It has been over a year since the installation of my outdoor ground mounted AM transmitter and, until today, there have been no problems when it was discovered that the transmitter was off the air...not a weak signal but no signal at all.
The transmitter was brought to the 8Z labs and the problem was obvious when the lid on the enclosure was opened revealing about a quarter of an inch of water in the bottom of this water proof box. The water could have entered where the RF and power/audio lines enter but the duct seal compound at these points was intact. Water could have entered through the power cable which is a simple vinyl covered multi-conductor cable where the water can creep through the cable between the outer jacket and the conductors but there was no evidence of this. The water could have entered if the cover gasket was not seated and, indeed, inspection of this gasket revealed that it had some gaps where it was not smooth. Though not definitely determined, this is the most likely path for the water entry.
The water did not damage anything but it did affect the PLL frequency generator most likely by entering the DIP frequency selector switch. The PLL frequency was 8.9 kHz rather than the usual operating frequency which could have been caused by water in the switch bridging the contacts which were supposed to be open. A few minutes of gentle application of a heat gun fixed the problem.
The next step will be to place a plastic bucket over the enclosure to make it harder for water to enter, or even better still, maybe install a mini sump pump in the box.