T start with, I would be checking to see if perhaps the standby 12V is low - also check the microprocessor B+ (perhaps 3.3 volts) and it's oscillator crystal and reset lines. The crystal typically is about 1/2 Vcc DC volts on either side, and the reset line should toggle LOW when the unit is plugged in, and rise to B+ pretty quickly and smoothly.
Also, you can just look at the relay turn-on line coming to the standby power board and see if there is any action there when you press the Standby / On button.
Come to think of it, that should be your FIRST step.
If, for example you see that line budge but not go much more - you could have a leaky relay drive transistor.