If you have replaced the power supply and still the system is not working properly, it can be an issue with the system board.
Try using a different power outlet.
Also, try turning on the system after removing all external peripherals.
If the issue persists, remove internal components one by one and try turning on the system to isolate the system.
Please refer to the service manual for removing the internal components.
NOTE: Internal components are to be removed and replaced by a professional only.
Thanks & Regards