need help to identify a failure on DIY guitar pedal (MXR Micro Amp)

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  • SrMrPixel
    New Member
    • Jul 2025
    • 6
    • Mexico

    #1

    need help to identify a failure on DIY guitar pedal (MXR Micro Amp)

    I'm building this pedal for the second time and it has the same failure. Voltage divider not working as it should. Before the 10M resistor it works, 4.5v, but after it the voltage drops depending on the value of the resistor I put on. 10M=0.45v 2M=1.9V and so on. It should not do that, that resistor is just for biasing the input signal.

    Any idea?

    Click image for larger version

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  • stj
    Great Sage 齊天大聖
    • Dec 2009
    • 31026
    • Albion

    #2
    schematic?

    Comment

    • SrMrPixel
      New Member
      • Jul 2025
      • 6
      • Mexico

      #3
      Originally posted by stj
      schematic?
      Just R1 is changed to 2.2M and a 47uf power filter cap is added

      Click image for larger version

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      • stj
        Great Sage 齊天大聖
        • Dec 2009
        • 31026
        • Albion

        #4
        use a 78L05 to get the reference,
        a voltage divider is dependent on current.
        or increase the current by using 10k resistors instead of 100k for R7,8

        Comment

        • SrMrPixel
          New Member
          • Jul 2025
          • 6
          • Mexico

          #5
          Originally posted by stj
          use a 78L05 to get the reference,
          a voltage divider is dependent on current.
          or increase the current by using 10k resistors instead of 100k for R7,8
          still that doesn't make much sense to me, based on other circuits that I've made and references that behavior is not normal. Maybe there is something wrong with the board but I can't find anything

          Click image for larger version

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          • stj
            Great Sage 齊天大聖
            • Dec 2009
            • 31026
            • Albion

            #6
            a leaky c1 or a bad opamp maybe then.

            Comment

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