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B688/D718

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  • B688/D718

    Working on a Fishman loudbox mini...

    Output is very distorted.

    DI sounds good. Voltages in PA are off.

    The pre driver voltages are good. Things get hairy at the drivers.

    Subbed 1k resistors for drivers from B to E. Now have:

    +/- 880mv Vbe on drivers. But also:
    NPN Ve -250mv
    PNP Ve -350mv

    Then I subbed 1k on output devices as well.

    Outputs
    NPN
    Vbe +46mv

    PNP
    Vbe -39mv

    What's the consensus here? Haha. I have results. But a little uncertain on interpreting them. From what I can gather... it seems that there is a B-E issue with the NPN output device. As if it isn't conducting or is conducting very little.

    What do you all think?
    Attached Files

  • #2
    Oh and the whole point of the post was to ask for suitable sub part #s for the haha. B688/D718 120v 8A power transistors.

    Comment


    • #3
      2sc5200/2sa1943? They're way over spec... But doable?

      Comment


      • #4
        Check your transistors with a DVM and see which are bad.
        "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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        • #5
          That's the thing they test good with the meter. So it's c9nfusing me as to what the problem is.

          Is there a measurement I'm not aware of? I checked the junctions. Good.

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          • #6
            Your bias on the outputs is way low . I don't think, at this point, that they're bad. Do you have a schematic of the output section you could upload? Can't see enough of the amp in your post #1 upload.
            "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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            • #7
              schematic:
              Attached Files
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


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              • #8
                Can you list the DC voltages on Q8,9,&10?
                "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                • #9
                  Well. Found that Q13 emitter solder trace had broken. I had though it broke when adding the 1k resistors. But after reinstalling all transistors and making sure that the emitter was soldered in... it's all working. Amp sounds good.

                  Q8
                  Vc +920mv

                  Q10
                  Vc -970mv

                  Thanks!

                  Comment


                  • #10
                    After double checking the outputs I see there may possibly still be an issue. Voltages with a speaker connected:

                    Q14
                    Vbe +450mv

                    Q11
                    Vbe -170mv

                    Without speaker connected these are +/-320mv

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                    • #11
                      Is it ok that with a load the Vbe voltages are skewed? And everything looks ok without a load?

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                      • #12
                        Is there DC on the output like that? What is the voltage across R71, R72 ?
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


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                        • #13
                          I just fired it up and there was no sound. And no bias voltage. Kept chasing my tail because taking measurements seemed to change the results. I was getting odd results on the differential pair so scraped some mask off their traces as well as D6/7 and soldered the holes to the traces... it's working again and output Vbe voltages are about +/- 300mv without a load and +180mv/-480mv with a load.

                          There is 0 DC on the output.

                          Comment


                          • #14
                            And those emitter resistor voltages?
                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


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                            • #15
                              0v across them... with and without a load. What should this reading be?

                              6.3mv on both ends of Re to ground.

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