At church we've got a Soundcraft Spirit LX7 24-channel mixer, about 7-8 years old, working fine until we took it apart a month ago for significant cleaning. ;-(
Everything works *except* for the gain pot on one of the channels (of course, *the* most frequently used channel... /me strangles Murphy). Over the last few weeks it's exhibited various symptoms, then worked fine for quite a while, then failed, then worked, etc...
The current failure mode, which seems persistent, basically has the gain of the mic preamp cranked way up. With the gain pot at "zero" (below unity), I get a signal that seems roughly comparable with a normally functional channel with the gain pot cranked all the way up. If you turn the pot clockwise, the gain goes up even further, well beyond what is useful or safe.
I haven't yet done exhaustive tests with the rest of the channel strip, but I believe that the failure is contained within the preamp. I know the circuit is pretty darn simple, consisting exclusively IIRC of little
8-SOIC opamps and passives. I don't remember if there were any discrete transistors around, but if so there aren't too many, and they're really tiny ;-(We'll have an oscilloscope to debug this, and of course there are 23 other channels to reference against, but does anyone have any early hints as to where to start looking, or what kind of failed component we might be looking at? Knowing the little I do about analog design, I strongly suspect a walking-wounded op-amp from some kind of un-noticed static discharge during our cleaning. It could also be a physical failure, either loose wire (unlikely as there are no wires in the
*middle* of the circuit, just on the input and output sides) or loose trace.Anyway, I'd appreciate any hints, and I'll try to make sure to post an update once we've dug into the board. We have a few months before we
*must* have it fixed, as there are spare channels I've moved that mic over to for the moment, but come July we're gonna need the *whole* board.TIA, Omega aka Erik Walthinsen