hi..before you replace the whole power supply try to isolate first. if the problem is in power supply or in the mainboard.you can test the power supply without the main board.since you have standby..search for the triggering pin in the power supply.usually the label is POWER_ON,PWR_ON,MAIN_ON,SW_POWER,ENABLE,DATA and etc.use 1k to 10k resistor,connect the resistor from standby volts to the triggering pin,and voltage check if there is output for 12v 18v or 33v for inverter.make sure to remove all the connector going to other board before you do it.if you get 12v output and some other output it means your power supply is working,probably you have a problem in the main board or in inverter board.