JBL L-40 Technical Help Requested

[ThomP]

Hi,

Thank you for reading this.

I'm new here and love this site. Hopefully some of you folks can help me with my old JBL speakers.

I bought a set of JBL L40 bookshelf speakers probably 25 years ago from the Appliance Store for $338 each or a pair... I forgot.

They've served me well although I've had to have the woofers re-coned once.

BTW, these are the 2 Way L-40s.... not the European 3-way variation that I hear exists.

Over the past years, however, I've been having trouble keeping tweeters in them. I bought original, factory replacements (033) from JBL, but they became obsoleted and I started putting in Morel MDT 033 tweeters http://www.morelhifi.com/products/raw_tweeters.html as a replacements. They sounded great, but they get fried too if I turn the volume up a bit too loud.

I am wondering, why do you think this is happening?

1. Could I have a "dirty" receiver/amp? (Technics SA-550) - It's rated at 70W/channel and the L-40 papers I have say that's the maximum suggested output for the amp. BTW, I was thinking of upgrading to a new receiver... a Sony ES-STR-DA2400ES which has 100W X7 for output.... Not a good idea for these speakers???

2. Have the crossovers gone bad due to the fact they they are an capacitive and inductive cuircuit and caps do change in value with age?

3. Am I just playin the things too loud? (It wasn't THAT loud!)

4. Are the Morel's a good match?

5. Am I getting some bass freqs sent to the tweeter somehow?

I think I'd love to get these things fixed again. I always liked them. Suggestions from you experts would be GREATLY appreciated!

Thom

P.S. Anyone thing those inline fuses are good? I see they sell tweeter fuses... Maybe I should invest in some.

THANK YOU!

I am only running the speakers with the system as I mentioned...

An SH-8055 Technics Graphic EQ
A few cassette decks and a turntable
And this receiver:

Technics SA-550 Receiver
Product Features:

Computer Drive
New class A
FM/AM Random 16Ch Preset Tuning
Manual and Auto Tuning
Stereoplex function
Subsonic Filter, High Filter, and Loudness options
Input Selector: Phono, Tuner, TV, CD/Aux 1, Video/Aux 2

[hjames]

Hi,

3. Am I just playin the things too loud? (It wasn't THAT loudf!)

I think I'd love to get these things fixed again. I always liked them. Suggestions from you experts would be GREATLY appreciated!

Thom

P.S. Anyone thing those inline fuses are good? I see they sell tweeter fuses... Maybe I should invest in some.

THANK YOU!

What gear are you playing them WITH?
Please to detail rest of system to get full picture of tweeter blowing technology currently in use.

Technics SA-550 - what are specs of this piece?-

JBL L40 (from jbl.com support pages)

SPECIFICATIONS First Manufactured 1977
Last Manufactured 1982
General
Recommended Power Amplifier Range 70 watts continuous, per speaker
Impedance 8 ohms
Woofer 10"
Tweeter 1"
Crossover Frequency(ies)1.8kHz
Sensitivity88dB (1 watt/1 meter)

Owner's manual - http://manuals.harman.com/JBL/HOM/Ow...nual/L40om.pdf

[grumpy]

The Morels should have been fine. That the tweeters keep blowing is
likely related to the amp and settings (55w/ch?).
Like to run the bass up or leave the loudness button pushed in?
Even if the bass driver can take it, the amp can clip without it seeming
"way" loud and take the tweeter out.

The L-pad should be able to make up the difference in JBL 033 vs Morel unit's
sensitivity, and absorb some of the power that would go to the tweeter
(L-pad setting? or even still working?)

... guessing your amp is clipping often enough to smoke tweeters now and
then.

[Allanvh5150]

I would agree with Grumpy to a point. Remember that the crossover point on the L40 is way down at 1.8Khz. All of the other cabinets that use the 033 are up around 3.7Khz. I would definately not play these cabonets loud. I have a pair of L86's and a pair of L150's that have the 033. I have never had an issue with them. On the off chance though, the crossover caps may have gone out of spec and the crossover frequency may have dropped. This is a longshot though and in my way of thinking, it is the low crossover point that is most probably causing the issues.

Allan.

[ThomP]

HJ,

The specs on the Technics SA550 (2 channel receiver) - It's a 70W per channel rated minimum sine wave RMS (20-20kHz .005% THD.
It also says: 1kHz continuous power output both channels driven 0.005% THD (75W per channel)

SMPTE Intermodulation Distortion: .01% (8ohms)

BASS TONE CONTROL: 50Hz +10db ~ -10db

TREBLE TONE CONTROL: 20kHz +10db ~ - 10db

Subsonic Filter: 30Hz -6db/oct

High Filter: 7kHz -6db/oct

Loudness Control (volume @ -30db): 50Hz +/- 9db

Low Frequency Damping Factor: 40 @ 8 OHMS / 20 @ 4 OHMS

I'd guess this to be vintage approx. 1989 (I'd have to dig out my receipt)

THANK YOU!

[ThomP]

Grumpy, Alan... THANK YOU for your replies.

Don't know if the L-pads are still working... I assume so. Way to check?

So this is probably more the louder BASS setting than the Treble setting?

Would that more efficient amp I was thinking of getting remove the clipping issue? The Sony ES2400 http://www.crutchfield.com/p_1582400...owAll=N&tp=179 is more powerful, but is it "cleaner?"

As far as the low crossover frequency... This may be right because I've been blowing tweeters for a long time... including the originals when I first got them. It's doubtful the crossovers went out of spec THAT early.

Could I replace the crossovers with one that operate at a higher freq? Should I even want to try?

I see in my JBL papers that they suggest max amp size is 70 W.... and that's what that Technics is... 70W per channel ... New Class A.... ?!?!?!?!?!

I hope Morel will repair that tweeter and I can try over again, but I'm tired of this... Maybe it's time to dump these L-40s if they're weaklings.

THANK YOU AGAIN!

[grumpy]

Don't know if the L-pads are still working... I assume so. Way to check?

turn them up and down... treble sound range should get louder and
quieter, and not drop out or crackle (at least not much).

So this is probably more the louder BASS setting than the Treble setting?

...only speculation on my part, given your described history with the speakers and a bit of knowledge about how tweeters often fry.

Ask Morel (or Madisound) about the application and crossover freq.
It's over 2x Fs and output hasn't dropped much crossed at 1.8KHz...
and you should be able to turn down the L-pads a "number" or two from
where they were with the 033's. Allan could be quite correct though.

Also, at the replacement price for current equivalent Morel tweeters
you could probably find a workable JBL ebay upgrade from the L40's

[pierce]

I had one piece of 1970s stereo gear go postal on me and deliver 70 volt square waves at 200khz regardless of the input signal. I couldn't HEAR it (maybe my dog could?), but smoke came out of the tweeters. We determined the problem when we plugged it into a dummy load on my friends workbench, and the load got _HOT_ fast, so we put a scope across the load and saw the square waves.

not saying this is your problem, but stranger things have happened. ok, this was a 250W/ch RMS Marantz Model 510 power amp. I scrapped the amp rather than trying to fix it.

[Allanvh5150]

I had one piece of 1970s stereo gear go postal on me and deliver 70 volt square waves at 200khz regardless of the input signal. I couldn't HEAR it (maybe my dog could?), but smoke came out of the tweeters. We determined the problem when we plugged it into a dummy load on my friends workbench, and the load got _HOT_ fast, so we put a scope across the load and saw the square waves.

not saying this is your problem, but stranger things have happened. ok, this was a 250W/ch RMS Marantz Model 510 power amp. I scrapped the amp rather than trying to fix it.

Yup, That'll do it. Mosfets have a tendancy to oscilate if untamed. The older Perreaux amps, allthough fantastic, have a couple of parts on the boards that can die. It makes no difference to the sound but the amp will oscilate at close to full power at upwards of 5Mhz. It can get very expensive in the speker department.

Allan.

[Allanvh5150]

Grumpy, Alan... THANK YOU for your replies.

Don't know if the L-pads are still working... I assume so. Way to check?

So this is probably more the louder BASS setting than the Treble setting?

Would that more efficient amp I was thinking of getting remove the clipping issue? The Sony ES2400 http://www.crutchfield.com/p_1582400...owAll=N&tp=179 is more powerful, but is it "cleaner?"

As far as the low crossover frequency... This may be right because I've been blowing tweeters for a long time... including the originals when I first got them. It's doubtful the crossovers went out of spec THAT early.

Could I replace the crossovers with one that operate at a higher freq? Should I even want to try?

I see in my JBL papers that they suggest max amp size is 70 W.... and that's what that Technics is... 70W per channel ... New Class A.... ?!?!?!?!?!

I hope Morel will repair that tweeter and I can try over again, but I'm tired of this... Maybe it's time to dump these L-40s if they're weaklings.

THANK YOU AGAIN!

With the L40 I don't think you would want to shift the crossover frequency, although I am not sure how hi the 117 woofer will go.

Allan.

[LRBacon]

Don't know if the L-pads are still working... I assume so. Way to check?

As far as the low crossover frequency... This may be right because I've been blowing tweeters for a long time... including the originals when I first got them. It's doubtful the crossovers went out of spec THAT early.

I hope Morel will repair that tweeter and I can try over again, but I'm tired of this... Maybe it's time to dump these L-40s if they're weaklings.

THANK YOU AGAIN!

Do you have a Volt/Ohm Meter or a Digital Multi-meter?
You could use one to check the L-pads to make sure they are working properly. Both tweeters went at the same time or one then the other?

You may very well be over driving your amp without realizing it and it is clipping. There also might be a mismatch between the amp and the speakers. Back in the early 70's my brother had a Marantz receiver and a pair of KLH speakers. When he would turn up the volume to what seemed like only a moderate level the speakers would start making a cracking sound. One day he had the KLHs hooked up to his friend's Pioneer receiver and they played fine at all volume levels. He got rid of the KLHs and got a pair of JBL L36s and the Marantz played them just fine.
You might think about upgrading to some JBL L36s, L110s, or L96s if you want to stay with a 10" woofer.
If you suspect the crossover network, I think I may have a pair of N40 crossovers. If you are interested send me a PM.

Larry

[Wagner]

Check the basics, your receiver is 25 years old and I'd be willing to bet has never had or needed any service or adjustments. Look for DC on the taps at idle. Warm up your amp for at least 10 minutes with volume all the way down, tone controls defeated or at "0" and no input.

Take a milli-amp reading with your meter at the speaker terminals. To do this either disconnect your speakers or use a set of unused taps after switching the selector for those taps.

You want to see 0-15 mV, but as much as 50 is OK. Anything more and there you go. If it takes a while to lose the tweeters then this is probably the culprit, you're cooking them them with DC. This is common with solid state amps as they age and is often over looked. It is gradual and a silent killer. The bias changes over time as the output devices age just like tubes so you don't notice it so much. After you have everything dialed in to where it should be you will be amazed at how much better an old amp will sound and your tweeters will thank you for it.

Take the amp to a tech and have the off set and bias adjusted.

Thomas
PS: My first post! Hope it does some good !

[Wagner]

Did you take a look at those taps for DC?
What did you find?
I failed to mention in my post above that you can do this adjustment yourself if you have a service manual, a decent multimeter and the confidence to follow the procedure.
It's usually pretty simple and straight forward. I don't think a consumer oriented manufacturer like Technics sold many inherently unstable designs.
Give this a try!
Good luck!
Thomas

[midlife]

I have gotten a pair of VERY clean L40's for my son. He is using an Adcom 535 amp and 555 pre-amp. The L40's do sound good but they seem to struggle on certain passages. Mostly on the vocals, for the most part musically they are quite nice. Do L40's have any known short comings. Or are there generally accepted upgrades for this model. I've taken the L-pads down to 4 from their previous #6 position and it has made them a bit more smooth.

[hjames]

These Things?
- JBL L40 2way speakers

(Tech Sheet has moved -
Was - http://manuals.harman.com/JBL/HOM/Technical%20Sheet/L40%20ts.pdf (dead link)

Now - http://www.jbl.com/resources/Brands/jbl/Products/ProductRelatedDocuments/en-US/technicalsheet//L40%20ts.pdf