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Mackie HR624 oscillating

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  • #1

    Mackie HR624 oscillating

    I've got a Mackie HR624 whose LF amp goes into high frequency oscillation whenever you try to drive a 4 ohm load. When you first power it up with a 4 ohm load, it does not oscillate, but as soon as you give it signal, it goes into high frequency oscillation and then continues to oscillate, whether or not you are giving it signal. Depressing the swamping defeat button stops the oscillation.

    It drives 8 ohm loads fine, and the rail voltages are correct.

    Any ideas?

    hr624_1.schm.pdf
    Tags: None
  • #2
    Feedback defeat stops it? I bet shorting across R73 ground resistor in the speaker lead also stops it.

    Is the stock speaker 4 ohms or 8 ohms? I found the hard way on some other Mackie power amps that if I ran an amp into 4 ohms that was intended for 8 ohms (or even 12 ohms oddly) that it went unstable as long as the current feedback was live. It would sit there at first, then as soon as any signal went through - like just a pulse - it would start and quickly grow into a rail to rail oscillation.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      That's exactly how it is behaving. The woofer measures 3.5 ohms with my meter and the spec for the speaker says its a 4 ohm woofer. So it should drive 4 ohms supposedly. I didn't try shorting the resistor yet.

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      • #4
        If that is stock, yes it ought to work then.

        Shorting that rsistor ought to do the same thing as the kill button.

        make sure R3 is not open on the output zobel. Explore that damping circuit, a gew resistors and a pot. Does turning the dampinbg control affect the oscillation?

        And it also could be one of your 7293 has gone limp on you. If so, I;d replace both.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          I can dial out the oscillation with the damping control. I already replaced both of the 7293's and it did not help. The components in the damping circuit all measure ok. Is it ok then, with the oscillation dialed out?

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          • #6
            Well, in my view it is never OK when a panel adjustment can render an amp unstable. Is that a trimmer or a panel control?
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              How about that R3 and the zobel network Enzo mentioned?
              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
                That is an internal trimmer. R3 is ok in the Zobel. The cap won't really measure in circuit. What is the proper procedure for the adjustment of the damping pot?

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                • #9
                  Oh, if it is a trimmer, then adjust it back so the amp doesn't go nuts. I have no idea what the official adjustment is. Make it work, that's my view.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #10
                    Thanks Enzo. Yeah, I agree.

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