orientation, oxygen source (oxygen or water) and impurity doping.
So this tells me that when you sing into a mic it affects the temperature, the room affects the temp..
Pressure is a big one singing into a capsule, oxygen source, oxygen and water, from breath and spit.
My money tells me this will get worse over time.

[klaus]

Experience, in my book, beats Google/Wiki infos every time. 

I've observed and dealt with the K47/67/87 corona defects since the late 1980s and have not found the issue getting worse with time; rather, the deterioration seems to arrest at some point.

But what do I know? Just spending the best years of my life with one type of corona or another...

[Derek Samuel Reese]

I know experience is king, but sometimes Google is helpful.

Since the deterioration seems to arrest at some point, how does one find out what stage of deterioration it is in at the present moment?
The seller bought this capsule four years ago, showed me pictures of the original sale and the capsule looked the same.

I am going to ask again if he would allow me to install the capsule and listen myself.
Thanks again Klaus.

Two different pictures at different angles, four years apart:

[gtoledo3]

The question is not new: how could could gold corrode? I confirmed the issue with Berlin. There was some kind of substrate material used to bond the gold to the Mylar. That material over time had an electro-chemical reaction causing the corona oxidization.

I read somewhere that some nickel (or mix that includes nickel) was used to enhance the bond of the gold. Possibly still being done. I think I read this on the old Neumann forum.

I am not sure that explains “corrosion”, but the discussion does bring it to mind.

[panman]

I've observed and dealt with the K47/67/87 corona defects since the late 1980s and have not found the issue getting worse with time in most cases; rather, the deterioration seems to arrest at some point, unless accelerated by environmental factors. 

I have one Gefell(RFT) M71 capsule, that has a really heavy corona oxydisation. I have had it since 15 years and it still works perfect no change within all these years. So, I am leaning towards, what Klaus says.