Please help fix my sub

Discussion in 'Archived Threads 2001-2004' started by Steve_D, Aug 1, 2001.

  1. Steve_D

    Steve_D Second Unit

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    OK, this may be like someone trying to describe a computer problem to me over the phone but its worth a try. Warning, I know nothing about these types of electronics, in computers if a board is bad you replace the whole board.
    The Sub is a 2 year old passive Citation 7.4, which has a 14" 18 lb JBL woofer built into a large heavy ported cabinet.
    The problem is after playing test tones for awhile (5-15 minutes)it gets about 7 db louder. In other words, as soon as I calibrate, its "gets warm", and throws my calibration off. This is very worrisome for a sub, as you all know if may be basically inactive for an hour then be called to perform!
    So here's what I'm pretty sure it isn't, I've tried 2 different 5.1 receivers, 2 different amps, and checked, rechecked, and rechecked all the connections.
    I opened up the sub and as most of you know its basically a binding post connected to a electrical board on to the woofer. All the wires and sodders looked and felt nice and tight. The binding post is so simple I don't think it could be that.
    So that leaves the woofer and the electrical board. This board consists of 4 white identical rectangles, each marked 10W25OmegaJ. All 4 of these rectangles seem to be connected to each other, and then to a large copper wound tube. Here, a picture may be worth a thousand words:
    [​IMG]
    The woofer has never been pushed to bottoming, and rarely pushed hard. If it's the woofer I may be SOL (Or SVS!), as I don't know where to find 14" JBL replacements.
    I guess I can take each part to a speaker repair place and have them tested, thought I'd check with the real experts first.
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    http://www.sdiver.org
    [Edited last by Steve_D on August 01, 2001 at 07:26 PM]
     
  2. Brian Fellmeth

    Brian Fellmeth Supporting Actor

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    I think that electrical board is a low pass filter with an inductor and 4 resisters. It is possible that one of those components has a temperature dependent change in impedance that may result in more juice getting to the driver. I believe it would be safe to bypass this board- since you are using the LFE out of a 5.1 receiver, its not needed since you receiver has already stripped high frequencies. So place jumpers directly from the binding posts to the lugs of the driver. Play it like this at low volume for awhile and check your amp frequently for excess heat (bypassing the filter board may decrease the total impedance of the sub and make a tougher load for the amp). If the amp is hanging in, gradually increase the volume while checking the amp for overheating. It the calibration drift is gone and the amp can play loud without getting hot, you can solve your problem by soldering perminent jumpers.
     
  3. Steve_D

    Steve_D Second Unit

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    Brian,
    Thanks for the advice.
    I buttoned everything back up tight and so far so good, played tonight without a hitch for several hours and no warm up time after reassembly. But, one thing...I realized in all the permutations I tried that I never switched the cable from the receiver out to the amp, so I used another RCA cable...so now I suspect maybe my $60 sub cable may have a bad sodder? One side does look a little suspect.
    Since I was enjoying my music with bass so much I didn't put it back in to see what would happen. I'll try switching cables again tommorrow. So, at this point either something was loose inside OR its the cable.
    If I rule those 2 things out (meaning I find myself still with the problem) I will bypass that filter next. As for impedence, the sub is rated at 6 ohms, but I noticed the driver was rated at 8 ohms. Is it possible the filter lowers the overall system impedence? Anyway, the amps I'm using is either a Marantz Ma-700 Or a B&K ST-140...neither should have much trouble driving a low impedence load.
    Now I have to go back to the shop where I took my other amp in and tell them oops, there is nothing wrong with the amp. lol.
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    http://www.sdiver.org
    [Edited last by Steve_D on August 01, 2001 at 11:51 PM]
     
  4. Steve_D

    Steve_D Second Unit

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    OK, still have the problem.
    I bypassed the board, and when I do I get a basically a hump around 55 HZ and another around 30 HZ...with nothing to speak of above 65HZ. So I'm guessing now the board somehow flattens the FR of my sub between 22 and 100 HZ.
    That's it....SVS time! I'm about to palce an order for the 16-46.
    However, if anyone can help me get this injured sub on its feet I would appreciate it. I guess I'll take the board to a shop and have them test it.
    TIA
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  5. Brian Fellmeth

    Brian Fellmeth Supporting Actor

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    Looking closely at the pic of the board and assuming there are no caps or other components on the other side, it looks like 4 resisters and a coil all in parallel, and this placed in series with the driver. At lower frequencies, the resistance of this circuit will approach zero, and at higher frequencies it will add R/4 ohms of impedance where R is the value of one of the 4 resisters. So I was wrong, its not a high pass filter but some kind of compensation for the driver to get rid of that peak at 55 Hz. I don't understand why you got less above 65 or the lower peak, but it appears that your unit needs that board and it appears to have a bad part. The cost of replacing those parts would be
     

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