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JBL L36 Decade crossover inductor mystery

ericj

ericj

Well-Known Member
These big (well, medium size) old JBL speakers followed me home from the thrift store the other day.

There are no stickers on them but I am pretty sure they are L36 Decade because they look like the L36 Decade, they have 125A woofers, LE5-6 midrange drivers, and LE25 tweeters.

Sort of excited about this find because finally, I have something with veneer that is both not so bad that it can't be refinished AND worth refinishing, and the woofers look like they were professionally re-coned not too long ago. And the L36 seems to be well-regarded.

When I went through the first one I found all the original (or at least original type) crossover parts in pristine as-new spec, and even photographed the results for the caps because I couldn't believe that electrolytics from the 70's would still have ESRs under 0.02 ohms.

Going through the 2nd one, the parts look the same - to memory, the first one is fully reassembled. And the tweeter looks like it knocked around someone's workbench for a while - sticker is damaged. Maybe not the original tweeter.

But my $20 chinese component tester tells me the inductor is 20mh? Actually 19.9mh.

The technical manual for the L36 Decade says it should be 2.0mh.

https://manuals.harman.com/JBL/HOM/Technical Sheet/L36 Decade ts.pdf

In disbelief, I re-tested it several times and fished inductors of known values out of the parts bins. Indeed, it still reads my 3.0mH inductors as 2.9mH, my 1000uh chokes as 998uH, etc.

Is there some other speaker I could be mistaking these for? The xover traces out correctly other than the way out of whack inductor value. And the tweeter cap in this one is a gonner at an ESR of 1.3 ohms.

Maybe they ended up in the thrift store because they never sounded right after they got back from repair? The only real problem I found in the 1st one was crud in the L-pads.
 
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ericj said:
But my $20 chinese component tester tells me the inductor is 20mh? Actually 19.9mh.

The technical manual for the L36 Decade says it should be 2.0mh.
Click to expand...

Are you sure you aren't misreading something here, a 20 mH crossover inductor would be huge, the biggest I can find at Parts Express for example is 10 mH.

A photo of this would be very helpful IMO.
 
HypnoToad said:
Are you sure you aren't misreading something here, a 20 mH crossover inductor would be huge, the biggest I can find at Parts Express for example is 10 mH.

A photo of this would be very helpful IMO.
Click to expand...

I re-tested it about 8 times. Used a different tester too. And pulled some brand new inductors out to make sure the tester worked.

The inductors on these boards are pretty diminutive to begin with. And they are EI-core, like you'd more usually see in a power supply.

Here's the picture I took of the crossover in speaker number 2 showing the 1.4ohm esr result for the tweeter cap:

20190804_174451.jpg

Here's the picture i took of the crossover in speaker number 1 showing the stellar 0.00 esr result for the woofer paralleled cap:

20190801_000247.jpg

From this angle they look like the same part, right?

Close up, from some other angles, something is janky about how the core is assembled on the inductor of #2.

There's a part number on the side that doesn't match the part number in the spec sheet, was only visible after i removed the bad tweeter cap.

Do you know, the caps in these cardboard tubes are bare rectangles with the leads welded to the ends, packed in what looks to me like playground sand, with a slug of wax on top? Not rolled, an enlongated cuboid shape. Folded.

It's late and i don't want to go back to the workbench right now to look at the part number.

Speaker #1 is fully reassembled so i will have to open it back up and re-extract the crossover to double check the inductor value. the only hassle there is the glue where the midrange wires go into its little enclosure.
 
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As implausible as it seems, i think the component tester is correct and the inductor is way wrong.

3.0 inductor is from a pair of Ross Hurricane H-218CD 1000W PA speakers. The other one had burst into flames at some point.

20190805_132246.jpg 20190805_132704.jpg
 
Yeah it is hard to say. Someone has clearly worked on these speakers (sharpie marks of "1" on the backside of all the drivers in the 1st one, "2" on the drivers in the 2nd one, plus the worn sticker on the #2 tweeter) but the capacitors are genuine JBL parts?

The woofer surrounds look new and whoever installed them really knew what they are doing. There was a scrap of old rotted surround inside cabinet #2.

They sound about how you'd expect - excessively bright with not much in the lower mids / midbass range. If you turn the mid and tweet way down they just sound disappointing.

The original caps are weird. I mean they appear to be films but the way they are glued to the inside of the cardboard makes them very vulnerable to damage from manhandling them.

An 18awg 2mh I-core is about 76mm long, which is about 15mm longer than the current distance from the rectangular (16.5uf) cap and the tag board.

So i guess i will be carefully heating the hot glue to see if i can shift those caps over without damaging them before i place my next speaker parts order, in case i need some 16uf films too.
 
I was able to shift things around by carefully heating the glue.

Got my parts in yesterday and have assembled. The bass sounds good to me now.
20190813_144156.jpg
 
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