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Mackie SRM450 V2 Link Problem

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  • Mackie SRM450 V2 Link Problem

    Has paid attention someone to the link that goes from +12VDC supply (does not measure +12V but -63.7VDC) GND pin/tab of the U34 (7812 IC) to the -75V DC supply? are this correct???? because on the minus side of the C208 and anode of D42 measure -61.3VDC and at the output of IC U34 measure -63.7VDC, so where´s the +12Volts to measure??? Are not this link (which I consider it a big mistake) the big problem/mother of almost all of this V2 problems presented ????
    THANKS and Very Good Day!!!!Edu.

  • #2
    I have always thought that the -75Vdc 'ground ' reference for the +12Vdc regulator was indeed goofy.

    But after all, it is relative, is it not?

    SRM450v2 Secondary Power Supply.pdf


    • #3
      It measures +12 with respect to the -75v rail, in other words it is 12 volts less negative than that rail.

      It was never intended to be a +12v supply for things to use.

      Look at the power amp drawing, just left of output Q3. See PWR_MUTE? Note that it is associated with the -75v rail. Now look left center on the power supply page. PWR_MUTE and +12v- are across the phototransistor in IC1. When AC mains power is applied, upper left corner, it charges C128 there just above IC1. That takes just a moment, and once charged, that allows the LED in IC1 to turn on and thus turn on the transistor side. That connects +12v- to the PWR_MUTE. That then turns off the mute circuit via U38. Of course that is only the power up mute, there are other muting circuits, and all must be satisfied before the amp un mutes, but the +12 over -75 supply is used there, and a far as I know, nowhere else.

      Since you do indeed have a 12v differential across that regulator, I'd say it was working.

      it is not a mistake, nor is it likely the center of all SRM450v2 problems. All the other secondary supplies DO reference ground, but this one is specially designed for this one purpose. Not understanding something does not make it a mistake.
      Last edited by Enzo; 04-30-2016, 05:13 PM. Reason: typo
      Education is what you're left with after you have forgotten what you have learned.


      • #4
        Thanks Jazz P Bass for your comment!


        • #5
          Hi Enzo, thanks a lot for your Great to much Appreciated explanation!!!