The power supply will be located on the main board, not a seperate
replaceable card. Someone must troubleshoot the circuit and replace the
faulty components.
Replacement boards are not made available or will cost more than the TV is
worth.

Some Sony sets did have separate power supply boards, and some boards are
available, but you're correct that it's not generally economical to replace
a whole board. Power supplies in particular tend to be easy to repair.