I will be happy to assist you with this question.
I understand that you have the STB 5V going into the main board.
This is all the main board needs to function at this point.
Now,please push the power button and monitor with the meter on Vdc,the power on command from the main board into the power board.This should be a pin on the power board connector,that should go to about 3-5V when power button is pushed,
This command is supposed to wake up the power supply ,and this should generate the other switched voltages.
My suspicion is that the main board is defective and will not generate this power on command.
The part# XXXXX this board is:
Unfortunately it is very hard to find,there is a very low availability for it.
If the Power On command is coming into the power board ,but still no output voltages from the power supply=defective power board.
Part # XXXXX this is:
A good place to start looking for parts is :
I hope this helps,
Please leave a positive feedback if you feel I did my best to answer your question.It will be highly appreciated.
Thank you and have a good night!