In article , Phisherman wrote:

My bad. It is the evaporator in the basement and located above the
furnace. There is a PVC drain, and I checked that there is water
flowing through the pipe to the outdoors. The outside of the
evaporator is cold, wet and dripping on all sides when the A/C is
turned on.

Too much humidity in the basement. Run a dehumidifier, or add a cold-air
return in the basement so that some of that humid air is sucked through the
evaporator and dehumdified.