help! 2245 with strange impedance

[franz]

Hello friends,

I got a tough question for the wizzards out there.
Today I received 2 used 2245H in pretty good shape.
However one of them that seems to be reconed shows a very strange z-behaviour.
In free air it exhibits a z -max of only 12 Ohm at around 14 Hz.
Its a very flat depressed curve. TSP calculated by CLIO are Qm < 0.5 !! Qt < 0.15 , R dc is only slightly above 4 Ohms.

Its Brother seems to be normal with about 150 Ohms at 18,5 Hz. TSP are quite o.k. also.

Have you ever experienced such a strange thing?
What do you guys think is the reason?
Are the mechanical losses too high?
Can a bad recone job be the reason for this?
Can this unit still be used properly? (besides needing a totally different alignment)

I know that´s a lot of questions but I just need to know what´s going on here.
Thanks anyway for all your input
(I could post graphs if anyone would like to know)

cheers

Franz

[Guido]

Hi Franz!

Seems you are new here, welcome!

Please post a graph of both z curves. This would help.

How did you measure the impedance?

[Ralf]

I tap on a not cleanly centered cone.

Maybe there is a short circuit in a few windings because there is a friction...

Reconing is the solution...

Greetings
Ralf

[franz]

Hi fellas,

thanks for your ideas so far, but what about Mr. Widget et al. ??

Has really noboby ever had a similar problem (or maybe heard of this) before?

One thing that came into mind was a demagnetized magnet but I never heard of this with ferrite units. Suppose nobody baked the frame in his oven before reconing it.
Didn ´t measure BL so far but I guess it might be low.


Any more ideas ?

Thanks

Franz

[Earl K]

Hi Franz

- I wish I had a pair of 2245h(s) so I could answer your question with some real hard data. Obviously the Zmax figure is "way-off" for a healthy compliance. Plus the Re figure looks suspiciously low. What was the Re (DCR) of your other woofer ? According to JBL Customer Service "Test Specs" circa 1982 , the DC resistance for a 2245H should fall in the range of 5.0 to 6.0 ohms .

- In the past , JBL issued to its' authorized reconing centers, information to help determine the health of a LF transducer. These "Transducers (LF) Test Specifications" came from the "Quality Assurance Department" . I have some of this info that is circa, late 1970s' and 1980s' .

- Here's an example of the type of info given out in the late 70s';
2230A : Zmax ( MIN - MAX) ; 180 to 300 ohms, Fs ( MIN - MAX) ; 14 to 18 Hz, DCR ( MIN - MAX) ; 5.7 to 6.9 ohms,

So;

What do you guys think is the reason?

- My guess is that there is something lodged in the gap.

Can a bad recone job be the reason for this?

- Yes , I've had a reconer not properly clean the gap - in my case, capturing a (new) chunk of metal in there. This "Gauss 18" recone lasted about a week before failure . It was a timely lesson in the " fallibility ? " of even the best .

Hopes This Helps <. Earl K