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Acoustic B200H Burning R32

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  • Acoustic B200H Burning R32

    Hey Folks.

    I replaced R32 on the power board which was burnt. The new one installed (470 ohms) started to smoke as well. Schematic attached.

    R32 (510 ohms) is not shorting to ground (in the pic it looks like one leg is touching a grounded heatsink screw, but it is not). The amp has the proper 4A fuse installed.

    Suggestions please?

    B200PowerAmpRevE08Apr2009-JA.pdf

    Click image for larger version  Name:	IMG_20240210_104537527.jpg Views:	0 Size:	3.05 MB ID:	993771
    Attached Files

  • #2
    IC2 pin 13 must have 0volts on it exactly as pin 12. Is that your readings?
    If it isn't I would replace the STK. If the voltage is correct one must ask, why no damage to R31.
    R32 MUST be 510R. Is IC6 OK is the next question.
    Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
    If you can't fix it, I probably can.

    Comment


    • #3
      Originally posted by Jon Snell View Post
      IC2 pin 13 must have 0volts on it exactly as pin 12. Is that your readings?
      If it isn't I would replace the STK. If the voltage is correct one must ask, why no damage to R31.
      R32 MUST be 510R. Is IC6 OK is the next question.
      I am seeing about 53 VDC on both pin 12 and 13.

      Not sure how to check IC6.

      Comment


      • #4
        In that case IC2 is faulty.
        Good luck finding a grnuine replacement.
        Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
        If you can't fix it, I probably can.

        Comment


        • #5
          Originally posted by Jon Snell View Post
          In that case IC2 is faulty.
          Good luck finding a grnuine replacement.
          Right.

          Comment


          • #6
            From your description, I would say IC2 is at fault if the voltages on pins 6, 7, 10 & 11 are correct and hope that the opto isolator IC has survived. (The 510R will have reduced the current as it failed).
            Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
            If you can't fix it, I probably can.

            Comment


            • #7
              Yes, good luck finding an STK that's not fake. I had to do this (see link), which isn't worth the effort in most cases.

              https://music-electronics-forum.com/...tronics/46254-
              "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

              Comment


              • #8
                I think I bought same STK recently that was legit. Somebody had some on ebay that he had got from MCM. I'll see if I can find the seller and if there are any more of them.
                edit: sorry, that was STK1050 that I bought recently.
                Last edited by g1; 02-13-2024, 01:40 AM.
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


                Comment


                • #9
                  One day I was faced with the problem of finding a quality chip. I found it. They sent it over. Two of them.
                  I put it in, turned it on, it worked. I more than doubled the power, and the chip lost its spirit.......
                  I didn't even bother inserting the second chip.
                  I assembled a small PCB, using parts from the original device as much as possible. It works and it screams!
                  In the schematic (see attached), I removed the protection transistors and used a single pair of output transistors at the output. It most closely resembles the inside of the chip.

                  Translated with DeepL.com (free version)​​
                  Attached Files

                  Comment

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