I will show you how to check your televisions power supply board on a dead tv set. This is a sony lcd tv KDL-40BX421. We have no standby light led on front of the TV, no picture or sound, and no Back Lite. The very first thing to do is check for our standby voltage (DC volts) using your multi-meter. If you do not have any standby voltage on any of the pins on the plug going to the mainboard from the Power supply; then most likely you have a bad mainboard. If you do have 3-5 volts standby then there are a few more check you need to make before you rule out the power supply completely. You have to jump the POWER ON or PS ON pin to the active standby voltage and see if the power supply board attempts to come on by simultaniously checking voltage on another line. For example, the 12v or 24v line. If those voltage(s) still do not come up then you definitely have a bad power supply board. If any of them do then we can assume that the main board is bad.
NOTE: THIS PARTICULAR TV HAS AN "AC OFF DET" LINE. IF YOU HAVE A DIFFERENT TV WITH NO "AC DET" LINE. JUST MAKE SURE THAT YOU HAVE STANDBY VOLTAGE!
