smps break down after two or three second

hi all
So i have a problem with a power supply of echographe aloka
the problem is when i start the power supply all is ok , all output voltage are ok but this is take for a three maybe tow second and then it slow down to zero (all the outputs) until the 400v capacitor discharge complitely.

i think for the first time that the control is not good so i replace the tl431 and his resistor and capacitor and the tow optocoupler PC1 and PC2
and nothing is happen
so secondly i try by changing tr2 and tr3 and the thyristor put1 and also nothing happen
when i test the same thing happen
and i verify all the component in the control with tl431 and pc1 and pc2
and also the primery i the 2sc transistor and all arround it


so any one have an idea

thankx a lot

Check the voltage on primary(LED) side of PC2 - if it is greater 0.1 v, but voltage on c12 is ok - may be leakage of zener D16. Also,if You have oscilloscope, check pulse voltage on all secondary electrolitic capacitors.

thankx
so the voltage of LED PC2 i about 1.12
voltage in C12 is 0.5
i will change the D16 and i tell you
thanks for help