How this SMPSU start up and working on CC mode?
At start up the TL494 is dead and primary of the drive transformer has a high impedance across it .Main switch halfbridge transistors  Q1 Q2 are biased slightly into class A due to the 150K bias resistors.They now have gain and form an oscillater due to positive feedback provided by the current overwind on the secondary of the drive transformer .The frequency of this prospective oscillation is on this circuit determined by main transformer saturation .The duty cycle of this start up oscillation is close to 50% ,in other words there is only a small amount of dead time .After a few cycles the TL494 activates because it now has Vcc .The TL494 now controls the power supply via the drive transformer primary .Output current is sensed along with output voltage and fed into the TL494 control loop in an orthodox manner .Under current limiting the average DC output voltage falls but Vcc which is peak rectified should stay relatively constant so the TL494 still runs .This old circuit was really clever in its day.It could be tarted up for greater reliability ,lower switching losses and outperform a lot of modern stuff .