Hi @sam65142 ,
Without at least a DMM (digital multimeter), it will be difficult to find the problem as nothing appears to be visually wrong with the power board.
The [https://www.manualslib.com/manual/808537/Onkyo-Dv-Sp502.html?page=1#manual|service manual] doesn’t have the schematics for the power board but on [https://www.manualslib.com/manual/808537/Onkyo-Dv-Sp502.html?page=26#manual|this] page it shows the various DC voltage outputs being supplied by it.
I’m assuming that in the Standby mode that there will only be the one standby voltage being supplied that will allow the Standby LED to be turned on (not directly by the power board but via the main board), whereas the other voltages will only be there once the unit has been turned on. Unfortunately the manual doesn’t state what the value of the Standby voltage is or have a schematic of the power board, so if there is no voltage being supplied on any of the pins on the output from the power board, you will have to start back at the input to the board and work through the circuit.
''Be safety aware'' if you decide to do this as you will be testing with exposed lethal AC voltage on the power board.
Hopefully a start.