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mackie amp help

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  • #1

    mackie amp help

    So I finally got into a 1400i power amp, blows all 4 10A rail fuses.
    Found their troubleshooting guide which recommends replacing ALL the output transistors.
    I'm a little hesitant because I don't know if the 2n6507 SCR's for protection are good.
    They measure ~30ohms end to end, anyone know if this is normal?
    I can't remember.
    That's the only continuity they show, no typical junction drop.
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  • #2
    Where are these 2N6057s, I can't spot them? I looked in the 1400i. The 1400i-2 has triacs across the outputs. I don;t see SCRs.

    An SCR is a diode with a turn on gate. They sound shorted.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Originally posted by Enzo View Post
      Where are these 2N6057s, I can't spot them? I looked in the 1400i. The 1400i-2 has triacs across the outputs. I don;t see SCRs.

      An SCR is a diode with a turn on gate. They sound shorted.
      And triacs can be used on a DC circuit without latch up.
      The farmer takes a wife, the barber takes a pole....

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      • #4
        Originally posted by Enzo View Post
        Where are these 2N6057s, I can't spot them? I looked in the 1400i. The 1400i-2 has triacs across the outputs. I don;t see SCRs.

        An SCR is a diode with a turn on gate. They sound shorted.
        The SCR's are not on the schem that's floating on the web, they must be a newer version.
        I found a troubleshooting guide that says bring up the voltage slowly with a variac and if there's any DC on the output to replace all the output transistors.
        It says the SCR's fire when they detect dc and short the rails and blow all 4 fuses.
        So, I found a bad MJE15032 pre driver on ch1, all the output transistors test good.
        I can apply full power with input to ch1, but the output looks funny.
        With input on ch2 it's still blowing the 4 rail fuses.
        So as suspected I think those SCR's are bad.
        Thanks, back to work.

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        • #5
          Floating on the web?

          I was downloading them straight from the factory.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            Yes Floating.....anyway, the SCR's are not on the schem I have, they are mentioned in a troubleshooting guide I found, the board is dated '96 but the parts numbers are also different from the schem.

            Anyway, removing the SCR's ch2 comes up okay, ch1 has open emitter resistors, and the base resistor to the bad pre-driver Q23 (Q28 on this board) is open.

            I'll replace the output devices and bad resistors and the SCR's, that should take care of it.
            Thanks.

            Edit: Here's the service manual.

            http://fileshare.eshop.bg/downloadsm...20M-1400i.html

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