Should I get output if I lift the NFB wire in a 74' JMP

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by john l, Feb 22, 2012.

  1. john l

    john l Member

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    Tittle pretty much says it all.
    I was going to try out a stripped down resonance mod where a .0047 cap is soldered in serries between the NBF wire and the speaker hot and I for some reason I get no output. This is how Im wired (- the mod of course)up until I find an impedience switch that fits the existing hole FWIW

    [​IMG]
     
  2. dazco

    dazco Member

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    Can't tell what i'm looking at exactly, but are you sure you didn't accidentally use the tap you are using (ex:the 8 ohm wire if u r using a 8 ohm spkr) instead of the NFB wire?
     
  3. john l

    john l Member

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    Yeah Im pretty sure. Heres a pic of the whole board but I dont know how much it will help.

    [​IMG]

    From what little info I dug up
    Grey should be the 16 ohm tap
    Green should be the 8 ohm tap
    Yellow should be the 4 ohm tap
    Orange should be the common
    Purple is the NFB that goes to the 100k on the board

    It just seems like I should get sound. I know lots of guys that used to do that to old fenders to wake them up a little.
     
  4. Blue Strat

    Blue Strat Member

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    Lifting the NFB should not kill the output. Put it back to make sure it's still working, check your work, and try the other end of the NFB wire. Either you've got the wrong wire or you disconnected something else, in addition, in the process.
     
  5. john l

    john l Member

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    I put it back as soon as I found I wasnt getting anything and the amp sounds fine, i can trace the purple wire right back to the 100k res right above the PI coupler in the pic. I really appreciate the help getting this figured out guys. Can anyone confirm my OT color coding is correct.
     
  6. SatelliteAmps

    SatelliteAmps Member

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    The OT coding is correct. The only way that lifting the NFB would get you no sound is if either something else in the amp is wired incorrectly, or you pulled more than just the NFB.

    Looking at the pictures a few times, I noticed the obvious, that there is no ohms selector. I also noticed the grey tap has a bit of heat shielding and is leaning up against the chassis. It is possible that the grey tap might have been grounded out. You may want to secure that and try again.

    The other issue is that on the lower tab (in the picture) of the hot lead of the 8Ω jack, you have what looks to be two wires soldered there that shouldn't be connected there. Those should be going to the 9 pin socket below the jack.
     
  7. john l

    john l Member

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    Thanks for the suggestions guys.
    A few fellas from the Amp Garage got me up and running. The presence pot (while still functioning) wasnt wired right so the feed back wire was acting as ground for the PI. Rewired the presence pot and now Im up and running. I appreciate help.
     

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