JBL xover cap

[joetech]

I have a pair of JBL L-88. I've had them for 35 years and they still sound great. I was thinking of changing out their one weakness, the tweeter. They are just like the famous Century L100 but without the midrange.
Someone in another forum suggested the best way was to update the cap (it only has a cap an L pad to the tweeter) because modern caps are much more accurate and hold their values better. OK sounds like a plan that would be relatively inexpensive and easy.
Today I opened them up and sure enough one of the caps appears to be leaking. They still sound good but could go south at anytime. Only problem is all they say on them is "mexico 10391". No value markings. I use to work for a company that made automotive test equipment and all of our caps came in with values marked on the sides.
Does anyone have any idea what the value should be? I could measure what I have but I've never measured capatance before. I looked it up and it looks very complicated and so what if I did? If these have changed I would be getting the wrong value.
I E-mailed JBL but I've asked questions before and it took days to get a half *** answer.

 

[bpape]

Do some more research - but from what I can find in the JBL Misc Parts list - part # 10391 is an 8mf cap.

Bryan

 

[Swerd]

I have a pair of JBL L-88. I've had them for 35 years and they still sound great. I was thinking of changing out their one weakness, the tweeter. They are just like the famous Century L100 but without the midrange.

Someone in another forum suggested the best way was to update the cap (it only has a cap an L pad to the tweeter) because modern caps are much more accurate and hold their values better. OK sounds like a plan that would be relatively inexpensive and easy.

Today I opened them up and sure enough one of the caps appears to be leaking. They still sound good but could go south at anytime. Only problem is all they say on them is "mexico 10391". No value markings... Does anyone have any idea what the value should be?

Hey joe

I have info that appears to confirm that JBL part #10391 is an 8 µF capacitor. See the attachment which shows the schematic diagram of a JBL L-100 crossover. The high-pass filter for the midrange is the same 8 µF cap. In the 3-way L-100 the tweeter has a 3 µF cap and an L pad.

If you want to simply replace the cap with a new one, you could get an 8 µF metalized polypropylene cap from Parts Express (part 027-426, $2.75) or Madisound for about $3.40.

Before you do that, you should check out this web page by a Danish DIY speaker builder named Troels Gravensen http://www.troelsgravesen.dk/JBL-L26.htm. He restored a JBL L-26 and designed a new crossover that seems to perform much better than the original design. The L-26 and the L-88 appear to have the same tweeter. Depending on what woofer you have in the L-88, you might be able to adapt Gravensen's new L-26 crossover. He answers email questions and his English is good.

I have a pair of old L-100s that I am planning to refurb with a new crossover that I hope will smooth out the peaky midrange, and possibly a new tweeter to replace the old JBL cone tweet. I'll post details about that when I know more.

 

[joetech]

Thanks Swerd
I've seen the danish guys site with the L-26. I tried to e-mail him but the e-mail listed on his sites doesn't work or at least not anymore. This is the same tweeter but a different woofer.
I got an answer from JBL but the schematic they sent me has a 123a woofer (right) and a LE20-1 tweeter (wrong). Mine is LE25-1. Visably the only difference seems to be a square vs round mounting flange. The curcuit in labeled LX-12-1 and mine is marked N88-1. The LX 12 shows a 6uF cap, .4mH inductor and a 2.5ohm resistor and L-pad. Mine only has a cap (from your message and the other is assume 8uF) and l-pad only.
Maybe there was a design change at some time during the time they were making these. I bought them directly from a dealer that was going out of business in Cincinnati in 1974.

 

[joetech]

Just heard back from JBL (that was fast) Apparently there was more than 1 version of the L88 x/o made. Yes the cap in a 8uF.

 

[Swerd]

Thanks Swerd
I've seen the danish guys site with the L-26. I tried to e-mail him but the e-mail listed on his sites doesn't work or at least not anymore. This is the same tweeter but a different woofer.
I got an answer from JBL but the schematic they sent me has a 123a woofer (right) and a LE20-1 tweeter (wrong). Mine is LE25-1. Visably the only difference seems to be a square vs round mounting flange. The curcuit in labeled LX-12-1 and mine is marked N88-1. The LX 12 shows a 6uF cap, .4mH inductor and a 2.5ohm resistor and L-pad. Mine only has a cap (from your message and the other is assume 8uF) and l-pad only.
Maybe there was a design change at some time during the time they were making these. I bought them directly from a dealer that was going out of business in Cincinnati in 1974.

Interesting. The L-100s I bought in 1973 have the same 123a 12" woofer and LE-25 tweeter that you have, only the L-100 adds the midrange driver. I am going to make an educated guess that the woofer in the L-88 has no low pass filter at all (similar to the L-100), and that the tweeter has a simple 1st order highpass filter at 1.5 - 2 kHz. If that is correct, you could directly borrow Gravesen's new crossover for the L-26 which has a 10" woofer. You could cobble together a unsoldered version of that crossover made with cheap components and try it without mounting it in the cabinet. You might have to play with several different resistor values to balance the tweeter level with the woofer, but you could do that by ear in a trial and error method. That 123a woofer is amazing - it is smooth (on axis) out to 6 kHz. Try and find a 12" woofer made today that does that!

The email address I used about a half a year ago for Troels Gravesen was troels.gravesen@stofanet.dk