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cooked resistors

moebuster

moebuster

Super Member
here you can see the pair of paralleled resistors from on of my JBL L-96 crossover boards.
IMG_1738w_zpsfosuu0lt.jpg


the woofers needed refoaming and so no test have been done to speak of though I did hook both cabinets up to a low volume source when I get them initially and all the speakers played.
should I assume that the resistors need replacing though the drivers function?

what are the resistors specific functions within the crossovers?
 
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doing some research and if I understand the schematic:
N96_zpshnkaolms.jpg

then I need to replace the resistors in R3=2 10 ohm resistors in parallel to equal 20 ohms at 10 watts.
someone with a better understanding of schematics than I ( not difficult) please confirm.
 
For resistors, unlike caps, the values sum in series, not parallel. See http://www.wikihow.com/Calculate-Series-and-Parallel-Resistance

Parallel 10s would result in a 5-ohm DCR. Series 10s would equal 20 ohms, but the resulting power-handling would be poor.

You could run two 40s in parallel to raise the power-handling capacity (probably what JBL did already there), or better yet, parallel 12-watt 40-ohm Mills.

OK, I see that Parts Connexion does not have 40s, but it does have 39s and 1s: http://www.partsconnexion.com/resistors_mills_mra12.html

So you could parallel all four (two 1s and two 39s) to get 20 ohms. The combined trunk of resistors would have a pretty high power rating.
 
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For each speaker, I would buy two 1s and two 39s, if you can't find two 40s. Either way, run them in parallel.

10 watts is OK; 12 watts would handle more power.
 
The resistors probably don't need to be replaced, although resistors aren't particularly expensive.

Going up to a slighly higher wattage resistor won't make them run appreciably cooler, especially if the replacement resistors are smaller in size. Smaller size but higher rating isn't unheard of these days considering material advances.

One question I have is, is that model of speaker known to burn the board like that or is it possibly something else going on/happened that caused it?
 
I don't know how you can tell which resistor in the schematic those two paralleled resistors correspond to without removing them so you can read the numbers on the bottom. Which is really the top, why in the heck did they mount these upside down?

Anyway, as to the specific functions, they are generally adjusting the volume of the tweeter and midrange (padding resistors) and/or contouring the sound from each driver (reducing signal only in a particular frequency range).
 
toxcrusadr said:
I don't know how you can tell which resistor in the schematic those two paralleled resistors correspond to without removing them so you can read the numbers on the bottom. Which is really the top, why in the heck did they mount these upside down?

Anyway, as to the specific functions, they are generally adjusting the volume of the tweeter and midrange (padding resistors) and/or contouring the sound from each driver (reducing signal only in a particular frequency range).
Click to expand...

great question! I thought the same thing.
it's not like they run cooler upside down.

so perhaps I don't need to replace them after all?
 
whoaru99 said:
The resistors probably don't need to be replaced, although resistors aren't particularly expensive.

Going up to a slighly higher wattage resistor won't make them run appreciably cooler, especially if the replacement resistors are smaller in size. Smaller size but higher rating isn't unheard of these days considering material advances.

One question I have is, is that model of speaker known to burn the board like that or is it possibly something else going on/happened that caused it?
Click to expand...

I have a pair of KLH3's that had the same burned board at the resistors,which were open and so I replaced them.

I wonder what causes the resistors to heat up and fry?
overdriving the amp/receiver?
 
i would replace all of them because they are cheap to buy and look a bit stressed ..
then when putting it all back together be sure the insulation is far enough away as i suspect this is why they have cooked . they need to breathe ..
 
Good idea, these will cost a buck each give or take so replace them. You could add a heat sink, or at least find a way to space them out a bit.

And yes they overheat due to excessive wattage being cranked through them.
 
petehall347 said:
i would replace all of them because they are cheap to buy and look a bit stressed ..
then when putting it all back together be sure the insulation is far enough away as i suspect this is why they have cooked . they need to breathe ..
Click to expand...

your right. they are cheap and 044 tweeters aren't growing on trees!

gonna go now and pull the crossover boards and see exactly they are and get them ordered.
 
so I got the board out and connected a meter to the resistors and they appear to be in spec-
the two paralled 10 ohm pair read 5.2 ohms.
IMG_1742xs_zpshurdfphx.jpg
 
That board is seriously cooked. Maybe a silly question, but is the trace side undamaged? Honestly I'd still replace them because of how extreme that looks.
 
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.
 
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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. 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...

doesn't the schematic show 2 10 Ohm?
 
an air gap under them should help .. not sure what to put under them to hold them up though .. couple of wedges of something that wont burn or transmit heat ..
maybe as suggested the speakers were driven hard .
have seem much worse including burned drivers and burned amp ..that's why i wont use passive crossovers in a high power situation .
 
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