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Wireless Quandry-RF in control cavity?

Discussion in 'Hardware, Setup & Repair [BG]' started by Jontom, Jun 10, 2002.


  1. Jontom

    Jontom

    Mar 11, 2002
    New York
    Here's my predicament: I have 2 Warwick Corvette Standard basses, 1 fretted +1 fretless. They have identical EMG pickups(J's) and BTC controls. What freaks me is that the fretted will not work w/my Shure wireless system,while the fretless works fine. The problem is that when I plug in the fretted bass, I can play for a couple seconds and then it totally craps out. The sound will go from bassy and clear, to trebly and distorted,to finally nothing. I've noticed that this will happen as soon as I touch a control knob of any variety. The folks at Shure said it is due to RF in the control cavity. Does anybody know how to get rid of it?
     
  2. You could try shielding the cavity, or maybe put a very low value capacitor say in the order of Pico farads across the output jack. Maybe.. never tried it, its often used in amps in negative feedback to cancel out RF.

    Can you change the frequency at which the wireless transmitter outputs???

    :D:D

    Merls
     
  3. Jontom

    Jontom

    Mar 11, 2002
    New York
    I cannot change channels on the wireless. And in addition, I also use the same cabling on the two basses...whats really freaking me out is that the system used to work fine w/the fretted bass.
     
  4. Does the fretted have active electronics?

    I'm just thinking this through. kinda weird. I'll still keep thinking on it.

    :D:D

    Merls
     
  5. Jontom

    Jontom

    Mar 11, 2002
    New York
    Identical EMG J pickups w/active EMG BTC preamps...in other words the same on the inside. Say Merls...if I put the capacitor in line,wouldn't that affect the tone somewhat?
     
  6. Hmm it might effect the tone. Not greatly. hmmm It might have something to o with the frets. Acting as small antenna's maybe? RF is high frequencies, a fret is probably the perfect 1/4 of a wavelength.

    Where's Pkr2! Any ideas?

    :D:D

    Merls
     
  7. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    I've been watching this one unfold and I'm about as stuck for good ideas as you are Merl.

    Jontom, does the bass work normally with a cord between the bass and amp?

    Have you tried putting more distance between the bass and the transmitter? If so, does the problem get better with distance?

    Did the problem occur all at once or over a period of time?

    Has any repair work ever been done in the control cavity?

    I understand that the bridge is usually not grounded with active electronics but I would still ground the bridge temporarily and see how it affects the problem. At this point any clue will help with the diagnosis.

    The cap across the input of the bass may yield a clue even if it doesn't correct the prob. No, the cap cannot go in series with the input. That would block the audio signal. The cap should go from input to ground with the very shortest leads possible. A cap in the lower PF value will not affect the tone.

    The purpose of the cap is to bleed the RF to ground before it reaches the amp.

    You can always temporarily shield the cavity with foil to see if that corrects the problem. If it does, a more permanent shielding can be performed.

    I realize that Shure built the xmtr and they should be the experts but I seriously doubt that thier diagnosis is right.

    Since you have an identical (for all practical purposes) bass that works properly, you could trade out the electronics from one bass to the other to see if the problem follows the preamp board. If the problem is there with the board from the fretless installed in the fretted bass, the problem will probably be a shielding or grounding prob.

    Try just unplugging and reconnecting any plug connectors to clean the connectors.

    I would try swapping out the preamp first. I have a gut feeling that you have a faulty preamp.

    Keep us informed, if you will, Jontom. If I can clarify any of the suggestions, feel free to PM or E-mail me.

    Good luck.

    Pkr2
     
  8. Jontom

    Jontom

    Mar 11, 2002
    New York
    pkr2 thanks for the "input"...to answer some of your questions the fretted(problem)bass works fine with a cord, I can touch and move my controls with no "crapping" out. It will "crap" out at any distance. It seems to have happened over time. Other than the initial installation of the preamp, not to much has been going on in the cavity,other than a battery change about 2x a year. I"ve thought about swapping the electronics, because the fretless is my "homebass" and the fretted is my stage bass...but I have other projects on the burner and haven't had time to. Thanks again.
     
  9. I would've thought a cap in series with the input wouldn't allow signal to pass. Afterall a cap passes AC and blocks DC, but the signal from a bass is well AC technically. Plus in amps they have "coupling" capacitors between the stages for this reason, so no DC gets to the next stage and amplified.

    I was unsure about doing the Pf cap from input to ground, because of the passing AC idea. It would pass the signal to ground. Although if you put it from Input to the pre-amp ground, it may fold into the negative feedback loop and definately damp out that RF. Its something thats done in amps. Place a Picofarad capacitor on the Collector to the Base of a transistor or the Emitter to the transistor.

    And yeah i guess with a very low value, it is in parallel with that "phantom cap" idea (see thread in this forum), and caps in parallel act like "resistors" in series. so yeah that should be fine.

    Hmm i just answered my own questions...

    :D:D

    Merls

    boy we're gettign technical now! ahh its great
     
  10. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    A high value cap will pass audio frequencies and block RF frequencies. A low (PF) value will pass RF but block, or seriously attenuate audio frequencies.

    It has to do with capacitive reactance.

    That's why electrolyctic (normally high relative value) caps are used in crossover networks and low value caps (PF) are used for the tank cap in an RF oscilator or RF coupling caps.

    Notice that the filter caps in a 60 or 50 hertz power supply(audio frequency) are always high value while the filter cap for a diode AM radio detector (RF frequency)is always a low cap value.

    I'm not sure if you have gotten into AC theory that far at this point in your classes, Merl, but if you want to play around with the formula for capacitive reactance it will probably turn some lights on for you.

    Confusing? Darn right it is, but that's what makes it fun. :)
     
  11. Jontom

    Jontom

    Mar 11, 2002
    New York
    I'm confused. I have a cap,but I don't think it will work. It says "New York Mets" on the top...
     
  12. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    LMAO!

    You're right, a Mets cap wont work. :)

    Pkr2
     
  13. Yeah i have done AC theory, but they never explained that aspect. Thanks Pkr2. Your post actualy reinforced my anger at the place where i am studying. Not only do they study bits and pieces, it is never indepth enough, or they go too indepth and forget other simple things. Not to mention cancelling my Audio course due to "lack of numbers" which is crap, as i got it reinstated with more people than before.

    *sigh* In fact Pkr2, yours posts have clarified many of the things i was "supposed" to know.

    Thanks!

    :D:D

    Merls