If the relay does not get energized then the ceramic resistors will blow because they are carrying more current than intended;
they can only carry the AC for a very short time, enough such that the voltage can be sensed and reported back to the controller, which can then pull the return side to ground to drive the relay coil.
Diodes D311,312,313, and 314 are used as a bridge rectifier to convert the AC to DC, where it gets divided down thru resistors and sent across a photocoupler (aka
Opto-Isolator), PC305, and thru the ribbon cable to the upper control board.
So there are multiple possible failure points for this critical function:
Presence of AC? check that off since it is blowing the resistors
Sensing of the AC?
Low voltage supplies? 5V used to drive the relay created on the upper control board
Relay return switching? transistor on the upper control board
Defective Relay? damaged contacts
IC501 is used to sense the HV DC on the Output side; your issue is on the AC Input side