ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 [2]  All   Go Down

Author Topic: JBL 4719A noises  (Read 1142 times)

Steve-White

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 830
  • Fort Worth
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.
Logged

Mike Caldwell

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2246
  • Covington, Ohio
    • Mike Caldwell Audio Productions
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 »
Logged

Steve-White

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 830
  • Fort Worth
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.
Logged

Richard Rempes

  • Newbie
  • *
  • Offline Offline
  • Posts: 4
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.
Logged

Steve-White

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 830
  • Fort Worth
Re: JBL 4719A noises
« Reply #14 on: August 08, 2021, 10:59:48 PM »

Very good sir!
Logged

Mike Caldwell

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2246
  • Covington, Ohio
    • Mike Caldwell Audio Productions
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

  • Newbie
  • *
  • Offline Offline
  • Posts: 4
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.
Logged

Craig Hauber

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 724
  • Mondak Sound Design - Plentywood MT/Grenora ND
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)
Logged
Craig Hauber
Mondak Sound Design
-Live PA
-Installs
-Theatre

ProSoundWeb Community

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


Pages: 1 [2]  All   Go Up
 



Site Hosted By Ashdown Technologies, Inc.

Page created in 0.021 seconds with 19 queries.