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Thread: B688/D718

  1. #1
    Old Timer
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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 Thumbnails Attached Thumbnails 20180205_143909.png  

  2. #2
    Old Timer
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    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.

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

  4. #4
    Supporting Member The Dude's Avatar
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    Check your transistors with a DVM and see which are bad.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  5. #5
    Old Timer
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    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.

  6. #6
    Supporting Member The Dude's Avatar
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    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.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  7. #7
    g1
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    schematic:
    Attached Files Attached Files
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  8. #8
    Supporting Member The Dude's Avatar
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    Can you list the DC voltages on Q8,9,&10?
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  9. #9
    Old Timer
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    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!
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  10. #10
    Old Timer
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    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

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

  12. #12
    g1
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    Is there DC on the output like that? What is the voltage across R71, R72 ?
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  13. #13
    Old Timer
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    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.

  14. #14
    g1
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    And those emitter resistor voltages?
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  15. #15
    Old Timer
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    0v across them... with and without a load. What should this reading be?

    6.3mv on both ends of Re to ground.

  16. #16
    g1
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    There would only be a couple millivolts across those resistors.
    You have no DC on the output and it sounds good?
    Listen to the decay of a quiet note. Any audible crossover distortion? If not I'd call it done.
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  17. #17
    Old Timer
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    All correct and it sounds good. Thanks for the help.

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