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Mackie RC4 speaker no sound

Discussion in 'Audio' started by thenonext, Apr 6, 2018.

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  1. thenonext

    thenonext

    4
    0
    Apr 4, 2018
    Hi,

    Recently I plugged in my guitar with pedals directly to the speaker TRS inputs. I played several days and than output sound started disappear at random then appear again. And then it gone completely. It died not right away.

    It's powering on. The signal is present in headphones, but no sound coming out from the speaker.

    I inspected the circuit visually and found a brown burned spot under the transformer (see picture attached). All other components look good.
    Can you guys advise, what component may be broken?
     

    Attached Files:

  2. duke37

    duke37

    5,301
    740
    Jan 9, 2011
    I do not see a transformer. There is a brown area from the reservoir capacitors, indicating excess current in the tracks. This leads to a capacitor, probably the capacitor feeding the speaker, which looks to have been boiled.

    Are there any fuses in the circuit? Do you run at absurd volume levels?
     
  3. thenonext

    thenonext

    4
    0
    Apr 4, 2018
    Thank you for the response. I removed the transformer, it was mounted under the circuit. I couldn't find any fuses. Run at normal volume level.

    So why they get boiled? Because I plugged in the guitar directly? Or there is short circuit somewhere?
     
  4. duke37

    duke37

    5,301
    740
    Jan 9, 2011
    Electrolytic capacitors go leaky after a considerable time particularly if they are abused. I think the bad one is the capacitor feeding the speaker so will not do anything if no speaker is connected. If you are VERY lucky, this is the only component to be changed. The grott needs to be removed. There could be faults in the power supply, amplifying transistors or the speaker. Get the DC voltages correct first.

    Do you have a schematic?
     
  5. thenonext

    thenonext

    4
    0
    Apr 4, 2018
    No schematic, I asked Mackie support to provide it but they refused.
    Should I try to replace those two big capacitors on the left?
     
  6. duke37

    duke37

    5,301
    740
    Jan 9, 2011
    Do not replace components on a whim, this is a way of introducing more faults.

    Remove the boiled capacitor. Clean up the board with meths. Check the tracks under the brown area for continuity. Check the speaker for resistance (between 4Ω and 15Ω)
    What active devices are used.
    Try to draw a schematic and measure the DC voltages at critical points.
     
  7. thenonext

    thenonext

    4
    0
    Apr 4, 2018
    Thank you! Will try to investigate it.
     
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