Destroyed front diaphragm on a brand new K67

[Delta Sigma]

I can't believe this one. I built a U67 into a brand new TLM67. I was just trying to measure 10dB down at 15kHz. After measuring, I left the room for a bit, then came back to find that my cheap ebay sig gen was putting out a tonne of signal on the calibration input. The output was clipping my interface input. I must've bumped the pot? I don't remember hitting it. No idea why I even left it hooked up. Didn't think anything of it last night. I pulled it off and left the mic on overnight to keep burning in the tube. Today, after changing C17 then going to retest, I had no output. The front diaphragm measured 500pF and it's visibly damaged. It must've been slamming the backplate.

Other diaphragm is fine, so I can use it in cardioid for now. Any recommendations to get it reskinned? I'm in Canada.

Adding insult to injury, my interface is dead too. It's likely unrelated, as it was working fine this morning. This Zen Q has been nothing but problems.

 

[Khron]

Since you can't really mess it up more, i'd try loosening the screws holding the two halves of the capsule together, to let (more) air in and see if that might unstick the diaphragm.

Since you're probably at the "nothing to lose" point...?

 

[Delta Sigma]

Definitely at the "nothing to lose" point. I'll give it a shot. Probably need to wind down a bit before trying anything. Not a good day.

 

[Khron]

At least the K67's out of AKG Perception 200's had an incomplete-circle brass spacer shim between the backplates, the gap being to equalize the air pressure inside & out. No idea if the Neumanns do as well, or whether it's an uninterrupted plastic shim between the halves.

But as mentioned, let some air in there and see if it does anything. Perhaps even consider separating the halves and lightly blow onto each backplate to "pressurize out" the diaphragm(s)? I'm a cheapskate though, and i'd rather try all the free and minimal-effort possible solutions, before throwing money at s problem...

"It's already f**ked, whaddya gonna do, f**k it up more?" - AvE

 

[k brown]

At least the K67's out of AKG Perception 200's had an incomplete-circle brass spacer shim between the backplates, the gap being to equalize the air pressure inside & out. No idea if the Neumanns do as well, or whether it's an uninterrupted plastic shim between the halves.

But as mentioned, let some air in there and see if it does anything. Perhaps even consider separating the halves and lightly blow onto each backplate to "pressurize out" the diaphragm(s)? I'm a cheapskate though, and i'd rather try all the free and minimal-effort possible solutions, before throwing money at s problem...

"It's already f**ked, whaddya gonna do, f**k it up more?" - AvE

I'm even crazier than you - I'd also put my mouth around the outside edge of the capsule and suck on it!

 

[Delta Sigma]

I'm even crazier than you - I'd also put my mouth around the outside edge of the capsule and suck on it!

Ha! I'll leave that as a last resort

 

[Delta Sigma]

I gave it a try, no dice. The two halves were easy to separate. The diaphragm didn't restore on its own. I tried putting pressure from behind and the diaphragm "pops" out easily, but returns to its same state, measuring around 500pF.

 

[k brown]

I gave it a try, no dice. The two halves were easy to separate. The diaphragm didn't restore on its own. I tried putting pressure from behind and the diaphragm "pops" out easily, but returns to its same state, measuring around 500pF.

Is it possible there's static electricity involved here? If so, any way to neutralize/drain that?

 

[Delta Sigma]

Is it possible there's static electricity involved here? If so, any way to neutralize/drain that?

I could try, but I think that the problem is that the signal swing was too large and it was hitting the backplate with 60V potential. Now the mylar is physically distorted, causing it to be too close to the backplate.

 

[k brown]

I could try, but I think that the problem is that the signal swing was too large and it was hitting the backplate with 60V potential. Now the mylar is physically distorted, causing it to be too close to the backplate.

I would be surprised if that's the case; polyesters are pretty hard to permanently deform - doesn't that require heat?

 

[Delta Sigma]

I tried grounding both the backplate and diaphragm to discharge any static. No change. I left it sitting open on my workbench, with the diaphragm facing down, to let the moisture (from blowing on it) dissipate. I went to put it away and a lot of the marks are gone! It still measure poor, but 160pF vs the 500pF that is had been.

I'm not expecting this thing to return to normal, but I'm gonna give it some more time and see what it does.

@k brown, you may be right about the mylar. Could it possibly be the gold that was holding shape? Maybe the Mylar is taking its shape back.

 

[Khron]

Give it some time, maybe also chuck it in a ziploc with a few sachets of dessicant. For all we know, there might even be some previously-ingressed moisture still keeping those stuck parts on there. There may still be hope yet!

 

[Delta Sigma]

Measured it this morning and it was down to 95pF with less visible marks. Got home from work and it was the same. Put pressure from the back again and there were still marks visible. Removed the centre screw (second time doing this) and put pressure from the back, all the marks disappeared. Measures 62pF now. Got it in the zip loc with the rear capsule and desiccant now.

 

[k brown]

Kinda sounds like it took the mylar awhile to 'bring the gold layer with it".

 

[terry setter]

Yes, we all make mistakes...
(see attached photo)

 

[Delta Sigma]

Yes, we all make mistakes...
(see attached photo)

What happened???

 

[k brown]

What happened???

. . . forgot the bloody pop screen!

 

[12afael]

You were recording Graham Bonnet? or a jet engine?

 

[Khron]

Measured it this morning and it was down to 95pF with less visible marks. Got home from work and it was the same. Put pressure from the back again and there were still marks visible. Removed the centre screw (second time doing this) and put pressure from the back, all the marks disappeared. Measures 62pF now. Got it in the zip loc with the rear capsule and desiccant now.

Hey, that's great news! To be "on the safe side", perhaps the affected side should be reinstalled as the "rear" (less frequently used), and the unaffected one used as the "front"?

 

[Tubetec]

Looks like you got away with it ,
Ive collapsed the diaphragm with over voltage , it sometimes looks fatal
but it always came back up after the voltage drained away ,

Ive used a hair dryer directly on mics and capsules , allowing the metal work to heat gently ,
and evaporating and blowing away any trapped moisture ,

To some small degree you can add a little extra tension to the mylar with heat , perhaps if its prone to collapse on one side , concentrating on that side with hot air can help , obviously you never want to allow it to melt or distort . I only tried this technique on a throw away capsule , and at that stage I didnt even have a LCR to keep an eye on the pF of the capsule .

Might be a useful way to balance the capacity of both halves in a mismatched capsule , just adding small degrees of heat to tune things .

I havent pulled a working LDC appart , but one thing struck me , it probably makes sense to mark the alignment of both the backplates and their respective membranes, spacers before you go at it . So everything goes back to exactly as is left the factory .