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Mackie HD 1521 thermal circuit tripping

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  • Mackie HD 1521 thermal circuit tripping

    Customer has a Mackie HD 1521 that he's been using for outdoor shows. After about 1/2 hour the thermal switch trips. I went though the amp and checked all the silicone components and all check ok. I ran signal to the amp for about a 1/2 hour indoors before the thermal light tripped momentarily a few times, making sure that no airflow was being blocked from the rear so the fan could get good air circulation. How is the sensitivity of the thermal circuit on these amplifiers? Is it normal for the HD1521's thermal circuit to trip so easily?
    Helping musicians optimize their sound.

  • #2
    How hot is the heatsink running? Maybe the thermal circuit is just doing its job?

    On the other hand, just because the trouble light says thermal beside it doesn't mean the problem is actually heat.

    What exactly is happening when this occurs. SOund goes away? A "thermal" light comes on?
    Education is what you're left with after you have forgotten what you have learned.


    • #3
      The fins on the heatsink do not feel any hotter than normal. When the thermal light trips the sound cuts out and it makes a sound similar to a CD that gets hung up. This only happens for a few seconds and then the signal resumes for a few minutes. it happened a few times like this after 30 minutes at Unity gain. When it's a thermal issue isn't the whole unit supposed to shut down until it cools down below the thermal threshold?
      Last edited by WholeToneMusic; 10-12-2010, 05:22 PM. Reason: added heatsink info
      Helping musicians optimize their sound.


      • #4
        That was why I asked if it was getting hot, it may not be a thermal issue. The thermal protection light may be coming on, and the amp acting like it is overhot, but it could be as simple as a busted leg on a thermistor.

        How an amp goes about protecting itself from overheating is a matter of what the designer wanted to do. SOme amps have a thermal switch that opens th main power and won;t reconnect until it cools down. SOme just disable the amp circuit.
        Education is what you're left with after you have forgotten what you have learned.


        • #5
          Thanks Enzo, I'll poke around some more to see if anything is open.
          Helping musicians optimize their sound.


          • #6
            I combed through the circuit boards again and did not notice anything broken. I had an HD1521 in last month that had a shorted Q1 so I thought maybe it was shorted also. I took it out of circuit and it tested fine. The only thing I noticed was that the heat sink compound was dried up. Since the boards were already lifted I applied some more bird doo to both rows. Ran the module at unity gain for 3 hours with no clipping.
            Helping musicians optimize their sound.


            • #7
              i have a mackie 1521 and all of a sudden i had no sound. it was lite up but no sound. the blue light in the front is on, and i believe the power light in the back is also on. i think this is a thermal circuit issue. where is the thermal circuit located??
              does anyone have a picture of one? or a place i can get a replacement, in case mines is out?


              • #8
                You should start a new thread. This one is 10 years old. I would not assume that it is a thermal circuit issue until you verify that via troubleshooting. Here is the schematic.

                Last edited by The Dude; 11-03-2020, 11:45 PM.
                "Yeah, well, you know, that's just, like, your opinion, man."


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