View Full Version : beloved jbl century 100's

bob newton
08-17-2004, 05:24 PM
I have 2 l-100's that I purchased new around 1973 along with a Sansui 9090 that has an output of about 110 watts per ch. My brother had the system for about 15 years and when I got it back ,It had 2 blown le5-2's and 1 blown le25. I replaced the bad speakers and 2 days later one le5-2 was blown again.So I bought 2 more but I cannot afford to replace speakers every 2 days. Back in the 70's I ran the speakers at full volume and full everything and never had any problem. Could the caps in the crossovers be bad? Any help is much appreciated in advance.

08-17-2004, 06:22 PM
Possibly, but how is the controls on the 9090 scratchy?

08-17-2004, 06:23 PM

Somethings up. I have a pair from 1974 that were virtually indestructable with a Dynaco ST-400 driving them. I would always give in first. I would see if the caps are shorted some how. Can't think of what else it could be. You have a meter??? Just replace them.


Lots of links associated with this thread.


08-17-2004, 09:28 PM
You might want to have the 9090 checked out. I've seen Sansui 8080s and 9090s in which the amp transistors and caps got "tired", and the bias, DC balance and distortion figures were pretty far out of spec. At low volume, they'd sound pretty much fine (sometimes a little "scratchy" or "raspy" at low volume), but as things got near max, they'd clip and do NASTY things on the output waveforms. Given the simple crossovers in L100s, that kinda shennanigans from an amp, will take things out in a heartbeat...

I'd recommend taking the amp in, and have a tech put it on a dummy load with an oscilloscope monitoring the output, and drive it at various levels. Bet he finds something at least a little amiss...


bob newton
08-18-2004, 07:13 AM
OK guys, Thats pretty much what I was suspecting. It is just hard for me to understand why the speakers could take so much abuse in the 70's and now burn out at less than half power. Although I am not a tech, I do have meaters and 2 scopes,including a dual trace. I also have enough knowledge to check the different components, which makes me kinda dangerious! And I have the service manual for the 9090. So, I guess I will spend the next few nites in my little shop downstairs.I welcome any advice or warnings,even if it is tecknical. I should understand it.
Does anyone know of anyone that repairs le5-2 speakers,and what it cost to repair them? The coils are open.

Very glad I joined this place! Nice to talk with people that love JBL as I do. And thanks for the fast replys.

08-18-2004, 09:54 AM
OK, but PLEASE now that you've made us aware of your problem, let us know what you find so we's can all learn somethin'.



bob newton
08-18-2004, 01:54 PM
I intended to. And I will be in touch for some new grill

08-20-2004, 11:07 AM
My Marantz 300DC power amp developed a similar problem reciently. The right amp would run hot while the left side runs cool. If your amp is like mine-dual mono, swap one section at a time to see which section is at fault. Measure DC mV as well as for DC voltage at the speaker terminals. Use a dummy load or a pair of "test" speakers for this test. If you measure DC, use the dummy load only.


bob newton
08-20-2004, 11:11 AM
What kind of dummy load? 8 ohm resistor ? how many Watt?

08-20-2004, 03:46 PM
My amp is rated at 150 watts/channel into 8 ohms. I am using a 100 watt, 8 ohm load resistor. Be careful. These resistors get very hot if you put a signal through the amp. What you should look for is any DC at the speaker (dummy load) terminals. Set your meter to DVC. If zero, set the meter to the mV position. This will tell you what's going on while the amp is at idle. Remember, there should be no signal input to the amp.


Charley Rummel
08-21-2004, 07:20 PM
Your Sansui 9090 DC voltage in your power amp stage needs serious balancing. Some solid state amps have pots on the board (like your 9090) which can be adjusted; otherwise the bias resistors on your output stage may be at fault. Don't waste any more money on your L100 components 'til your 9090 is checked out.


bob newton
10-14-2004, 04:22 PM
OK, I finally got around to checking the 9090DB I measure .02 vdc on one channel and .04vdc on the other. Is this enough to cause the problem? If so , where do I look? Gotta be a cap somewhere in the circuit. I am using the sansui speakers for the testing and it doesn't seem to have any effect on them. The sansui's are rated for 70 watts and the l-100's 50 . Am I putting too much into the l-100's? Come on guys, I need your help!

:yes: :cheers: :duck:

10-14-2004, 07:55 PM
I'm not an ET, nor can I answer the 1st question. I can say that If you had a 300 watt amp it wouldn't be to much as long as you didn't drive the speakers beyond what they were intended. I think everyone will agree that having twice the wattage output in an amp over watt your speakers were designed to handle would be a good thing!

bob newton
10-14-2004, 09:12 PM
Yeah, Thats what I thought too. The amp is rated at 110 wpc but I heard that the 9090 has been measured at 150 wpc. Does anyone know just what the watt limit is for L-100? Factory specs of 50 watts is out to lunch! It is a very conservative number. Everybody knows that the speaker can handel much more. But how much more? And I already know that it must be clean power.:yes:

bob newton
11-12-2004, 08:09 PM

bob newton
11-12-2004, 11:52 PM
Not for nuttin, my JBL's L-100's are puuuuuurrrring!! The problem was da amp!! The Sansui 9090 had bias problems. YOU SHOULD HERE THEM NOW!! The L-100's WOW!! Thats what I call GETTING IT ON !! Question for BO, Is there life after Century 100?:duck: :no: :wave:

11-13-2004, 07:33 AM
Question for BO, Is there life after Century 100? If you stay in the family, it only gets better...

bob newton
11-13-2004, 07:50 AM
If you stay in the family, it only gets better...


All I ever heard from JBL is L-100 and the cearity is uncanny compaired to other brand spealers. Its hard for me to beleave that they could make something better -- louder maybe , but better sounding? Hard to beleave. Maybe someday I'll get to hear one of these "IMPROVED JBL's"

Question - Does the 4311B sound better than the L-100? I know one has ALNICO and the other has the other type magnet. And I suppose the crossovers are different too. But is the 4311B suppose to be an improvement over the L-100?:cheers: