Re: Optiplex 755 only turns on through power supply button, no video output.
Despite the working power supply test button, I tried replacing the power supply anyway just to see. Lo and behold, it worked! That button is a little liar. However, it appears that SATA ports 0, 1 and 2 are broken, and the BIOS only recognizes SATA3. So I need a new mobo anyway, on top of the power supply, and a new I/O panel it seems....yikes.