Using CM-31 mics. How do I reduce hum?

I have a hum problem with six CM-31s hung over a stage. When we bring up the lighting dimmers, we hear a hum that is traced to the mics.

In my theater, the female mic connectors are in metal boxes over the stage, and are tied to grounded electrical conduit.

Reply: The differing ground potentials between the conduit and the mic's pin-1 ground (which is grounded to the mixer's ground) apparently created a ground loop that caused hum.

Try removing the set screw in the CM-31 housing that grounds the shell to XLR pin 1. This would break the ground loop and stop the hum.

Also, the CM-30 and CM-31 have long cables that are medium impedance unbalanced, making them potentially susceptible to hum pickup. If the hum does not stop after you remove the set screw, cut the CM-31 mic cables short (about 3 feet long) and re-solder them to the mics' XLR connectors. Then you can run long low-impedance cables from the mics' XLRs to the mic connectors overhead. The low-impedance balanced mic cables are much less sensitive to hum pickup.