JBL LE85 Damage From Drastic Crown D60 Failure

[Jjnet]

My tri-amped vintage JBL/Crown system failed with a sudden massive noise out the right speaker side. Imagine a smoke alarm battery warning buzz multiplied by about 10000. The 15" D130F woofers and 077 slot tweeters survived because their power amps were fine, but the LE85 midrange driver lost 6 db or so in volume relative to the left side. We've tried adjusting the Rane crossover and replacement power amp settings, as well as swapping cables and channels, but something significant has changed in the LE85.

Is a serious drop in volume typical of diaphragm/coil damage from this type of amp failure? I would have expected heavy distortion instead. But perhaps that's because I've only heard blown out woofers in the past. I can replace the 2420 diaphragm with a JBL D162421, but I would like to be more certain that this is the problem before I order it.

The next question is: if I have to replace the right LE85 diaphragm, should I plan on replacing the diaphragms in both sides for consistency? All the components are 40 plus years old and have been used as our home sound system. Thanks for any advice.

 

[phase_accurate]

Difficult to tell. Is its DC resistance still the same as on the other driver ? If you are mechanically skilled you may probably swap the diaphragms and see - or better listen - whether the problem has moved or if it stays in the same place.

Since this is a driver with alnico magnet, the magnet might have lost some of its magentisation. Although I can't really imagine that this could happen to a horn driver. But I've heard that it happens to woofers that have been abused. A good driver repair specialist should be able to re-magnetise a driver like that.



Regards

Charles

 

[weltersys]

1)Is a serious drop in volume typical of diaphragm/coil damage from this type of amp failure? I would have expected heavy distortion instead. But perhaps that's because I've only heard blown out woofers in the past. I can replace the 2420 diaphragm with a JBL D162421, but I would like to be more certain that this is the problem before I order it.

2)The next question is: if I have to replace the right LE85 diaphragm, should I plan on replacing the diaphragms in both sides for consistency?

1) Usually some point of the voice coil burns entirely, resulting in an open circuit and no sound. A fracture around the surround could result in a reduction of level, but usually with buzzing.
2) The 2420 is a half roll surround, the 2421 a diamond pattern surround, which extends the HF response. They won't sound the same. That said, if your ears are similar vintage as the components you are using, the difference above 10kHz won't be very noticeable ;^).

Also, due to the (different) work hardening of the aluminum over years of use, it would be hard to find two 2420s from different systems with identical response.

As far as a loss of magnitization, since the voice coil is not burnt entirely, the driver was not subjected to enough long term power to affect it, as Charles points out, that generally only happens with woofers, their thicker voice coils don't burn so quickly.

 

[kevinkr]

I originally had 2420 in my system, and would recommend replacing the diaphragms in pairs so that they continue to match well. Most of the aftermarket replacements are terrible so avoid the temptation to get cheap ones.

I'd recommend a moderately large film capacitor between the mid and high drivers (sized so that the 3dB point is at least a couple of octaves lower than your XO frequency) and their respective power amps to protect against DC and out of band signals. (Accidental cable swaps, XO malfunctions, etc.)

 

[eriksquires]

Just for the sake of understanding the problem.

You have no passive crossover right? There's no resistors caps or coils between the amp and the suspected bad driver?

If you answer no, then take the diaphram out from both midranges. Measure the DCR, or better yet the impedance sweep.

If reasonably close, swap the diaphrams and see if your volume comes back.

Best,


Erik

 

[badman]

Just for the sake of understanding the problem.

You have no passive crossover right? There's no resistors caps or coils between the amp and the suspected bad driver?

If you answer no, then take the diaphram out from both midranges. Measure the DCR, or better yet the impedance sweep.

If reasonably close, swap the diaphrams and see if your volume comes back.

Best,


Erik

No need to pull the diaphragms for this. Test them installed and if the quiet driver has a much higher measured Qes then it's demagnetized.

 

[Jjnet]

Thanks for all of the near instantaneous helpful replies. I will brave the 3 feet of snow on the ground to march out to our storage shed and get one of our stepladders to pull down the working left LE85 so I can compare DC resistance between the two. We're in the middle of yet another major winter storm here in SW Colorado, so I will try to do it before another foot of snow falls today.

My ears are just as vintage as the speakers, and even when I bought them in my early 20's I could not perceive above 18khz while my wife could from the same system. Even so, I should probably replace both diaphragms if I replace one.

No passive crossovers, just a Rane 3 way active crossover and three vintage Crown amps, 45 year old DC300 for the bass, newer D75 and D60 for the mids and highs respectively. In the past we used a pair of D60's for the mids and highs, and the older D60 powering the two LE85's is what blew. In hindsight it's remarkable that the DC300 and speaker components have been trouble free for 45 years. The Rane is our second active crossover and the D75 and D60 are from the last ten years, but otherwise the components have been unchanged from my mid-20's to mid-60's.

I will have to get some additional stuff to measure Qes, but if that's what it takes to determine the required solution, then I will get on it today. Thanks again for all the expert help.

 

[badman]

You can determine the impedance at various frequencies to plot an impedance profile and determine Q from that- it requires a tone generator and an ammeter.

Or a driver tester.

 

[eriksquires]

So, Badman, don't you think it would be best to try to measure the coils first?

I'm not very knowledgeable about horns, but I thought the compression drivers could be removed and they could be tested independently first. Is that a bad idea?

As for impedance sweeps, Dayton DATS V2 for the win!



Best,

Erik

 

[Jjnet]

Thanks for the info. I've got a tone generator and ammeter and should be able to figure out how to measure the impedance from the web in a couple of days.

 

[weltersys]

Thanks for the info. I've got a tone generator and ammeter and should be able to figure out how to measure the impedance from the web in a couple of days.

Simply run a tone sweep, you probably will hear buzzing from the voice coil dragging or the torn surround. The distortion will be most obvious at low frequencies, 100-400 Hz. The output of the tone generator is probably adequate for the buzz test, a fraction of a volt is plenty.

 

[badman]

So, Badman, don't you think it would be best to try to measure the coils first?

I'm not very knowledgeable about horns, but I thought the compression drivers could be removed and they could be tested independently first. Is that a bad idea?

You said to remove the diaphragm. You don't have to remove it to test the coil, nor do you have to remove it to test the impedance, the change from a magnet discharge or damaged coil will show up when installed, though you definitely want to test the driver disconnected from the XO.

Testing just the diaphragm requires disassembly of the compression driver, which is completely unnecessary.

 

[Jjnet]

The LE85 is recommended to crossover from the 15 inch woofer at 800 hz. Is there a risk of damage feeding raw 100 to 400 hz tones.to it?

It occurred to me that I could swap the diaphragm with the good one to determine if the diaphragm is the issue. I'm waiting on delivery of a non magnetic screwdriver before I consider touching the diaphragm. I'm sure that experienced souls can do without, but I'm sure it would be safer for me to use the non magnetic tool.

 

[nigelwright7557]

I had a pair of eminence speakers on a amplifier.
I noticed one day that one of the speakers wasn't as loud as the other.
When I checked DC resistance of the voice coils one was lower than the other by a couple of ohms. The amplifier must have put out DC on one channel and fried the voice coil shorting out some turns.

 

[weltersys]

The LE85 is recommended to crossover from the 15 inch woofer at 800 hz. Is there a risk of damage feeding raw 100 to 400 hz tones.to it?

It occurred to me that I could swap the diaphragm with the good one to determine if the diaphragm is the issue. I'm waiting on delivery of a non magnetic screwdriver before I consider touching the diaphragm. I'm sure that experienced souls can do without, but I'm sure it would be safer for me to use the non magnetic tool.

I forget whether the LE85 are 16 or 8 ohm, (16 reads about 12 ohms DCR, 8 around 6) but they easily handle 1 watt of sine wave (4 volts or 2.83 volts) down low, though you don't need to test at a watt to hear buzzing, which is caused by the voice coil or former rubbing in the gap. You do need to slowly sweep down low, as the driver will be loud enough to potentially mask any buzzing at higher frequencies. I use the direct output of my tone generator, probably only a volt or so.

The magnetism is highly concentrated in the gap, I have never used anything but a steel screwdriver for removing or installing diaphragms. The screws are (should be) stainless steel, so don't transmit much magnetism through them.

Your damaged driver may actually have a few coil turns melted off the former, it may be possible to varnish them back in position, though unlikely. It is also possible that the loud volume cracked off a bit of the bakelite phase plug material, and it lodged in the gap.

If your LE85 drivers are "virgin" you will need to heat a screwdriver to melt through the wax screw covers.

The gap should be cleaned using masking tape, don't stop till the tape comes out completely clean. When installing diaphragms they should be tested with the back cover off for buzzing, if any buzz is noted, partially loosen the screws and tap the diaphragm to align it in the direction that eliminates the buzz.

Unfortunately, far too many people just slap diaphragms in without aligning them, sometimes you can get lucky and no adjustments are needed, but typically they need some tapping, and the amount of crud in the gap can be amazing.

Art