L-300 077 question

[Domino]

I am the original owner of a pair of JBL L-300 Summits and I just got the Rives Test CD and tried it. I am noticing that the highs above 10khz are down by quite a bit on the Radio Shack analyzer on both speakers especially compared to my other speakers which consist of a pair of IMF Monitors and a pair of JBL 4313B's. I'm using the corrected tones on the CD that are made for use with the RS analyzer. The sound coming out of the 077's just doesn't seem to be what it should be even with the level turned all the way up. I wonder if I have deteriorating crossovers or what.
Any ideas or similar experiences?

[Rolf]

I am the original owner of a pair of JBL L-300 Summits and I just got the Rives Test CD and tried it. I am noticing that the highs above 10khz are down by quite a bit on the Radio Shack analyzer on both speakers especially compared to my other speakers which consist of a pair of IMF Monitors and a pair of JBL 4313B's. I'm using the corrected tones on the CD that are made for use with the RS analyzer. The sound coming out of the 077's just doesn't seem to be what it should be even with the level turned all the way up. I wonder if I have deteriorating crossovers or what.
Any ideas or similar experiences?

Hi and welcome to the forum.

Are you sure there is any sound at all? Try the highest tone you know you can hear and put your ear on it. You should also hear the 077 on normal music if you do the same.

Remember, the higher the tone, the more important to sit on the right place.

Rolf

[johnaec]

I will say that when I first got my 4315A's the level of one of the 2405's was way down - swaping the drivers showed that the problem was in the crossover, since both drivers were way down when in that cabinet.

I happened to have some spare 4315 crossovers so I replaced the main cap for the UHF in the flakey crossover and it immediately brought the level back up. I don't know if it was the cap itself or bad solder joints, but it definitely solved the problem.

Your situation is somewhat unusual, in that both sides are down in level. I'm not sure the best way to track that down...

John

[Domino]

The fact that both speakers sound exactly the same is puzzleing me also. Maybe they are ok but they definately don't have as much energy above 7k or so as the 4313's do.

[JuniorJBL]

another tweet of any kind that you might be able to swap out?
This would require careful (and low level) testing. If the new driver sounds low then you may have crossover problems. As was said before do the 077's have any output? You should determine this first.
Hope this helps.
Some of the other forum members may have better paths for you to follow.;)

[Audiobeer]

With both sides being down....well the chances of both speakers suffering the same thing makes me lean towards the source. However you stated that you stepped back and hooked up your other pair and the problem goes away? I would double check and clean all your connections. I would then turn your L Pads back and forth a few times on the upper frequency side. If that doesen't work I'd have to say your 1 in a million in that a component in your crossovers both went bad at the same time. That's a long shot. Could you have blown the tweeters some how? P.S. This is advice from a dummy. The experts here may help you. They have me in the past.

[edgewound]

Try cleaning the L-pads with something like Deox-it....maybe they're dirty and intermittant.

[Domino]

I don't have another tweeter to try without taking something apart. There is no doubt that the highs in the 4313's have a larger output in that region than what I'm getting from the 077's. I have tried working the L-pads back and forth.

[edgewound]

Try the simple solutions before quasi major surgery. You could check the Voice Coil DC reistance to see if its more likely the crossover, L-pads or the tweeter. Have the L-300's been dormant or used regularly? Did you have...or maybe your kids...heh, heh;) have a party lately where amp clipping could have taken the tweeters or L-pads out? Do you smell anything burnt? Maybe corrosion from dormant 077? Lotsa possibilites.

__________________

[Baron030]

Hi Domino

While, you may really have a problem with your 077 or crossover network. There is one thing that you should be aware of when working with the Rives Test CD. Each "1/3 octave" test tone is really nothing more than a single sine wave frequency. Since, that test CD does not contain "1/3 octave pink noise", each test tone is very prone to room resonances. As a result, your Radio Shack sound pressure meter can be giving you some false reading. The main advantage of using pink noise over a single sine wave is that room resonances are averaged out over a "1/3 octave" range of frequencies. Also, the slight variations in a driver’s sensitivity can be misinterpreted by using a single frequency test tone. Since, most of us don’t have access to an anechoic test chamber. Here is a possible work around for room resonances. You could perform your frequency response tests outside with the enclosure lying on the ground with the drivers pointing skyward and at least 30 feet from any building. My own experience with the Rives Test CD is that I have recorded loudness versions of over 10 dB just by changing the microphone’s position by a few inches. So, indoor measurements with the Rives Test CD are really very questionable. I hope your problem turns out to be a testing isssue and not a real problem with your 077.

[John]

Why don't you just spin a copy of "Dark side of the moon" and then let us know if it sounds the way it should?

[Mr. Widget]

As Rolf suggested, the first thing you should do is determine if anything is coming out of the 077s. If you have an FM tuner play inter station hiss or a noisy LP, or even your test CD. Remove the lens from the LE85 and put a small pillow in front of the horn. Now play the speakers and listen to the 077 directly.

Once you determine if there is sound coming from the 077, then you need to determine the source of the problem. We can help you trace the problem by systematically eliminating possible problems. With a properly working pair of L300s the 077 should be able to hurt you, if overly cranked.


Widget

[Domino]

There is definatley sound coming out of both 077's. I can also vary the level with the brilliance pot. I have a pink noise generator in an old ADC spectrum anaylzer that I have had for years. It has the calibrated mic and everything. The screen has a row of leds for every octave the highest of which is 16K. I'll try hooking that up and see what I get. I'll even try a different power amp when I rotate the L300's back into the line.

[Domino]

I followed your advise Baron 030 and what you say is very true about the test CD. I retested the speakers at about 1 meter distance with the analyzer and the Rives test CD and this time used a tripod for the analyzer and the results were much better. The 16k was now about -6 db and the 20k about -8 or so with the Lpad set at "5". At that setting the 12k track was +4 db. All compared to the 1khz track. Moving the analyzer made large differences in the readings as you said. Enough of this. I'm confident once again with the speakers.
Thanks again to the forum for all the ideas!
:)

[Mr. Widget]

Enough of this. I'm confident once again with the speakers.


If I understand what you are saying is that you were happy with the speakers, but became concerned after making a measurement? I guess all well that ends well.
Another example of why we shouldn't rely on meters and such...

Widget