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cooked resistors

all my speakers have insulation on the resistors, kind of impossible to keep it away...never seen that before...something else going on...
 
Thanks for the advise.
i'll disassemble both L-pads.

ordered these earlier-will they do the job?
http://www.ebay.com/itm/160460119409?_trksid=p2057872.m2749.l2648&ssPageName=STRK:MEBIDX:IT



SoundOfSound said:
I would inspect the R4 L-pad. If it were to go open or get to a hi impedance state because of it being dirty it would force a lot of current through R3.

Also check those mid drivers for damage.

PE has some 20 Ohm resistors that will do the trick.
http://www.parts-express.com/dayton-audio-dnr-20-20-ohm-10w-precision-audio-grade-resistor--004-20

No affiliation.
Click to expand...
 
moebuster said:
so- do I need a 20 ohm at 10 watt resistor?
Click to expand...

If there's no problem buying new parts, sure. However, the two 10 ohm, 10W resistors in series would accomplish the same thing from a technical standpoint.

Myself, I sorta like things to be the way they're supposed to be unless there's a really good reason to do it differently.
 
I'm not sure if I follow you but the "good" cabinet I opened up to compare to has only one (I presume) 20 ohm resistor in place so I wanted to do the same. I could have used the two 10's but it was going to require fiddling with finding something to extend the resistor leads.


whoaru99 said:
If there's no problem buying new parts, sure. However, the two 10 ohm, 10W resistors in series would accomplish the same thing from a technical standpoint.

Myself, I sorta like things to be the way they're supposed to be unless there's a really good reason to do it differently.
Click to expand...
 
It's a long way of saying the two 10 ohm, 10W resistors in series will work, but I'd probably get the 20 ohm, 10W resistor because that's the way it's supposed to be.
 
right of course-I'd gotten that.
so the cause may have been a faulty L pad?

my KLH3's had similar but no where near as much damage to their board and they don't have L pads...
 
Zonker92 said:
I think someone went in there and incorrectly put those in parallel. They should have put two 40s in parallel, or one 20 (as original), or two 10s in series. The result was 5 ohms instead of 20. I believe that this may have diverted power away from the midrange driver and put two much current through those resistors. I'm not a crossover expert though. That's just my guess.
Click to expand...

You are absolutely correct. That resistor, instead of being 20 ohms, was effectively 5 ohms- which dissipated something like 4 TIMES the power of the original 20 ohm resistor, as well as screwed up the response of the midrange (the crossover is NOT designed for that low of an impedance load).

The recommendation to use two paralleled 39 ohm or 40 ohm resistors is right on. That will give extra power handling, and will give the right crossover response. Given its shunt position across the driver (effectively), the difference between 39 and 40 ohms per resistor, will not be audible. In fact, the allowable tolerance of those resistors is greater than the difference between 39 and 40 ohms, anyway...

Regards,
Gordon.
 
soooo...
I should buy what? (I'd already ordered "stock" 20 ohm at 10 watt resistors)
if possible I want to use a single resistor in the original position
be it 20 or 40 ohm.
please direct me here. I'm not clear on what to do at this point.

edit:is the suggestion to parallel 2 40 ohm resistors to yield a 20 ohm load
because it will offer better power handling for the mid drivers?


GordonW said:
You are absolutely correct. That resistor, instead of being 20 ohms, was effectively 5 ohms- which dissipated something like 4 TIMES the power of the original 20 ohm resistor, as well as screwed up the response of the midrange (the crossover is NOT designed for that low of an impedance load).

The recommendation to use two paralleled 39 ohm or 40 ohm resistors is right on. That will give extra power handling, and will give the right crossover response. Given its shunt position across the driver (effectively), the difference between 39 and 40 ohms per resistor, will not be audible. In fact, the allowable tolerance of those resistors is greater than the difference between 39 and 40 ohms, anyway...

Regards,
Gordon.
Click to expand...
 
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It's unclear what is the root cause of the burnt board. Sure, at some point something there got really hot, but that's not the root cause. The root cause is what caused that something to get hot in the first place.

In the absence of that root cause, and with the presumption that JBL basically knows what they're doing, my suggestion is to put in the functional equivalent of what the schematic calls for. That's 20-ohm, 10W resistor.

The most convenient (and original) way is with a 20-ohm, 10W resistor.
 
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Thanks who-

that's what I thought but....
anyway-
just came in from the garage to look up some info on the L pads
I have them out and of course one(midrange) from each cabinet looks pretty toasty.

any input on how to test/verify that they are working with a meter?

I do see that the inner coil on at least two of the L-pads is loose on one end.
 
Cool, Gordon; thanks! Even a blind hog finds an acorn now and then.

Moe, two 40s or 39s in parallel should sound the same as one 20, but with more power capacity. Your call, as to which you prefer. One 20 should be OK I guess.

On the L-pads, test the DC resistance between legs 1 > 2 and 2 > 3 (they are stamped with numbers); each forms one pathway with variable resistance. One circuit should climb smoothly as the other falls, and vice versa. The resistance should not climb or drop suddenly except at one end of the rotation, where it should climb to infinite DC resistance. Each pad should show roughly the same operation as the other on each set of legs.

But the damage you describe would make me replace them anyway.

 
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GordonW has it spot on, I don't know why I glossed over that when the OP originally posted the schematic. Yikes.

I don't see any doubt about how the board got toasted. Someone replaced a 20 ohm resistor with 5 ohms. I suppose the 20 ohm could have gotten fried by overdriving the speakers in the first place, who knows.

It's clearly supposed to be 20 ohms and if the maker designed it that way it will probably be just fine assuming all the other components are OK. If you're concerned about it get the 2x40's and parallel them, but it doesn't seem critical to me.
 
Re: L pads, you should be able to measure across the two outer lugs and get the total (max) resistance. From the center lug to either end should give variable resistance as you turn the shaft, if it's working OK. Edit: I see Zonker beat me to it.
 
If we're subbing parts, dissipation-wise the two existing 10 ohm, 10W resistors in series provides the same dissipation capability as two 40 ohm, 10W resistors in parallel.

So, if the choice is to increase dissipation capacity, the existing ones could be re-used.
 
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True, but they were wired in parallel and are possibly cooked. I don't think I'd reuse them, considering the low cost of replacements. JMHO.
 
They show no signs of heat damage and test in spec.

I do not believe those particular resistors caused the heat damage to the board.
 
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