ATX stby work proper but no output

i have i ball xpx-252 psu in which i get standby voltage perfectly, & also get power on volt but i cant get final output of 5,12,3.3 etc
i have checked al componant all is well. i have found that power transformer not get voltage in it's primer nor secondary , i dont know how to check drive pulse in circuit.
please guide me.
i am beginner.