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? for EEs and Kramer fans: ac voltage across gnd?

Discussion in 'Pickups & Electronics [BG]' started by bongomania, Feb 17, 2006.


  1. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    I just pulled my Kramer XL7 (DiMarzio high-output p'ups, 9v preamp) out of storage, gave it a fresh battery, and started to play. I kept feeling little electric shocks as I played! Very subtle little shocks, but enough to make me wonder what's up. So I got out the DMM, and found that there is about 280 mV ac across ground. The preamp grounds to the bridge (and the strings, etc.). It doesn't read anything for VDC. And I tried the same measurements on my other bass (Ibanez MC980), also with a 9v preamp, plugged into the same amp (PJ Briefcase) with the same cable, and did NOT get any ac or DC reading across ground.

    Any ideas? Is there something obvious I should look for, or certain test points I should be sure to read? Thread links I may have missed?

    FWIW, I cannot hear any extraneous noises like hum or buzz or static; in fact it sounds perfect. ??? Thanks!
     
  2. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    280 mV across ground and what?
     
  3. fookgub

    fookgub

    Jun 5, 2005
    Houston, TX
    What exactly are you measuring across? There should not be any ac from the output to ground when you're not playing. I don't trust the AC measurements much on DMMs. Do you have a scope? Are you measuring with the bass unplugged? Have you tried another bass through your amp?
     
  4. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    Good point, both of you, about "what am I measuring across?" I held the black probe in my bare fingers and touched the red probe to various points on the bass (strings, bridge, pickups, control plate, screws). Now that I type that out, it sounds illogical and foolish- what was I grounded to? Anyway, I got a consistant 260-280 mV when the red probe touched the strings or bridge, but not when it touched the pickup poles or control plate or screws. I was measuring with the bass plugged in to an amp.

    Unfortunately I don't have a scope, but I have access to one on Saturdays, so I'll check it out in that context. I did specify that I had measured another bass in exactly the same setup.

    Thanks for any further thoughts!
     
  5. fookgub

    fookgub

    Jun 5, 2005
    Houston, TX
    You should ground the black probe to the strings or bridge and measure using the red probe. Check the output for DC (and AC if you like, but I don't really see the point) and check all your grounds. Your strings, bridge, knobs, output jack shield terminal, preamp ground, and battery negative terminal should all be grounded and continuous with each other. If your bass is shielded, check that too, along with anything else you think should be grounded. With the bass plugged in, measure DC voltage and continuity from the bass ground to your amp chassis.

    Also, what kind of amp do you have? You're not using a three to two terminal adapter ("widow-maker") to plug it in, are you?
     
  6. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    I mentioned the amp in the first post (it's a Phil Jones Briefcase), and I also mentioned that I did not get these electrical readings measuring an equivalent other bass using the same amp and cable. Meaning it's not the amp. I'm not using an adapter, but I do use a Furman PS-Pro which tests for wiring faults.

    So since I don't have this issue with my other bass (also active 9v), using the same cable, it pretty much has to be a problem internal to the Kramer. Unless, and I haven't checked this out yet, the Ibanez doesn't ground to its bridge.

    I'll go test it all out as you describe.
     
  7. bongomania

    bongomania Gold Supporting Member Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    OK, so I've done some more testing, and I'm even more confused than before. With the bass plugged into the amp, and the black probe on the strings,
    red probe to pickup poles=0V
    red probe to metal control plate=0V
    red probe to metal switches=0V
    red probe to output jack nut=0V
    red probe to nothing (mid air)=40mV ac, 0VDC
    red probe to chassis screws of the amp=400mV ac, 0VDC

    I re-tested the other bass, and got very different results this time; I don't know if it was my incorrect use of the black probe last time, or what, but this time I got:
    red probe to nothing (mid air)=25mV ac, 0VDC
    red probe to amp chassis screws=300mV ac, 0VDC
    everywhere else=0V

    Although I still get weird electrical sensations off the Kramer, and NOT off the Ibanez. And I found that the Ibanez does ground to the bridge as well. I also unplugged both basses from the amp and put the red probe to the cable plug tip, and got 0V both times.

    Clearly there are some basic issues here that I don't know about yet. Maybe it's the amp after all, since ac voltage is present btwn amp and strings for both basses. My power supply is clean. Any further thoughts?
     
  8. fookgub

    fookgub

    Jun 5, 2005
    Houston, TX
    You really should test for continuity as well as DC voltage. Could you describe how you're getting the shocks (ie: where your hands are and what your touching)? 9V should not be enough to enough to feel, so I have a feeling your amp is involved somehow.