The AO output on the SE7600 controller is either off or on instead of modulating.
SE7600F Room Controller
The SE7600 Room Controller is expecting a Discharge Air Sensor to be connected in order to detect minimum heating temperatures. While this is missing it will not modulate the heat based on room demand.
Install a 10k Type 2 Discharge Air Sensor between the DS an Comm terminals or add a 10k resistor to allow the unit to modulate heat based on room spacetemp.