EDIT: To avoid confusion I underline that I am not the original poster but I have repaired a virtually identical power supply with similar symptoms.
Thanks to your help I fixed mine: C912 had failed (short circuit).
More details :-
I have a DAV-DZ280 with a 09DAV-POWER-MR-MP-12 (instead of -13) power supply board which is virtually identical.
The symptoms were similar: it didn't power on and mine had 0 V on all outputs.
I checked the resistors suggested in another forum and all were OK.
However there was only about 200 mV on the emitter of Q922 and even less on VCC of IC921.
The 320 V rail was OK and I noticed that the heatsink of IC921 was pretty warm.
I measured the resistance from VCC to GND of IC921: only about 7 ohms so obviously something was very wrong in the vicinity.
I read above that capacitors in Sony products rarely fail so I thought "****, IC921 must be dead".
(Particularly as, while searching for IC921's data sheet, I saw that someone was trying to sell them on Amazon at £71.00 + £12.00 shipping
)
Otherwise C912 seemed the most likely to fail so I unsoldered it and found with relief that it was indeed short circuit.
I replaced it and my DAV-DZ280 came back to life
Thank you all for your invaluable help.
I hope that this account of my experience helps others in turn.