2405H question

[martin2395]

I bought a pair of 2405H's yesterday and I noticed that they don't have the alignment pins anymore so how is the diaphragm alined?
Does the routing on the horn and the phase plug align it automatically when you bolt all the parts together?
I have to admit that I was kinda surprised when I tried to put the 2405H horns on 2405A - they don't fit as they are missing holes for the alignment pins!

By the way is a "85U-JBF 33752" a genuine 2405 dia? They measure 7.0 and 7.3 ohms.

[Mr. Widget]

You need to align them with a test sweep and very carefully tighten the phase plug to keep it properly aligned. I'd bet the vast majority of reworked ring radiators are not properly aligned.


Widget

[martin2395]

Hmm, I tested mine and they seem to sound fine on a TTG until 18KHz when I don't hear them anymore.

[subwoof]

33752 is the correct JBL internal part number.

If not aligned, you will always be able to hear a low frequency artifact and/or lower output.

Above 18Khz you need to use the canine method.

connect one lead while under test and see if the dog wakes up.

[ivica]

You need to align them with a test sweep and very carefully tighten the phase plug to keep it properly aligned. I'd bet the vast majority of reworked ring radiators are not properly aligned.


Widget

Hi Mr.Widget,

What would be your suggestion the way to align 2405 diaphragm, as some large angular rotation is not possible owing to the interconnection wires are short. Moving in radial direction is very, very limited owing to polarization pin ( on 2405A/077), or in the top-plate milled circular-shape place (2405H)?

regards
ivica

[martin2395]

I wonder that too, twisting the phase plug too much can destroy the fragile alu dia too.

[spkrman57]

I wonder that too, twisting the phase plug too much can destroy the fragile alu dia too.

Be very careful!

Regards, Ron

[Mr. Widget]

Hi Mr.Widget,

What would be your suggestion the way to align 2405 diaphragm, as some large angular rotation is not possible owing to the interconnection wires are short. Moving in radial direction is very, very limited owing to polarization pin ( on 2405A/077), or in the top-plate milled circular-shape place (2405H)?

regards
ivica

I am not suggesting that you rotate the diaphragm, I am suggesting that you center it accurately… which is not trivial. If you do not have test equipment you have to do this by listening to a test tone through the diaphragm. You'll hear a buzzing as you get near the edges of the voice coil gap.

After you have centered the diaphragm you have to very carefully hold it in place with the phase plug as you tighten the screw in the back. If you don't do this correctly the phase plug will rotate slightly wrinkling the diaphragm.


Widget

[martin2395]

Mr.Widget,
My golden dia's measure around 7 ohms, the blue ones were around 6 ohm when I measured them last time.
Has JBL changed they way they were made or is it simply due to age? The blue ones were from '78 and the golden ones are made in '91.

[Mr. Widget]

Mr.Widget,
My golden dia's measure around 7 ohms, the blue ones were around 6 ohm when I measured them last time.
Has JBL changed they way they were made or is it simply due to age? The blue ones were from '78 and the golden ones are made in '91.

It could be due to manufacturing changes... I'm not sure, but looking at this plot, there is a fair amount of unit to unit variation.


Widget

[ivica]

I am not suggesting that you rotate the diaphragm, I am suggesting that you center it accurately… which is not trivial. If you do not have test equipment you have to do this by listening to a test tone through the diaphragm. You'll hear a buzzing as you get near the edges of the voice coil gap. After you have centered the diaphragm you have to very carefully hold it in place with the phase plug as you tighten the screw in the back. If you don't do this correctly the phase plug will rotate slightly wrinkling the diaphragm. Widget

Hi Mr.Widget,

I can imagine that it would be possible to move a diaphragm in radial direction on the 2405A/077 in the way that You have enlarge the hole where the the pin has to pass, but on 2405H, radial movement would be possible if somehow the outer aluminum ring has been filed before the installation....

On the attached figure that can be seen, the differences between 2405A vs 2405H top-plate.
Here on the:
http://artificepower.blog26.fc2.com/...tegory-11.html

there has been suggested that a kind of "shimming" 2405/02 diaphragm has to be done.
Owing to Your large experience , is such "shimming procedure" suggest-able ?

regards
Ivica

[martin2395]

All that effort just to drop an aftermarket shi*y dia in it