Vizio 75P-C1 Won't turn on

I don't think you have all of the model number as I can't find anything on a 75P-C1 or have I ever heard of this model. Without the correct model it's a little hard to give good advice but will try, 1st thing to do is look on the power board for 5 volt standy pin and see if you see around 5 volts there, I think you will find it's ok considering you've already replaced the power board with no luck, if it's missing you received a defective power supply, What I suspect you have is a bad EEPROM, that's an IC located on the main board, one thing you can try is taking the board out the TV and put it in the freezer for an hour, after it's cold replace it in the set and see if it will power on, in most cases if this IC is bad using freeze spray will make the set work, confirming it's the EEPROM that's defective, but freezing the board will confirm if this is what's wrong. You have 2 choices if it's the EEROM that's bad, in most cases you can buy an IC from shopjimmy.com and solder it in, although it takes some advances soldering skills to replace it as it's very small, other option is replace the main board.

It's possible that the issue could lie with either the T-Con board or the mainboard. However, it's difficult to determine the root cause without physically examining the TV and conducting further tests. Here are some steps you could try:

1. Check for any visible damage to the T-Con and mainboard.
2. Check the connections between the T-Con, mainboard, and power board to ensure they are properly seated and secure.
3. Replace the T-Con board with a new one and see if that resolves the issue.
4. If replacing the T-Con board does not resolve the issue, consider replacing the mainboard.