You stated that you replaced the wick in the Trailing A unit. You must be very careful, there is a thin, plastic inset that contacts all four walls of the smoke unit chamber. It is there to prevent resistor contact with the chamber wall. This is most probably where your short is.
After you replace the wick, or do any internal work on smoke units it is wise to check the resistors and make sure there is no contact with the outer wall. Always check the resistors for dead shorts prior to connecting the smoke unit and powering up.
Bruce
