JBL PB12 Won't Turn On

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  • joltman
    New Member
    • Dec 2021
    • 3
    • USA
    #1

    JBL PB12 Won't Turn On

    I have this sub that broke years ago which gave me an excuse to get a new one. I didn't get rid of it because I was hoping I could repair it. I used to get noise from the sub when nothing was playing. Then one day it just wouldn't turn on. Today, I took the amp board out and took some photos. It doesn't look good, but I was wondering if I could replace some items that look scorched.



    You can see the names for the components in this photo. The scorch marks make me feel like these components are toast.



    This photo makes me think the board is actually cracked. I'm wondering if it would even work if I did replace the resistors and transistor on the board. And finally, here are the main filter caps (3300uf, 80V) and diode bridge rectifier:



    You can also see the fuse in that photo, but it looks fine and not burnt out. Am I crazy to try and fix this board? Thanks for the help!
    Attached Files
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    • Per Hansson
      Super Moderator
      • Jul 2005
      • 5895
      • Sweden
      #2
      Re: JBL PB12 Won't Turn On

      I don't think the board is cracked, it looks like the PCB has carbonized, the voltage found a new path to travel...
      Usually when you have this type of damage there will be more parts destroyed.
      So the best advice is to check those big resistors, then follow the path from the obviously dead transistor and see how much else in its path is destroyed...

      Attached Files
      "The one who says it cannot be done should never interrupt the one who is doing it."

        Comment

        • joltman
          New Member
          • Dec 2021
          • 3
          • USA
          #3
          Re: JBL PB12 Won't Turn On

          Thank you for the reply! I'm thinking I'm going to replace several components. This is the list I'm going to replace:

          Diode Bridge Rectifier: The one on the board is an RS804. I think this is the suitable replacement part.

          Filter Caps: The current caps are 3300uF 80V. I think these would be a suitable replacement. I couldn't find any Panasonics at Digikey.

          Components near carbonization:

          Transistor 1 (Q3): Currently, MPS W56 221 (PNP). I'm not really sure how to find a replacement for this.
          Transistor 2 (Q2): Currently, MPS W06 226 (NPN). I'm not really sure how to find a replacement for this.

          2 x Large Caps (C17 & C18): Currently 200uf 50V. Best I could find are these.

          R6 & R9: This might sound dumb, but I'm not understanding the color coding on those resistors. If I'm doing it right, I'm seeing gigaohm resistors???

          C4 & C9: 22uF 25V. I think this will do.

          C2: 4.7uF 50V. This one maybe?

          Can anyone confirm what I've got above to make sure I'm not crazy? Also, I would love some assistance on the resistor color coding and definitely on the transistors. Thank you!

            Comment

            • Per Hansson
              Super Moderator
              • Jul 2005
              • 5895
              • Sweden
              #4
              Re: JBL PB12 Won't Turn On

              It's brown-red-red-gold, it has just been heated up so much that the color bands got burned up... You can make a sanity check with a multimeter: it may still measure ok.

              The reason I know this is because the service manual was available with a 2 second Google search
              R9 should be rated for 3W while R6 5W, I would just go for 5W for both:
              https://www.digikey.com/en/products/...SSJ1K2/9926228

              The transistor was also found with a quick Google search, links here.
              Just look for something similar with the parametric search on Mouser, Digikey or your favorite place.

              Useful links:



              https://www.diyaudio.com/community/t...for-42.219734/

              https://cdn.badcaps-static.com/pdfs/...003411a40c.pdf
              https://cdn.badcaps-static.com/pdfs/...b45330ebb1.pdf

              https://www.mouser.se/ProductDetail/...wdKTZT9Q%3D%3D
              https://www.mouser.se/ProductDetail/...EeECjleg%3D%3D
              "The one who says it cannot be done should never interrupt the one who is doing it."

                Comment

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