I checked the power resistors and Zenar diodes, seem all working. I got resistor reading both direction of a diode around 100 or so Ohm I suspected faulty but checked again all good (I should have had a good note of all measured carefully, it's confusing when turning the board back and forth, measuring here and there). Also I used a damp wipe to clean the surface of some components, also the board as far as I can reach (and cleaned the dust over the silk screen print to read the text) - of course very carefully and when not live.
The most strange thing was at one point I got a voltage across pins 6/7 of the controller around 10V, and about 200V across R1S14 (1/4W 270k). This was very different from previously (i.e. now seemed working as it should). Surprised (also confused), I unplugged the power and discharged the 400V cap, reconnected the PSU board with the main AV processing board and the front panel (it's needed because it sits between the power switch and PSU). I pressed the button and it worked (the power LEG lit up)! See photo.
With excitement I put the boards back to the chassis expecting all will be good now. Just to be cautious, before hooking up the DVD drive, I tested by pressing the button again. It failed to turn on! And the voltage across these jobbies all disappeared! (330V or so DC after the rectified still there, i.e. the power is definitely plugged.)