Sunday, January 13, 2013

Dang it, JBL!

I've had a pair of JBL 4312 Control Monitor speakers for about 20 years now having bought them at a pawn shop for $75 in the early 1990's - a good price even back then!  At that point the speakers were already about a decade old, but they were in reasonable shape aside from some minor scratches on the cabinet and a small tear in one of the woofer's cones.

A very light sanding of the scratches with some fine steel wool and the application of a coat of finishing oil made the scratches completely disappear while the hole in the cone was repaired with a bit of white glue and some paper, the fix being mostly done from the back side with the grille cloth making the imperfection normally invisible.
Figure 1:
Before the rebuild - the bass driver with the problem.
Click on the image for a larger version.

I have used these speakers almost daily since I bought them and they have always produced excellent sound.  At some point about 10 years ago I borrowed a laboratory-grade audio spectrum analyzer and a noise generator and determined the L-pad settings that yielded ruler-flat response (according to the analyzer) over their entire frequency range.

A month or so ago I noticed a bit of what sounded like "clipping" from one speaker - but not quite clipping.  I didn't think too much about it until that same speaker started "barking" on certain bass frequencies so I popped the speaker grill and started pushing on the woofer's cone.  I observed that it would bind up when I pushed on the upper and lower edges - but not so much on the left or right edge.

Hmmm...

I'd run across this before on speakers with foam surrounds:  Over time, gravity can cause the foam surround and the spider (the accordion-like structure near the voice coil and the magnet) to sag from the cone's own weight.  The fix for this is usually pretty simple:  Remove the speaker, rotate it 180 degrees so that what was the bottom is now the top and then re-install it in the cabinet.

Except that these speakers don't have foam surrounds, but rather fabric-based accordion surrounds (similar to that of a speaker's spider) coated with a latex "damping" compound that also provides an air seal for the cabinet's tuning.  Another point of concern is that when "cone droop" occurs, pressing on the bottom edge usually produces less binding than if pushing on the top edge - but it was about equal in this case.  Nevertheless, I turned the speaker180 degrees to see if this would help.

Figure 2:
Making the first cuts in the 2213H woofer!
Click on the image for a larger version. 
A week later, it was worse so I started looking into what it would take to rebuild the speaker(s) and found that I could get an OEM JBL rebuild kit for a 2213H woofer for 2-3 times the cost of an after-market kit.  Sending the woofer away for a rebuild with an aftermarket kit was also an option, but it would be even more expensive, costing several times what I paid for the pair originally, not to mention that a rebuild with a JBL kit would be even more!  I also noticed two types of kits:  Those in which the voice coil, surround and spider were already attached to the cone, and those in which they were all separate and "some assembly was required."

I looked into what people had to say about the quality of the aftermarket rebuilds and couldn't come up with anything conclusive:  There were quite a few people that said simply that nothing could be as good as the JBL parts - end of story - but there was a dearth of anything other than opinion as it seemed that no one (that I could find) did any before/after measurements or compared a rebuilt speaker to an original in any other than a subjective way.

I finally ordered a pair of rebuild kits for the JBL woofers from McKenzie (reconekits.com), just one of several places that supply such kits and a company about which I couldn't find any general credible dissatisfaction.  This kit was one of those with all of the pieces already attached to the cone and was a bit more expensive than the others, but since this was my first "cone job" I decided to spend the extra $5-$10.  In about a week's time over the Christmas week holiday a box appeared and inside the parts were well-packed - although the shim had a slight amount of latex goo that had leaked from the bottle of "damping fluid" - probably due to the air pressure from changes in altitude during shipping:  This was easily cleaned off, however.

Figure 3:
The mysterious foam ring!
Click on the image for a larger version.
After another week or so I finally got some time to start the task of rebuilding the speaker.  While I have replaced dozens of foam surrounds on speakers owned by me and by others and have made "readjustments" on other speakers to remedy "cone rub" problems, I'd not before done a complete "cone job" so I was a bit apprehensive - but I figured that it was probably easier than a foam surround since I was starting from scratch rather than trying to make adjustments to a speaker that had already "settled in" over time.

So, I began to cut.

Once the wires are unsoldered, the job of removing the original cone is somewhat brutal: Cutting away the foam edging, gaskets and then the spider to finally liberate the original cone and voice coil from the basket - the metal frame that holds the magnet and everything else.  Once this was removed I spent 15-20 more minutes carefully scraping away the original adhesive - a fairly hard, epoxy-like substance -  that attached the surround and spider to the basket.

Immediately after pulling away the original cone I noticed something peculiar:  A ring of foam inside the speaker surrounding the hole in the center of the magnet.  At first I couldn't figure out what this was for but then it occurred to me that since the JBL voice coil is "vented" that this might be part of a foam filter to keep out contaminants.  When I touched the foam, it disintegrated into tiny blobs that stuck to my finger and I now knew why the speaker had "failed"!
Figure 4:
The goo left on the inside of the voice coil from the
disintegrated foam filter.
This was the actual cause of the speaker's failure!
Click on the image for a larger version. 

In looking at the inside surface of the voice coil (see figure 4) I saw the remains of the rest of that foam filter in the form of a sticky, tarry goo.  What had happened was that as this foam disintegrated into small bits, it fell down onto the voice coil and accumulated, reducing the clearance between it and the magnet assembly and finally causing what had appeared to be a jammed solenoid.

Had I known exactly what was going on when the speaker "failed" I may have been able to "repair" the speaker by removing the dust cover from the center, clean the goo from the voice coil with careful scraping and an appropriate solvent and then use tape to remove the remains of the foam so that no more would break loose and fall into the gap.  

(At some point I may see if I can get another surround, spider and dust cover for this cone/voice coil assembly as it is still in good shape, otherwise.)

One of the oft-touted advantages mentioned online about the woofer used in the 4312 was that it didn't have those foam rubber surrounds that often fail - but this was worse:  The same fate will likely befall 4312 owners of similar vintage (and also those owning speakers with similar woofers!) when the foam hidden inside, out of view, will likely disintegrate and jam their bass drivers, too!


Dang it, JBL!

By the time this speaker was made I would have expected that the folks at JBL already knew about the way this type of foam would disintegrate, so why did they use it in that application?
Figure 5:
The bare basket after the foam and adhesive had been removed.
Click on the image for a larger version. 


It didn't matter, now, though!

These speakers were already about 30 years old - a reasonable service life - and now I was knee-deep in the rebuild process so I proceeded to clean up the basket and the magnet assembly, covering the gap on the magnet with tape to prevent the pieces of glue and spider remnants from falling into it.  There was a very good chance that there were foam particles within the magnet gap so after removing the glue remnants elsewhere I attacked it with sticky-tape, sliding it around in the gap to sweep up any loose particles.

The "inside" of the magnet gap of the 2213H is hollow and just "below" (inside) the gap and inside this cavity there could have been an accumulation of particles of foam.  With tape still in the gap, I turned the speaker basket upside-down and around while rapping on the magnet assembly with a rubber-coated screwdriver to knock particles loose, repeating this process several times.  After doing this I soaked a small piece of cloth in lacquer thinner and, using a small piece of copper or brass sheet to avoid scratching the metal (or sticking to the magnet!) I pushed the cloth into the gap and moved it around its perimeter:  This dissolved and removed the last deposits of foam goo that were stuck to the magnet itself.  One final sweep with sticky tape confirmed that the gap was now clean and I was ready to install the new voice coil/cone assembly.

Figure 6:
The new cone assembly installed with shim, but before the gasket was
installed to hold the surround flat.
Click on the image for a larger version. 
The next step was to put a thin, even layer of glue on the inner ring where the spider would attach, and then on the larger, outer edge where the speaker's surround would be attached.  The next step is a bit tricky:  Installing the speaker cone and voice coil assembly - with the wires lined up with the speaker terminals - in the gap without getting the adhesive on the voice coil or my finger and then onto the cone itself.  If you manage to put your finger into the adhesive as I did, it would be a good idea to clean it immediately so that none of it is transferred it to the cone or something else and then check to see that there's still enough of it left to glue the spider/surround in place.  Once it was installed, the shim was inserted to center the voice coil about the magnet and then the cone was rotated back and forth slightly to distribute the glue and to "settle" the alignment of the cone and voice coil and then, finally, carefully aligned with the speaker terminals. 

At this point I had a problem - one that I'd encountered previously when replacing foam surrounds:  The  surround would not lay flat in the glue along the edge!  When I've replaced foam surrounds in the past I could generally find a dinner plate of the right diameter to hold the foam flat against the basket, or failing that I would use a dozen or three steel nuts (for 1/2"-3/8" bolts) around the perimeter to weight it down.

Because this surround was a loose-knit fabric rather than foam, the glue was soaking through it and I didn't want to glue a dinner plate or steel nuts to it!  Instead, I installed the speaker's gasket as it would handily hold the surround down - and it needed to be glued down, anyway.  Finding a flat cutting board in the kitchen that was just large enough to completely cover all of the surround and gasket, I laid it atop everything and then put some weight on of it.  This gasket supplied was in four segments and I had to tape the joints together (using "transparent" tape from the dispenser in figure 7) to keep them lined up.

Figure 7:
Weighing down the gasket to keep everything flat!
Click on the image for a larger version.  
After making sure that everything was where it should be I placed an incandescent desk lamp above it to add a bit of heat to the winter-cool basement room where I was working and then left it for 48 hours.

Two days later:

After allowing the glue to set I removed the cutting board and weights and then carefully pulled the tape off the gaskets.  Even with due care, a bit of the gasket came off with the tape, but small bit of white glue put this back together.  After removing the shim I pushed on the cone in several places around the perimeter and observed that it seemed to be nicely centered and moved freely, with no interference!

After gluing the dust cover over the voice coil I applied a bit less than half the bottle of the latex "damping solution" to the surround.  In addition to making motion a bit smoother, this effectively "air-proofs" the surround to help maintain the acoustic properties of the speaker's ported enclosure.

After letting the glue holding the dust cover and the latex damping solution set for an hour or so I flipped the speaker over to expose the back vent.  I cut a circle from a scrap of synthetic grille cloth from a previous speaker project and then, using RTV (silicone) adhesive, I glued it into place.

As noted above, this vent - which keeps dust and pieces of fiberglass insulation used as damping material inside the speaker cabinet out of the voice coil gap - had originally been a small circle of foam on the inside, behind the dust cap, but I decided to put it on the outside, on the back of the speaker as is often done with other JBL speakers, instead.  I then allowed the RTV, damping solution and glue holding the dust cover to cure overnight.

The next day, the rebuild was complete and I decided to measure the various parameters of the speaker that could be easily quantified with equipment that I had laying around and compare them to both the published JBL specifications and the other, original (not rebuilt) JBL 2213H driver as I wanted to see how different the readings were.

I first measured the DC resistance of the original, now-removed voice coil, the "new" after-market voice coil, and the one in the still-working original speaker:  They were all 4.4 ohms.

The next measurement - free-air resonant frequency - was not possible on the original speaker cone since it was "jammed" with the foam goo, but I measured this on the newly-rebuilt speaker and the other, still-original speaker:  The new speaker measured 28 Hz, the "other", original JBL speaker measured 21 Hz while the JBL specification was 24 Hz.  I attribute the 28 Hz resonance of the newly-rebuilt speaker to be due, at least in part, to the fact that the "new" speaker's components (surround and spider) were still very stiff in comparison with the "other" original speaker and I would expect the resonant frequency to drop slightly with time.  It's worth noting that either of these two frequencies are well below the acoustic cutoff frequency of the speaker/enclosure itself.
figure 8:
The rebuild is complete - ready to install!
Click on the image for a larger version.  

Another parameter was the voice coil inductance.  I'd forgotten to measure this on the original speaker cone/voice coil before rebuilding it (this measurement would only be valid if installed in the magnet gap) but I found that the newly-rebuilt speaker and the other original were within 15% of each other at both 120 Hz and 1 kHz and centered about the published JBL specs.

The final measurement was that of the impedance:  JBL doesn't mention the frequency at which this value is taken - or even the value itself - so I measured it using an LCR bridge at both 120 Hz and 1 kHz and the rebuilt and original speaker were both around 5.7 ohms  Other parameters such as Vas, Q, etc. are harder to measure and I wasn't going to go through the trouble to attempt doing so.

In closely inspecting the original cone removed from the rebuilt speaker, the new one supplied by McKenzie, and the one in the other, still-original 2213H driver I found them to all be slightly different.  The McKenzie cone material isn't quite as stiff as that which I removed during the rebuild, but I noticed that the cone material of the other speaker - the original JBL (and, incidentally, the one in which I'd repaired the hole) was much thicker and heavier than either of the other two, possibly explaining why its free-air resonance frequency was lower!  Online I'd read about how others had mentioned differences in seemingly-identical JBL parts, but I was surprised to see this much difference on speakers that appear to be of the same vintage.

Other aspects of the "original versus McKenzie" components look to be identical:  The same thickness of substrate on which the voice coil is wound, the the same gauge of copper wire, equally-heavy material in the spider and accordion surround, identical techniques to bond the voice coil assembly, spider and surround to the cone and identical-looking glue on these components.

How does it sound?  From what I can tell, they sound "dead-identical" to each other, but that's a fairly subjective analysis.  See below for a cursory analysis using an audio analyzer.

At some point I'll probably rebuild the other bass driver (woofer) - the one with the repaired hole - but I was able to inspect the foam filter through the vent in the back of its magnet and it appears to be both intact and non-sticky, indicating that it isn't in imminent danger of falling apart and causing it to fail.

Final comments:

In reading the online forums, it's apparent that some people are really picky and passionate about using exact replacement parts.  It's worth remembering that building speakers - especially mass-producing them - is not necessarily an exact art and most designs, particularly ported ones like this, are quite forgiving in the fairly wide tolerances involved - and which is one of the reasons why the speakers work so well!
 
I'm not really a rabid purist and since the most important parameters are pretty close to those posted on the official JBL data sheet (which includes a number of Thiele-Small parameters) I'm pretty confident that it will work about as well as the original.

What remains to be seen is if this aftermarket replacement will hold up for another 20-30 years!

 A few observations of the 'specs of the JBL 4312:
  • These are 6 ohm speakers, not 8 ohm as a lot of reviewers online seem to think:  Most modern solid-state amps don't care about the difference between 6 and 8 or 4 ohms, but check your manual to be sure!
    Figure 9: 
    The finished and functioning speaker, with the
    grille removed.
    Click on the image for a larger version. 
  • The crossover network in this speaker is very simple:  There's nothing on the bass driver, only a series (10uF) capacitor and L-Pad on the midrange, and an L-Pad and 3uF/200uH "L"-type high-pass network on the tweeter.
  • The low-frequency limit of the speaker is specified as being 45 Hz (@-6dB) - a fact verified on the Ivie IE-30A based, within the limits of its 1/3 octave resolution (see below.)
  • The maximum amplifier power is specified as being 150 watts.  The various drivers in the speaker itself are rated for far less than this in terms of continuous power, but practically any "sane" volume setting on such an amplifier will be quite safe as these drivers can easily handle the occasional peaks.

Update (21 January, 2013):
I was able to borrow an Ivie Technology IE-30A audio analyzer with matching microphone and, putting a white noise source into the audio amplifier (and verifying that it was also "flat" at the speaker terminals) I saw that the speakers were also quite well-matched with to each other - the newly-rebuilt 2213H bass driver and the OEM JBL driver.
This test was done in the middle of my living room with the speaker about 18 inches off the floor on a pad so room resonances cannot be ruled out.  It was too cold (about 12 degrees F) outside to lug everything into the yard to attempt measurements in an environment much less susceptible to such vagaries!

[End]

This page stolen from ka7oei.blogspot.com

2 comments:

  1. Hi - I noticed in your post that you said you were able to borrow someone's Ivie IE-30a analyzer. I'm quite desperately looking for one to purchase right now. Could you please lead me in the direction of the owner so I could make an offer on his? Thank you! ~ Lindsay, Los Angeles, CA (lindsaymorf@gmail.com)

    ReplyDelete
  2. In checking, that's the only tool of that sort that the owner has, so he's unwilling to let go of it at this time: If he sold it, he'd be in the same boat as you - looking for another! Sorry...

    All I can suggest is that you do what you've probably been doing: Checking EvilBay occasionally to see if something shows up.

    Although not perfectly calibrated, there are a number of "Sound Card" programs like Spectran and Spectrum Lab out there that do a credible job. If you can find a microphone supplied with calibration data I *know* that you can apply reverse calibration curves in the "Spectrum Labe" program (which is free!) and get accurate, very high-quality data from that.

    As far as a white noise generator: Any reasonable-quality sound card along with most audio editor programs (Adobe, Gold Wave, Audacity) will produce extremely good, flat noise with which speaker/amplifier systems may be tested.

    Best of luck!

    ReplyDelete





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