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Help diagnose component gremlin

Discussion in 'Amps and Cabs [BG]' started by Eric Moesle, Mar 2, 2004.


  1. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    Ok, I've got a gremlin in what I THINK is my QSC PLX 3002, but it might be in my Eden Nav preamp or the cabling. While As I attempt to track it down through process of elimination, curious as to what y'all think.

    The Symptom: During the past few gigs when running at volume, the sound would cut out, not entirely, but would significantly reduce, sounding a tad distorted, but only for a minute or less, then it would mysteriously swoop back in. Sounds like the amp temporarily chokes for lack of power. Happened once, no big deal, just a gremlin. Next gig happened twice, then last Saturday it increased to four or five times, and the reduction in volume is getting worse, nearly to total silence the last time it happened, and the cutouts are getting lengthier. When it kicks back in, it "sneaks" back in gradually over a second or two, not abruptly like someone just flips a switch.

    I can't get it to reproduce the problem at home when I crank it up, it only happens at gigs when I can't get around to tinkering in the middle of a song.

    I changed all the cables a few weeks ago after it first happened, no effect (everything's permanently mounted in a rack anyway, but I figured it couldn't hurt to try). Still the problem. Output LEDs on my preamp stay the same when it happens as when its pumping at volume, but the output LEDs on the QSC reduce significantly when the gremlin hits. Still not indicative, as the preamp might be sending a lower signal out AFTER its LED stage. Hmmm. Its definitely NOT related to any instrument or pedal problem, its happening in the preamp or poweramp, I just can't nail down which one.

    My setup: out of the Eden Nav into the QSC via XLR, two lines each to a different channel, nonbridged, 30hz filter on, clip limiter on. Usually only one channel used, usually channel one, into a 4x10 cab thats rewired for 2.6 ohms. Some larger gigs I add a second 4x10 cab though channel 2 (cab rated at 4 ohms). When the gremlin happened once when I used both cabs, it only affected ONE channel/cab, not the other. But when I ran just the one single cab through channel two only, it happened on channel two just like it did on channel one. The last gig I used just the 4 ohm cab, and it still did it.

    This weekend I'm going to try to disengaging the clip limiters and 30hz filters to see if that has any effect, and perhaps for one set just bypass the entire Nav preamp and go into the poweramp from my Sansamp Acoustic DI that I usually use for my EUB.

    I really can't afford to just ship the power amp out to QSC for a diagnostic since I play 2 to 3 nights a week and can't do without it. It very well may be the output from the pre, who knows. I'd really like to track down the gremlin and confirm that it is indeed the power amp first.

    Curious as to your thoughts? :meh:
     
  2. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    I'm no whiz, but could it be weak power at the venue(s) you play? Maybe you get better AC at home that at the club?
     
  3. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    Good point, I thought of that, but I use a Furman PM-8 that shows both input voltage and output amps on its front panel, and they're all fine when the gremlin rears its ugly head. This assumes the Furman is accurate, of course.