Chances are, someone yanked on the mic cable somewhere, probably where it plugs into the board, and tugged the
shield wire loose. I doubt it's the board itself.
Simple troubleshooting steps. First, if you have anything plugged into the insert
jack on the board for that
channel, unplug it. If the hum goes away, it's either the
insert cable or the gadget you're inserting.
If the hum stayed, swap that mic's
XLR connection with the one next to it. If the hum stayed on that
channel of the board, then the board is at fault - possibly a cold-solder joint on pin 1 of the
XLR. If the hum followed the mic cable to a new
channel, the board's okay and it's either the cable or the mic. itself.
If this is a permanent install, chances are the cable plugged into the board is relatively short and goes to a wall-plate with a bunch of XLRs that are, in turn, wired to jacks on and around the
stage. Go to the wall plate and swap the same two cables you just swapped at the board. If the hum stayed on its new
channel, it's the cable between the board and the wall plate. If it went back to its old
channel, it's either the installed wiring to the
stage jack or the cable from the mic to the
stage jack or the mic itself.
Do the same swap at the
stage jacks. If the hum stayed on the same
channel of the board,
call an electrician - your installed wiring has a problem. If the hum moved to the other
channel, the installed wiring is good, it's either the mic or the cable between the mic and the
stage jack. Swap mics. If the hum follows the
microphone, you've got a bad mic. If it stays on the same
channel, it's a bad cable.
John