Subwoofer produces static sound

Hello,

for some reason my subwoofer(JBL 138) started producing static sound when it is not plugged in. I just brought it down, and it worked for the first day, then it started creating a fairly loud static. The subwoofer still works fine when plugged in(LFE) however at low volumes it is very discernible. I've tried different electrical sockets, all producing the same static. I've mostly run out of ideas to fix it, it does feel unlikely that it suddenly produced an error inside the actual subwoofer, however not impossible. What might be the cause of the issue?

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Hi,

Does the static sound mechanical (rattling vibration type sound) or electrical (crackly sound)?

If mechanical it could be a loose fitting for the actual driver causing the vibrating rattle etc.

If electrical you could have faulty capacitors on the power supply rails in the amp. Check for bulging or leaking capacitors and possibly for any obviously heat stressed components on the pcb.

Here is a link to the service manual which provides a schematic diagram and a parts list which may be of some help.

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@jayeff

Thank you for your answer. What makes me quite curious is the first time it happened it worked to punch it(trust me, it was after attempting other solutions first!); it stopped buzzing for a day before it came back.

It does sound electrical, static mostly, like the sound of disturbance although I've tried to plug it in everywhere.

If I touch the speaker membrane it vibrates, it is a generated sound in some way if that is helpful.

I tried to pull it apart but found all cables were glued which were kind of a nuisance since I lack most tools, however if I can rule out all other issues than electrical then it is probably a requirement.

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Hi,

If 'punching it' stopped the buzzing to me that points to it being something loose.

It could still be a loose electrical connection but usually they come back fairly quickly to the faulty condition whereas a loose mechanical connection might not vibrate loose again for a long time.

The membrane does vibrate, that is how the sound is produced ;-) The frame holding the membrane of the driver (speaker) should not, although it might feel like it because you are feeling the membrane's vibrations. You need to feel if the frame moves when the membrane is not working at all.

Can you access the circuit boards easily? Even to be able to see them using a torch etc. might be useful.

Be sure to have the power totally disconnected when you are working on it.

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Haha of course ;) but I meant that it wouldn't be any coil whine or anything like that.

Of course the power is disconnected, and I also wait some time when working to allow discharge.

I can partially access the back, however due to the cables being glued into the frame I can only pull it out a few centimeters.

I am just dubious about it being something blown electrically since punching it fixed it the first time :p

If it is a loose electrical connection would that be difficult to fix? Just curious, I am a student and while I can borrow some tools my arsenal is fairly small.

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Hi,

With a loose connection I was thinking that it might be a dry solder joint either on the board or perhaps on the driver as well.

To repair this you would need good access to everything inside the cabinet and be able to get the board out to inspect it.

By the sounds of it that won't be easy and then you have to put it all back as it was to get the sound 'right' again.

The tools that you'll need would be a soldering iron (and solder), a good magnifying glass and a strong light (to closely inspect for dry solder joints as they may not be very easy to see with the naked eye, plus whatever tools you need to dis-assemble the unit correct screwdrivers etc.

Good Luck.

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Very curious, I left the subwoofer on over the day and when I got home it works just fine. I have no idea why and if it will be permanent but quite happy nonetheless.

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