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Hammond L-100 noise from preamp

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  • Hammond L-100 noise from preamp

    Hello, I'm in the process of modding my L-143-1. I'm removing the six voice percussion with the intent of having effects pedals wired to the cheek block switches. I've removed the rhythm board and power supply. There's a considerable amount of noise, mainly hum/buzzing and a loud crackling. Before anyone tells me that I've gone about the removal wrong, I'd like to clarify that these noises were present prior to any mods. The humming I've found to be related to the low frequency output that would normally go from the rhythm board to the pre amp.

    I'm planning on rebuilding the pre amp and removing the rhythm input, following the service manual schematic. The source of the crackling is still unknown. I've replaced the power amp for unrelated reasons and the crackling followed. The large filter cap in the pre amp has also been replaced. The volume soft switch affects the crackling and the hum, as does the expression pedal. I have a couple questions, hoping you guys can help me out with diagnosis:

    • Connecting the output from the pre amp to an external amp give mostly static noise, with a tiny bit of organ sound. I've read on here that the signal from the input and output of the pedal should be the similar, can someone confirm this?

    • Anyone have solutions to the crackling? I've heard that R206 could be the problem. I'm suspecting a bad ground somewhere, though I'm not sure where to start looking.

    Thanks! :->
    M3, L-143-1

  • #2
    Expression :
    LDR1 is a photo resistor lited by a light bulb and the light is shutted off by a moving screen.
    This resistor vary from a low value (May be 500Ohm ?) to Infinite in the black dark.
    In the dark the signal cross the pedal without any change as long as the impedance of the load should be negliged.

    Noise :
    If you remove V11 12AU7, percussion is disabled. This is often a source of problem.
    Clean the tube socket, cinch connectors and tube pins.
    Hurt gently each with the handle of a screwdiver looking for sensitive one.
    May be you have a faulty tube (Gas).