Frankentweeter ? 2404H and 2405H crossover from JBL??

[sebackman]

Dear all,

I just bought a pair of what I thought to be normal 2405H tweeters. When they arrived it turned out that the motor is a 2404H with a 2405 diaphragm and a 2405 front.


The motor is stamped (the read badge is gone and this is where it should have been) "2404H" and "MAY 05 1993". The casting number is X60754. The box they came in says 2404HPL-L

The diaphragm is stamped "2405" and "APR 28 1993". The diaphragm is silver with the outer ring being gold/yellow-ish. No steering pins.

Since the 2404H uses the 2405 diaphragm this seems to be right.


The front is a standard 2405 diffraction horn held down by a die cast flange with 4 screws. There is no marking on the horn and the casting number on the square mounting flange is "51780 P".

They seem to be in good shape and I will measure them today.


Did JBL ever sell 2405H units with 2404H motors? Is there any difference in the motors? Do you believe that there is any sound quality or any negative practical impact of this combination?


There is also like a thin “film” on parts of the diapragm like if it has been water on it that dried out. Do you think this will impact sound? Most of the "aluminium ring" is shiny but these areas are a bit matte. They are from 1993 so maybe it is time to exchange them anyway.

Do diaphragms age, ie lose sound quality och sonic performance with age?

Best regards
//RoB

[Rolf]

As I can understand your post, the diaphragms needs to be changed.

Dear all,
I just bought a pair of what I thought to be 2405H tweeters. When they arrived it turned out that the house is a 2404H with a 2405 diaphragm and a 2405 front.

The motor is stamped (the read badge is gone and this is where it should have been) "2404H" and "MAY 05 1993". The casting number is X60754. The diaphragm is stamped "2405" and "APR 28 1993". The diaphragm is silver with the outer ring being gold/yellow-ish. No steering pins. Since the 2404H uses the 2405 diaphragm this seems to match.

The front is a standard 2405 diffraction horn held down by a die cast flange with 4 screws. There is no marking on the horn and the casting number on the square mounting flange is "51780 P".

They seem to be in good shape and I will measure them today.

Did JBL ever sell 2405H's with 2405H motors?
Do you believe that there is any sound quality or practical impact of this combination?

There is also like a thin “film” on parts of the diapragm like if it has been water on it that dried out. Do you think this will impact sound? Most of the ring is shiny but these areas are a bit matte. They are from 1993 so maybe it is time to exchange them anyway.

Best regards
//RoB

[hjames]

As I can understand your post, the diaphragms needs to be changed.

Unless the film is Aquaplas ...
its 'sposed to be applied in a thin film, and its used to take the harsh edge off diaphragms.

Pretty hard to make judgements sight unseen.

[sebackman]

Thank you.
I will try to post pictures later. Is Aquaplas transparent on the tweeters? Does it have a color on the tweeters.

On my 2451SL's it is a real difference in color compared to the standard diaphragm. The SL ones are gray/black-ish compared to the more silvery standard titanium ones.

What do you think about the combination 2404H and 2405H horn?

Best regards
//RoB

[sebackman]

Here are a few pictures





Any ideas?

Regards
//RoB

[1audiohack]

The motors are the same. The ferrite assemblies don't have guide pins.

In my experience / opinion, before the diaphragms fail they " tighten up" due to work hardening of the aluminum. The response gets more jagged and the efficiency slides. If they sound good to you, run 'em.

I have never seen a coated 07X -240X. That certainly doesn't mean they don't exist however.

[grumpy]

'coating' just looks like oxidation to me. I would leave them be, perhaps
getting new diaphragms as time and finances permit (to have as spares
or, if you have access to measurement equipment, until they can be
measured)

[sebackman]

I think you are right, it’s probably oxidation. Is there any general rule how that may effect the sound?

Yes I do have MLS equipment with a EW M30 mic. I will try to do some measuring tomorrow.

I'm still currious as if JBL ever sold 2405H's with 2404H motors, or if something happened over the years....

Best
//RoB

[Mr. Widget]

The motors are identical so there is no reason JBL would use a 2404H motor on a 2405H. I don't think the HPL versions had foilcals... only printing on the magnets like your seem to have. They undoubtedly were changed along the way by one of the owners. I have 2405Hs that I put 077 acrylic phase plugs in for aesthetic reasons... these things get changed around over the years.

I'd expect replacing the diaphragms would be a good call, but may not be necessary.


Widget

[sebackman]

When I took the phase plug off I noticed that there are no markings or steering pins to realign the phase plug. How do you normally get the phase plug back in the right direction. The easy way would be to mount the horn and make certain that it optically centres in the slot. Is that good enough? Is this the result of having a 2404H motor and a 2405H phase plus or is it the same on all?

Both units have lost some paint over the years, both on the horn and the phase plug. How do one normally restore the paint? One alternative would be to glass blast the phase plug and the horn and powder coat them. How has this been done on your units? Is there anything particular with the paint it self or can I use any semi matte black paint. Is there any reason not to use glossy paint?

How sensitive is the alignment at reassembly? The pieces match reasonable well but can be moved slightly before the screws are tightened, including the phase plug. At 20kHz the wavelength is about 1,7 centimetres or 0.678”, so if the diaphragm centres and there is no noise over the audible spectrum is it really sensitive that the phase plug and horn is absolutely entered and if so how is this done?

Where can I get a pair of acrylic phase plugs?

All the best
//RoB