JBL L300 rebuild

[CRMagIII]

Need advice.. I am in the process of rebuilding a pair of factory original JBL L300s built on 9-14-1976. The bass drivers are 136a's with 2231A cones. Both of these need new foam surrounds. I have both drivers ready for new surrounds, one of them had a smashed dust cap so I have removed it. The voice coil gap of this unit seems to be a bit tighter than the other unit by feel. Upon inserting the spacer shims in preparation to center the surround I noticed that the shims in certain places around the coil seem to catch on what feels like a loose bottom winding of the voice coil. with all shims removed I am unable to move the cone vertically without VC rub. The other speaker does not exhibit this at all (nice free movement). If my theory is correct I would like to remove the cone and VC, reattach the bottom winding and re-assemble, so as to preserve the original VC & 2231a cones. So my questions are:
Is it possible to remove the spider in such a way that it can be reused? If so how?
If not can I obtain OEM spiders for 2231a and should I only replace the bad one or would I need to do both speakers?
Due to cost I am not ready to recone both speakers with 2235 as many have done.

[script56]

You can use acetone to remove spider glue.

[CRMagIII]

The Acetone worked best for removing the spider from frame. Pulled cone, spider, and VC out. Lucky me no loose coils as I thought I would find. Found what appeared to be a drill flashing stuck in gap, possibly from manufacture, who knows. Was able to remove it and inspect rest of gap. Question is there is an area on the VC that the enamel has scratched off due to the rubbing. What is the best type of insulation and source for it, to re-coat this area without adding much mass or thickness?

[macaroonie]

Probably nothing at all as long as there is no short circuit. You should measure overall DCR ( prob about 6 ohm + or - ) and also if you can , test for winding to adjacent winding shorts. Small scale stuff I know.

Good luck

[remusr]

I would try to refoam and recap first and see how they play. You could be opening a pandora's box removing spider, regluing coil....might be better if there is a problem after refoam just to do the JBL 2235 recone. You probably will be unable to hear the difference on music between the 2231/136A 16Hz & 2235's 20Hz fs. Your 136A's alnico magnets likely need a recharge too and I think that needs the cone out.

[audiomagnate]

I would try to refoam and recap first and see how they play. You could be opening a pandora's box removing spider, regluing coil....might be better if there is a problem after refoam just to do the JBL 2235 recone. You probably will be unable to hear the difference on music between the 2231/136A 16Hz & 2235's 20Hz fs. Your 136A's alnico magnets likely need a recharge too and I think that needs the cone out.

It sounds like it's too late for that. I hope it all goes back together OK. You're going to love them when you're done. Recapping L300's is fairly involved. You might want to consider keeping the the original crossovers stock and building the Nelson Pass crossovers on a new board.

[ratitifb]

and I think that needs the cone out.

[audiomagnate]

Is there a way to tell if 136A's need recharging, or do they loose a certain amount over time, like a radioactive decay?

[ivica]

Is there a way to tell if 136A's need recharging, or do they loose a certain amount over time, like a radioactive decay?

Hi audiomagnate, try to measure drivers TS parameters, and if they are in JBL purposed values, everything is OK.
So getting BL value would indicate if recharging is necessary. The procedure how to do that can be find......

For my opinion one of the most interesting explanation is given in the:

http://www.bksv.com/doc/bo0202.pdf

[ "Determination of the Thiele - Small Parameters Using Two-Channel FFT Analysis" -by Christopher J. Struck,Brüel & Kjaer Instruments Hoffman Estates, Illinois
where the solution for the BL factor is given too. or

http://www.aes.org/e-lib/browse.cfm?elib=3067
["The Measurement of Loudspeaker Driver Parameters " - Hoge, W.J.J., AES Convention:58 (November 1977) Paper Number:1287 , Publication Date:November 1, 1977 ]

http://www.americanradiohistory.com/...DB-1981-08.pdf { from page 41 to page 46 } But other procedures can find such as:

http://sound.westhost.com/tsp.htm

http://www.mh-audio.nl/downloads/read-measuring-tsp.pdf

http://documents.jordan-usa.com/Famous-Articles/Thiele_small_parameters_measurement.pdf

http://www.sonicbeacon.com/Sonic_Bea...t_Software.htm


Regards
Ivica

[ratitifb]

Is there a way to tell if 136A's need recharging, or do they loose a certain amount over time, like a radioactive decay?

There are few reasons why alnico demagnetizes, power abuse and/or shock mainly

- over time : the flux drops very slowly (2 – 3% per decade, only 1dB sensitivity loss over 40 years)
- thermal losses : high temperature related loss, some reversible, some not
- power abuse : eddy current inside magnet material due to voice coil flux modulation. Possible loss depends on magnet structure geometry and voice coil hight & diameter (15” with 4” voice coil can lose 2 – 3dB from a single high power “pop”)
- shock : knoking the magnet structure can cause a dramatic reduction in magnetic energy
- disassembly : open magnet structure, such as to recentre a shifted pole piece, then the magnetic field collapses instantly.

Usually drivers that have been abused become “dull” or “slow”, sounding dreadful.

For my opinion one of the most interesting explanation is given in the:

http://www.bksv.com/doc/bo0202.pdf

[ "Determination of the Thiele - Small Parameters Using Two-Channel FFT Analysis" -by Christopher J. Struck,Brüel & Kjaer Instruments Hoffman Estates, Illinois where the solution for the BL factor is given too.

Nice print but B&K only describes the added compliance method and not the added mass method commonly used as in lab. on large drivers.

[ivica]

Nice print but B&K only describes the added compliance method and not the added mass method commonly used as in lab. on large drivers.

Hi ratitifb,

The mentioned "added mass method" is explained some other explanations, so fs, Re, Mms, Qes can be get.
For example here:

http://sound.westhost.com/tsp.htm

where first Re has to be measured, and then fs, Qes, Qms, Qts, and later Mms and Vas

{ ......"Measuring Vas (equivalent air compliance), Method 2 The second method is to use an added mass, M1. Typically modelling clay or Blu-Tak is simply stuck
to the cone close to the voicecoil, and the change of resonant frequency allows you to determine
the moving mass of the cone. Armed with this, you can then calculate the Vas.....
...For speakers less than 200mm (8"), use 5 grams, for 200mm use 10g, and for 250mm (10") or larger, use 20g.
You may need to add more if the mass chosen does not reduce resonance by at least 10%. The mass must be measured accurately! Even a small error can cause a large variation in the calculated Vas, so a precision scale (accurate to at least 0.1g) is essential. You also need to measure the effective cone diameter. This is generally taken as a measurement that includes half the surround. Again, an inaccurate reading will make a big difference. Because of this, the test box method is probably more accurate. You don't need to worry about extremely accurate measurements that have a profound effect on the measurement result. Still, the added mass method is quick and convenient, so has been added. The Fs of the speaker in free air has been measured, so simply add a suitable mass to the cone and re-measure the resonant frequency. This becomes Fs¹.

First, measure cone diameter so that effective cone area can be determined. Measure the diameter, including half the surround. The measurement must be in centimeters for this calculation. Divide by 2 to get the radius ...

Sd = pi * r²

Calculate cone mass ...

M = M1 / (( Fs / Fs¹ )² -1 )

Next, determine Cms ...

Cms = 1 /(( 2 * pi * fs )² * M)
VAS = CMS * d * c² * Sd²

Assume the following ...

d = density of air = 1.204 kg/m3
c = speed of sound = 345 m/s

Let's do a sample calculation using the same driver as before. ...

Sd = pi * r²

Next, we measure the driver's resonance with an added mass. The mass was carefully measured,.....
(note that it was indicated above that some drivers will need a lot more mass than may be indicated -...... has a heavy cone) ...

M = M1 / (( Fs / Fs¹ )² -1 )

Now we can calculate Cms, using the values shown above ...

Cms = 1 / (( 2 * pi * fs )² * M )

Now that we have everything needed, VAS can be calculated, using the default values for air density and sound velocity ...

Vas = Cms * d * c² * Sd²

Then, knowing fs, Re, Mms, Qes then BL can be calculated as mentioned in the attached figure:

http://www.audioheritage.org/vbullet...1&d=1391003332


Regards ivica

[ratitifb]

thanks ivica for sharing your experience.

Actually it's the same way but more "aesthetic" to directly estimate the compliance or its inverse (stiffness) from M1, Fs & Fs¹ without knowing the M value as follows :

K = 4*pi²* M1 / ((1/ Fs¹)² - (1/ Fs)² )

[audiomagnate]

Why wouldn't a simple sensitivity measurement work? Lower flux = lower output, no?

[grumpy]

Seems simple at first ponder...

Sensitivity at what frequency? using what mic/cal/pre? in what enclosure? in what environment?

(not so simple unless you have a consistent procedure and baseline).

Fairly consistent results can come from something like the Smith & Larson woofer tester
or equivalent (USB device and Windows program).

For a shop with a magnetizer, it's probably just as easy to do the remag.

Probing the gap directly often requires a destructive process (cone kit comes out).

[mech986]

Great Plains Audio and Orange County Speaker can both do alnico remags if you don't mind the cost of shipping. OCS can also do the refoams as well.

Edgewound at Upland Loudspeaker Service can also do the refoam and remag and his service is also very good.