dimmers come up for no reason within a show, then stay on until powered down and restarted

This is a strange one. My board op was running cues and found that midway through the show a number of lights from different dimmer packs came on during a cue that was supposed to be a blackout. And they stayed on.  She turned the breakers off and on again, and the problem disappeared. Had an earlier prob with lights coming on after power down, but that may have just been improper shutdown.  What about this lights up at random and staying on?  Does this sound familiar to anyone? 

  • As you turned dimmer breakers off then back on to fix it, this must be an issue with the dimmer packs themselves.
    That is unlikely to be anything to do with the DMX cable or the console, it's almost certainly internal to the dimmer pack.

    I'd suggest getting in touch with the support for the dimmer pack about this one.

    Lights coming on after shutdown is likely to be a different issue. That symptom can sometimes imply a DMX wiring fault - either broken cable or missing terminator resistor.
    DMX will often continue to "mostly" work with broken cables, only intermittent flicker or other issues.

    There should be a 120 ohm resistor between pins 2 and 3 at the very end of the DMX daisy chain and nowhere else.
    For DMX fixtures it's often done with an XLR plug, for installed dimmer packs it's often done with a small switch to enable/disable this. Check the manual to be sure.

    (A DMX splitter with nothing connected to its "Thru" counts as being the very end of the first daisy chain, each of the outputs is a separate daisy chain.)
    Easy first thing to check is for proper DMX cable termination:
    Turn everything off, then unplug the DMX cable from the back of the console and measure the resistance between pins 2 and 3.
    You should read around 100-130 ohms.
    If it's higher then you have a broken cable or there's no terminating resistor fitted at the end of the cable.
    If it's lower than you have a shorted cable or there's more than one terminating resistor fitted by mistake.