I see a lot of SMD resistors in series on the input side. Most likely is that there are high value or open ones there in the start up supply to the controller, which would explain the behaviour after power cut of the resistor slowly going higher in value till it was essentially open. Simple to check in circuit as well, just go along the input side resistors and look on resistance range for any that are either at marked value or higher value, though you might have to wait 10 seconds for capacitors to change. Easiest to only check where there are 2 or more in series, as this will be high voltage withstand. Replace with a standard leaded part as temporary test ( and put on a thick insulating sheet to test out of chassis) and then replace all of the same value on the primary side when you find which 47k plus resistor is open circuit.
The second hand refurbished ACER I am using to type this on had an annoying power supply whistle after a while. Rather than get a warranty replacement I simply went and got a cheap ( not the cheapest, a little better than the cheapest PSU I have had reasonable results with before) replacement PSU and removed it from the standard footprint case and shoehorned it into the custom ACER case. Took the fan as well, just did a quick mod to have it run all the time at full speed, rather than the default "run to lightly brown" that the thermally controlled fans are set to do out of the box. Even though the case is a third smaller the board did fit, and aside from having to solder on the mains leads to the socket it was easy enough. As a plus the new board did come with the optional input filtering and EMC chokes and capacitors, along with a wired in fuse, something ACER omitted from the original. Bonus was the extra 12V leads and the extra SATA power leads as well, as opposed to the original having exactly 1 power lead with SATA, IDE and FDD sockets along it. Also added 2 extra case fans to shift air around inside, seeing as it now has 2 HDD inside instead of 1, and they do benefit from extra cooling.