JBL 120Ti driver issue ... Please help

[max_thx]

Hello, I have a pair of JBL120Ti speakers which I enjoy very much. Recently, I realized that the low freq driver in the right channel plays significantly lower in volume that the left channel. I tried to swap the channels on the speaker terminals and this indeed confirmed that the issue is not the amplifier. The faulty driver is not completely dead.. just much lower in volume. I've done some searches on this particular model but so far the issues reported are related to the tweeters... My tweeters appears to be working fine.

Do any of you know how do I go about troubleshooting this issue? Could this be due to the low-pass filter components going bad?? I have some basic circuit knowledge and soldering skills... Thanks in advance!

-Max

 

[goodolpg]

Try swapping the woofer into the other cabinet, if still lower that would indicate the woofer is at fault.
By the same token, if you swap the "louder" woofer into the other cabinet and it is then lower in output then something in that channel is messed up. Either the speaker itself or the source (receiver, amp).

 

[avguytx]

Or possibly the crossover. But without seeing a schematic for it, its hard to say if there's something in the low pass filter that could've failed.

 

[max_thx]

I just did the swap of the woofers... it's weird because now they are working perfectly balanced and I have no idea why... I also measured the impedance at the driver terminals and both are around 6 ohms. I checked the crossover circuit for any obvious signs of bad components and they seem to be perfect visually. I did notice though, there were a few shredded bits of black foam like material under the woofer. Not sure where they came from, is this a bad sign? Perhaps this foam like material got stuck near the coils - causing it to not work properly?

-Max

 

[imral3]

Glad it's now working for you! Maybe a loose wire that is now secured via the swap?!?

 

[max_thx]

Thanks! Loose connection is possible but unlikely. The bare wire ends were secured with the spring type connector at the drivers and they were on pretty tight.

 

[Gazdatronik]

Thanks! Loose connection is possible but unlikely. The bare wire ends were secured with the spring type connector at the drivers and they were on pretty tight.

The bite doesn't last forever especially between dissimilar metals. Sounds like you cleared out the cobwebs.

 

[SterlingOne]

Thanks! Loose connection is possible but unlikely. The bare wire ends were secured with the spring type connector at the drivers and they were on pretty tight.

Pretty tight? The other day I noticed no left channel woofer output from my JBL L100t3's. Removal of woofer revealed spring loaded connector failed to retain bare wire ends. BTW, that disintegrated black foam could be from your woofer surrounds. The foam surrounds usually only last about 30 years before they crumble, causing need to re-foam.

 

[goodolpg]

If you did nothing except swap the two woofers, then it sure seems there was a "bad" connection to the one.
I chased a problem with a tweeter that seemed to have low output.
Put in a known good tweeter, still low or no output.
One day, who knows why, I pulled the tweeter and tried again. Ah-ha!
There is putty where the tweeter wires go through the baffle, some of the putty had gotten into the cable end (push on type) and was the reason for no/low output.

I also had a situation where a fuse holder was the problem. The pinch type holder just wasn't making a good solid connection with the fuse on one end. Never hurts to clean and check the fuse and holders once in a while.

 

[max_thx]

I think you folks might be right. I acquired these speakers from the original owner - and they haven't been serviced whatsoever. The connectors were left alone for several decades... I should probably reattach the terminals on all the other drivers as well. Thanks for all of your inputs! They sound absolutely amazing now!

 

[SterlingOne]

I think you folks might be right. I acquired these speakers from the original owner - and they haven't been serviced whatsoever. The connectors were left alone for several decades... I should probably reattach the terminals on all the other drivers as well. Thanks for all of your inputs! They sound absolutely amazing now!

Check the woofer foam surrounds for deterioration. There's no way original foam surrounds would have not begun to crumble. You can replace them yourself or have it done professionally by the Speaker Exchange or Simply Speakers.

 

[max_thx]

This is the amazing part... the original foams look really good and still soft. I don't know if this is because of the mild weather in San Diego or how they were stored. The original owner had them hooked up in the garage and play them for many years.

 

[LHorn]

The foam in the cabinet could be from a previous surround failure and has been refoamed since...unless you know the original owner in fact didnt service them. I have been dabbing the terminals with Deoxit whenever I work on connections for preventive maintenance.

You should post some pics...you dont see the 120Ti's very often.

 

[EarlK]

This is the amazing part... the original foams look really good and still soft. I don't know if this is because of the mild weather in San Diego or how they were stored. The original owner had them hooked up in the garage and play them for many years.

If your 128H woofers looked like this from the back;

then the foam bits you found might have been from the internal foam filter that's glued over the that VC vent ( blow-hole ) .

When new and fresh, the foam looks like this ( top-row, middle pic within the collage ) ;

That 1980's era foam also disintegrates ( like the surrounds ) eventually crumbling away.

It can lodge in the VC gap restricting movement of the cone ( & that'll reduce some of the woofers over-all level, especially in a three way with the mids handled by another driver ) .

At high user levels the foam crumbles can be reheated turning them into "gum" ( in the gap ) that can ( in extreme cases ) burnout the VC .

Exercising the woofer with a 20hz tone ( face down put propped up on some standoffs to allow cone movement ) will bring the particles to the center of the dust-cap where the ( truly brave ) can vacuum them out with a computer ( vacuum ) attachment ( for computer keyboard cleaning ) .

If the back of your 128H looks like this ;

> then all the foam-filter bits are still trapped inside the VC area waiting to cause you problems ( they really need to be removed ).

Those foil-cals are removable with a heat gun loosening the glue holding the metal cal > then a gentle pry with the edge of a blade to lift the first edge, then sliding the putty knife underneath the foilcal as the heat does its job of fully releasing the glue bond . Since the glue seems to re-activate somewhat it may be heat-glue.

 

[max_thx]

Here you go!






They are serving as computer speakers at the moment (and a place to hold my plant...) Except for the minor dent on one of the mid drivers (first photo) and missing peg + beam on the grill, they are in pretty good shape. My plan is to construct the peg with a 3D printer and replace the cloth.

 

[max_thx]

Earl, you totally nailed it! My woofers look like your first picture with the blow hole. The crumbling foam found inside the cabinet do look like the one shown in your second picture - at least the colors match. I'm not sure if I'm brave enough to use a vacuum to suck them out. Is there a way to take these apart? And to replace the foam? I assume it serves some purpose, correct?

-Max

 

[SterlingOne]

OP, the pics clearly indicate the speakers have indeed been re-foamed with new foam surrounds; but, not done expertly as evident from the glue pulled at woofer edging. BTW, there are some suction techniques which might get out that cap dent if it bothers you. If not bothered by cosmetics there would be no point to refoam again unless the woofers do not move freely, a condition which would be caused by not properly centering woofer when refoaming. Also, if you were to buy speakers today from JBL that sound as good as yours, you'd be buying the 4429's at $5000 a pair. One more thing, the midrange speakers appear to have new cones or units are non-OEM products as wiring leads are on underside of cone.

 

[EarlK]

One more thing, the midrange speakers appear to have new cones or units are non-OEM products as wiring leads are on underside of cone.

Here's a look at an OEM recone kit.



Those 104H cones look correct to me.

The 104H is a very different beast than the 104H-2 ( which does display the VC lead wires on-top of the cone ).

 

[EarlK]

Earl, you totally nailed it! My woofers look like your first picture with the blow hole. The crumbling foam found inside the cabinet do look like the one shown in your second picture - at least the colors match. I'm not sure if I'm brave enough to use a vacuum to suck them out. Is there a way to take these apart? And to replace the foam? I assume it serves some purpose, correct?

-Max

The foam serves to filter ( stop ) metal particles from finding their way into the VC gap and scoring ( shorting ) the voice-coil ( resulting in the need for a recone ).

The filter can be re-located to the back of the magnet for easier maintenance ( application ).

I'll post a pic of the vacuum attachment that I have successfully used.



Okay, here's a pic of the kludged vacuum attachment for use inside the dust-cap area.

A lot of the suction is bypassed with the arrangement.

TME, there's just enough suction to get the job done.

 

[LHorn]

Nice 120's.