If the power on your FMi C Series transmitter seems to drift up or down in output power over time or whenever my air handling system turns on, the problem may be due to the WINDOW ADJ R145 on the transmitter control board being set too wide.
The Window adjust is located on the transmitter controller board. You will need to open the front transmitter control panel. The Window Adjustment is near the 9V battery on the transmitter control board.
To verify if the Window Adjustment is set properly, look at the AGC SCALE on the Exciter. To get to the menu on the exciter, go to the OPER MODE, then Select IBOC Menu. When this adjustment is set properly (usually 3 turns Clockwise from fully counter clockwise) you should see jitter of the last number or maybe the last two numbers of the AGC SCALE on the FXi Exciter.