Wiring help needed to reduce RFI noise.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by winger, Feb 4, 2008.


  1. winger

    winger Member

    Messages:
    8
    Joined:
    Jan 6, 2008
    Location:
    Netherlands
    Hi all,

    I'm currently in the process of rewiring my girlie 6 space rack.
    It contains :
    Rexer wireless receiver
    Omnitronic RLD-8 power distributor
    Boss GT-PRO (waiting for an Axe FX Ultra 8) )
    BBE 386 Acoustic pre amp
    Marshall 8008 power amp

    In the rear I placed a patch panel for power in- out (Neutrik Powercon's), midi in- out, effects loop send and return (Neutrik locking jacks), speaker out L&R (Neutrik locking jack's). I still have 4 spaces left for balanced line out's and IEM inputs.

    It's all working out beautifully, except for the midi connections.
    When I connect these to the MFX, my wireless receiver starts to humm/hiss. The status led for the B channel also lights up, indicating it's receiving something. :?

    The midi connectors have only been wired to the pins. (Shields to the middle ones. The screws used to mount the midi connector should provide ground through the patch panel?)

    What should I do here ? Extend the shield wire to the connector chassis ? Or are there other / better sollutions?

    Thanks !
     
  2. donnyjaguar

    donnyjaguar Member

    Messages:
    4,234
    Joined:
    Nov 4, 2005
    Location:
    Canada
    I'm not at all familiar with this gear, but a ground loop still comes to mind.
     
  3. AR-305

    AR-305 Member

    Messages:
    881
    Joined:
    Dec 20, 2007
    Location:
    NW North Carolina
    You might try putting a couple of magnetic RF sheilds (like the kind on your computer monitor cable) on those midi cables in the rack.
     
  4. winger

    winger Member

    Messages:
    8
    Joined:
    Jan 6, 2008
    Location:
    Netherlands
    I was thinking of a ground loop as well. The midi chassis is touching the panel through it's mounting screws. The midi cable itself is grounded through the midi connections in the rear of the MFX.

    I don't think I should cut the shield connection on pin 3. But should I connect the shielding wire to the connector ground, or isolate the chassis mounting screws from the patch panel?
     
  5. winger

    winger Member

    Messages:
    8
    Joined:
    Jan 6, 2008
    Location:
    Netherlands
    Redid the cable routing. (It was following the signal lead loom at first, now it's dangling loosly.) Measured every pin to ground on the pin and chassis sides. I found no continuity. But still the noise is there. I'm at a loss here. [​IMG]

    Regarding the diagram I posted, it's one I found online and used for reference myself. The cable used is a normal midi cable cut in half so I could resolder it at the proper length. (It's 4-core + shielding.)
    The shielding is connected to pin2 (on the diagram), the other four are the normal cores, soldered to the corresponding pins as measured from the cable plugs.

    Any idea's? [​IMG]
    Help !!
     

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