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Author Topic: JBL 4719A noises  (Read 1637 times)

Richard Rempes

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JBL 4719A noises
« on: August 07, 2021, 12:56:49 PM »

Hoping someone can help with subwoofer rattling issue. I have four 4719A cabinets loaded with original 2441 8 ohm drivers. Half are reconed, half are original. Obviously these cabinets are pretty old and I'm not the original owner, so I'll guess on the safe side and say they've seen quite a bit of use. Last year we noticed a very un-musical hard (presumably) wood-on-wood slapping sound on BD hits and full-on rattling with the lowest of bass guitar/synth notes. This is happening with all four cabinets, regardless of the original and/or re-coned 2241s loaded in them. I pulled every driver and checked for VC rubbing - all seem OK. We finally tried an unorthodox test that involved me "bear hugging" the cabinets while they were making this noise from the back and compressing my body as tight to the enclosure as possible. The hard slapping/rattling stopped. All four yielded the same result. On to tearing the drivers and the back panel insulation out to inspect the internal 2x4 bracing. Everything appeared tight - even removed every accessible wood screw to verify boards were glued to the back panel. I find nothing loose, so my decidedly uninformed conclusion is that there is insufficient bracing in these cabinets. Have any of you experienced this issue with these older 4719A cabinets and if so, what did you do to rectify the problem? Add bracing? Fix some weird thing that happens with these cabinets that is not obvious? Scrap the cabinets and put the drivers into something lighter and better sounding? I've attached a screenshot from a Sketchup drawing I made that shows the bracing with the back off.
« Last Edit: August 07, 2021, 01:06:51 PM by Richard Rempes »
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Steve-White

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Re: JBL 4719A noises
« Reply #1 on: August 07, 2021, 04:08:06 PM »

Drivers can rattle, enclosures can rattle.  Most likely the plywood in the cabinets has dried out and de-laminated over time.  Maybe from moisture as well.

Rather that try and explain how to get loose plywood lamination's back together (which I would undertake in my shop without hesitation) and then get 25 other recommendations.

I'll give you this - pull the drivers out and grills.  Rap them with a rubber mallet to be sure.  Call around to local cabinet shops and have a pro work on them for you.  No big deal to repair, maybe coat a paint on the inside when repaired.

Rock on.
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Paul G. OBrien

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Re: JBL 4719A noises
« Reply #2 on: August 07, 2021, 05:24:49 PM »

The removable rear panel and 2x4 bracing is a red flag to me, I don't know for certain but that doesn't sound like an OEM JBL cab to me. These things should be made from baltic birch so the cab alone should be about 100lbs.
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Steve-White

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Re: JBL 4719A noises
« Reply #3 on: August 07, 2021, 05:31:25 PM »

The removable rear panel and 2x4 bracing is a red flag to me, I don't know for certain but that doesn't sound like an OEM JBL cab to me. These things should be made from baltic birch so the cab alone should be about 100lbs.

You're probably right - the OEM JBL stuff is pretty bullet proof.
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Helge A Bentsen

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Re: JBL 4719A noises
« Reply #4 on: August 07, 2021, 06:30:55 PM »

FWIW I bought 12 subs at a auction last year, two of them made a lot of noise when I tested them.
Initially suspected the drivers, upon further investigation I found that the baffle was loose in both of them.
Tightening them up with glue and screws solved this.
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Tim McCulloch

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Re: JBL 4719A noises
« Reply #5 on: August 07, 2021, 06:35:20 PM »

It's been a long time since I looked inside a 4719 but I don't recall 2x4 bracing, nor was there a removable rear panel.

I'm 98.7% certain the noise you're hearing is coming from loose wood joints, and not necessarily where you expect them to be... (Helge has found loose baffle boards, I found loose handles)

On the 2241, removed from the cab... you need to check for a loose spider.  If the adhesive has partly released, the coil former/cone will not be properly controlled. Double spider suspensions present additional opportunities for this.
« Last Edit: August 08, 2021, 01:13:59 AM by Tim McCulloch »
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Steve-White

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Re: JBL 4719A noises
« Reply #6 on: August 07, 2021, 08:51:43 PM »

2241
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Richard Rempes

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Re: JBL 4719A noises
« Reply #7 on: August 07, 2021, 11:53:26 PM »

The removable rear panel and 2x4 bracing is a red flag to me, I don't know for certain but that doesn't sound like an OEM JBL cab to me. These things should be made from baltic birch so the cab alone should be about 100lbs.

Paul - just to clarify, these are OEM 4719A cabinets. The pic I posted is a Sketchup CAD drawing I made with the back panel removed to show the bracing. Each loaded cabinet weighs 200lbs. The backs of my 4719A cabinets are not removeable! I've included some pics of the 2x4 interior bracing from the actual cabinet. All of the 2x4's run from top to bottom, side to side or front to rear, and all have a 1/8" to 1/4" gap between the end of the 2x4 and the panel they run up to. I assume this is by design?
« Last Edit: August 08, 2021, 12:30:04 AM by Richard Rempes »
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Mike Caldwell

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Re: JBL 4719A noises
« Reply #8 on: August 08, 2021, 12:58:18 PM »

Did you try it without the grills on the cabinets.

Try Steve's rubber mallet test, put a little force behind it when you swing the mallet, the slightest loose thing in a speaker cabinet can make a loud noise and not just on sub cabinets.
Using a tone generator may help localize the noise.

You mentioned "bear hugging" the cabinet made the noise stop, that more or less rules out the woofers.

That said by today's standards the 2241 is not the most high power sub driver.
When you running the system how hard are you driving it and how much cone movement/excursion are you seeing on the woofer?

Paul G. OBrien

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Re: JBL 4719A noises
« Reply #9 on: August 08, 2021, 01:05:17 PM »

Those 2x4s are not original... it even looks like PT lumber but that could just be the pic, but in any event the stamp on the lumber makes it a hardware store item not something JBL would use. And I don't see what purpose they serve if they don't extend all the way to the panels, the whole idea is to prevent flexing of those big panels so they have to be a tight fit and must be attached as permanently as possible with high strength glue.

If I were doing this I'd use sculped interlocking plywood braces but to do that well one of the outside panels has to come out, the baffle would be the part I'd remove. You could also try redoing the current method with lumber or large dowels but again those have to fit tight and I don't know what luck you will have doing that through the driver openings. I suppose you could also just glue some shims in there to take up the gap.

« Last Edit: August 08, 2021, 01:17:24 PM by Paul G. OBrien »
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Steve-White

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Re: JBL 4719A noises
« Reply #10 on: August 08, 2021, 04:46:31 PM »

Thump on the braces as well - you need to determine where it's broke before applying the fix.  Braces, de-lamination or both.  Braces loose will be easy to correct.  If it's de-lamination get some help from somebody that knows what they're doing to assist you with it.
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Mike Caldwell

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Re: JBL 4719A noises
« Reply #11 on: August 08, 2021, 06:47:18 PM »

Those 2x4s are not original... it even looks like PT lumber but that could just be the pic, but in any event the stamp on the lumber makes it a hardware store item not something JBL would use. And I don't see what purpose they serve if they don't extend all the way to the panels, the whole idea is to prevent flexing of those big panels so they have to be a tight fit and must be attached as permanently as possible with high strength glue.



In the picture it looked like just the stringers on the back panel stopped short of connecting to the top and bottom end panels.
The 2x4's tying the sides and the front baffle to the back panel together should certainly be connected at both ends.
I kind of remember JBL SR cabinets of that era using what looked like raw 2x4's for bracing. Having
rebuilt a few of the double 12 X 2 mid hi boxes of that same series I can say the construction at least in those boxes was not great, a couple of hard knocks would break the horn support loose.

I always felt the 4719 was tuned too low for what a 2241 could really do at live sound levels, it was something like 25 to 30hz.

« Last Edit: August 08, 2021, 07:01:34 PM by Mike Caldwell »
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Steve-White

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Re: JBL 4719A noises
« Reply #12 on: August 08, 2021, 07:14:23 PM »

^^^ 25-30 HZ would be more towards movie theatre tuning - probably should have been around 35 HZ or higher for PA.
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Richard Rempes

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Re: JBL 4719A noises
« Reply #13 on: August 08, 2021, 10:57:36 PM »

Those 2x4s are not original... it even looks like PT lumber but that could just be the pic, but in any event the stamp on the lumber makes it a hardware store item not something JBL would use. And I don't see what purpose they serve if they don't extend all the way to the panels, the whole idea is to prevent flexing of those big panels so they have to be a tight fit and must be attached as permanently as possible with high strength glue.

If I were doing this I'd use sculped interlocking plywood braces but to do that well one of the outside panels has to come out, the baffle would be the part I'd remove. You could also try redoing the current method with lumber or large dowels but again those have to fit tight and I don't know what luck you will have doing that through the driver openings. I suppose you could also just glue some shims in there to take up the gap.

Paul, Mike, et.al.
Thanks for all of the insights and suggestions. Turns out all four of the baffle boards were slapping the two 2x4"s that are in the center running from the baffle board to the back panel. I could re-create the slapping by pulling on the baffle board in the center. IMO, it's a design flaw in that it appears the only thing connecting these pieces were staples (half of which missed the 2x4's) and failed glue that attempted to fill a 1/8" gap. Two long wood screws (with large washers) driven thru the baffle into each of those 2x4s corrected the issue.
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Steve-White

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Re: JBL 4719A noises
« Reply #14 on: August 08, 2021, 10:59:48 PM »

Very good sir!
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Mike Caldwell

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Re: JBL 4719A noises
« Reply #15 on: August 09, 2021, 06:22:59 AM »

Paul, Mike, et.al.
Thanks for all of the insights and suggestions. Turns out all four of the baffle boards were slapping the two 2x4"s that are in the center running from the baffle board to the back panel. I could re-create the slapping by pulling on the baffle board in the center. IMO, it's a design flaw in that it appears the only thing connecting these pieces were staples (half of which missed the 2x4's) and failed glue that attempted to fill a 1/8" gap. Two long wood screws (with large washers) driven thru the baffle into each of those 2x4s corrected the issue.

Did you pry the gap open a little and work some glue in before running the screws in?
May not hurt to back up all the cross bracing with some screws before they crack loose.

Richard Rempes

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Re: JBL 4719A noises
« Reply #16 on: August 09, 2021, 09:33:53 AM »

Did you pry the gap open a little and work some glue in before running the screws in?
May not hurt to back up all the cross bracing with some screws before they crack loose.

Mike
That step will be phase two. At this point I just wanted to verify that loose baffle boards was the source of the problem. All of the interior bracing already had screws, so I pulled and then reseated/tightened them. Phase two will be a more aggressive disassembly that will involve shimming or replacing every bracing member that does not tightly contact adjacent surfaces, which is practically all of them. Again, thanks to you and all who commented for the words of wisdom. All four cabinets are at least useable at reasonable volume levels now.
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Craig Hauber

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Re: JBL 4719A noises
« Reply #17 on: September 01, 2021, 01:16:48 AM »

Mike
That step will be phase two. At this point I just wanted to verify that loose baffle boards was the source of the problem. All of the interior bracing already had screws, so I pulled and then reseated/tightened them. Phase two will be a more aggressive disassembly that will involve shimming or replacing every bracing member that does not tightly contact adjacent surfaces, which is practically all of them. Again, thanks to you and all who commented for the words of wisdom. All four cabinets are at least useable at reasonable volume levels now.
I remember the port tubes coming undone on those boxes and falling back loose inside.
The tuning would obviously be way off but it definitely showed the glue used wasn't the greatest and years of humid outdoor shows and being baked by sun took its toll.
When they were working properly I still felt like there was a ton of flap and rumble out of those boxes, but no real clean "hit" or sense of impact.  The Concert Series 4842A that it seemed like the SR sub was a "copy" of actually sounded much better to me (imho)
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Craig Hauber
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Re: JBL 4719A noises
« Reply #17 on: September 01, 2021, 01:16:48 AM »


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