2404 Diaphragm failure diagnosis

[Mike F]

Seeking opinions as to why these diaphragms failed.



And



These units were pulled from a set of 2404s in 4628 enclosures.
Interestingly, both still maintain a DCR of 6 ohms despite the nasty looking wrinkles and tears in the rings.


According to the JBL specs, the tweeters are crossed over at 3k which seems rather low.
Would over excursion be the culprit?

[sekess]

I can't be sure. But to me it looks like the phase plugs might have twisted the diaphragms upon installation. I understand that when the screw is being tightened that holds the phase plug - if the plug is allowed to twist, it can crease the diaphragm.
From the picture, it looks like it could have been from that. That's just my guess. I've never actually seen that happen. So, I could be way off here.

Steve

[Eaulive]

I would agree on that, seems like mishandling of some kind. I don't see how this kind of damage could have been caused by anything else.

[Mike F]

I can't be sure. But to me it looks like the phase plugs might have twisted the diaphragms upon installation. I understand that when the screw is being tightened that holds the phase plug - if the plug is allowed to twist, it can crease the diaphragm.
From the picture, it looks like it could have been from that. That's just my guess. I've never actually seen that happen. So, I could be way off here.

Steve

That is entirely plausible. We do agree that the `frams should not look like that right?
This is the first time I see a ring radiator type and it just doesnt seem right.

[edgewound]

Seeking opinions as to why these diaphragms failed.



And



These units were pulled from a set of 2404s in 4628 enclosures.
Interestingly, both still maintain a DCR of 6 ohms despite the nasty looking wrinkles and tears in the rings.


According to the JBL specs, the tweeters are crossed over at 3k which seems rather low.
Would over excursion be the culprit?

Radial distortion of the diaphragm looks like SEVERE over excursion, or maybe the phase plug was loose not clamping down the center ring.

Could also be just a hatchet job of installation, but they were done in the 1980's so they might be the originals.

If this were the 2405 diaphragm they'd be shattered, most likely.

[Allanvh5150]

I would have to agree with Edge. I have seen this in many 2402's used in SR. Do you know where these came from?

Allan.

[Mike F]

Radial distortion of the diaphragm looks like SEVERE over excursion, or maybe the phase plug was loose not clamping down the center ring.

Could also be just a hatchet job of installation, but they were done in the 1980's so they might be the originals.

If this were the 2405 diaphragm they'd be shattered, most likely.

Allow me to open a can of worms


6 or so years ago, a friend acquired these speakers but the top end never seemed right. It was determined that not only did they not have identical `frams, one being a 2402 and the other a 2405, but the phase plugs also differed in diameter. 1.6" and 1.71"which leads me to believe there were more than one version of the 2404.



It`s no wonder the two measured differently with those phase plugs and `frams that have different effective radiating surface areas.
He sourced another pair of 2404s as spares but unfortunately the `frams in those did not match either,
one being a 2405 and the other of Chinese origin however, both phase plugs were of the 1.71" variety.

That said, what would be the correct diaphragm to use, a 2402/D8R075 or 2405/D16R2405 with the 1.71" plugs?
In either case, what would you say to bumping up the x-over frequency little too?

[1audiohack]

The 1.7" plug goes with the D16R2405 diaphragm with a minimum suggested crossover of 5kHz. The 1.6 plug is for use with the D8R075 with the minimum suggested crossover of 3kHz. All the other parts are identical save that the later ones have the four inner metal horn mounting screws re-oriented as to access them with a hex key with greater ease, this does also re-orient the pockets on the back of the inner horn to clear the diaphragm solder terminals and screws making the early and late magnet/inner horns not swappable.

There is a thread titled "Ring Radiator Differences" or something like that with lots of pictures, measurements and likely more information than anyone would ever need.

[subwoof]

The conical plug was also used in the 2403 and it has a DIFFERENT angle along with the diameter. Check to see if the slopes are the same to see if one was swapped by mistake. I have one of each in my wtf box of tweeter parts.

On the wrinkles, whoever thought that putting a screaming vocal through one of these ( aka 4612 @ 3K ) was nuts. These wrinkle like a geriatrics privates in cold water when slammed like that.

Also the 2402 / 075 diaphram is used in the 2404H-1 ( note the subscript ) so check your tags on the back. The cabinet-only OEM models had no magnet tire and a white letter stamp.

sub

[Mike F]

Unfortunately the foilcals for all 4 drivers are no longer present as they were all at some point "fixed" but I do recall seeing at least one driver with a 2404h-1 stamp and one without the magnet tire, not sure if they were on the same driver though.

So it would seem likely that the failures could be attributed to: improper torqueing of the phase plugs, over driving or perhaps a combination of the two.
I think once the new `frams will be installed, it would be prudent to bump the crossover frequency up to at least 4 or 5 Khz and maybe even sharpen the slope. Any objections?

I`m not sure how well behaved the 2118 is at the top of it`s pass band, but that may be a small sacrifice to make in the interest of protecting the fragile tweeter in a system performing Sound and Musical Instrument reinforcement duties.

[1audiohack]

The conical plug was also used in the 2403 and it has a DIFFERENT angle along with the diameter. Check to see if the slopes are the same to see if one was swapped by mistake. sub

Is there more than two plugs? JeffW let me take one of his virgin 076's apart, (he bought them new) and it had the 1.7" diameter 1" tall 78 degree phase plug, the same as the 2404 that uses the D16R2405 diaphragm. The 1.6" is 82 degree included angle. Either plug will physically work with either diaphragm but they are different for a reason.

All the best,
Barry.

[subwoof]

the internal space that the phase plug "mates" with is unique to the 2404 and the 2403 and are NOT interchangeable.

I have never seen a different diameter 2404 plug but rest assured I am gonna look at the stash the very first day it gets back to 50 degrees....

the 2118 will behave just fine at 4K - remember that the MAJOR culprit in diaphram damage is when an amplifier ( or it is TOLD TO ) is driven to the point where it clips. Drive it hard enough and the output is almost a square wave and guess what? the leading and falling edges of the waveform show exactly what the diaphram movement should be.

Basic physics says that the mass can't move instantaneously from FULL excursion points...oh and by the way, the diaphram will just sit on top of the waveforms and cook...and cook...and cook.

sub

[1audiohack]

There are for sure two different base diameter phase plugs used in the 2404's, 1.720" and 1.600". I have 23 2404's of which seven were purchased new and the new ones were/are assembled as follows, drivers with D16R2405 diaphragms have 1.720" base diameter phase plugs, drivers with D8R075 diaphragms have 1.600" base diameter phase plugs.

The used drivers I bought were mixed as often as not.

[Mike F]

Many thanks to all who replied, hopefully this thread will be of use to someone faced with the same issues in the future.

[ivica]

The 1.7" plug goes with the D16R2405 diaphragm with a minimum suggested crossover of 5kHz. The 1.6 plug is for use with the D8R075 with the minimum suggested crossover of 3kHz. All the other parts are identical save that the later ones have the four inner metal horn mounting screws re-oriented as to access them with a hex key with greater ease, this does also re-orient the pockets on the back of the inner horn to clear the diaphragm solder terminals and screws making the early and late magnet/inner horns not swappable.

There is a thread titled "Ring Radiator Differences" or something like that with lots of pictures, measurements and likely more information than anyone would ever need.

http://www.audioheritage.org/vbullet...l=1#post298650