The TNY277PN must be working since it generates the 5V output. In your case a little low at 4.74V.
However it also generates the voltage to run IC808 and IC902 via another winding from the same transformer (T902) which is then rectified and smoothed by D904 and C912 but only measures 10.8V.
IC808 requires a minimum of 11.5V and typical 12.5V to turn on which it's not getting. The voltage at D904 is too low which I think may be the reason that it is not running IC808 to boost the voltage to 380V
When checking voltages in the primary side, use the large (100µf/450v) capacitors negative lead for ground.
Check the voltage across the large capacitor, in standby the voltage should be around 325vdc. When you connect S/B to +5v that voltage should go to around 380vdc. check it. and if it does not change, check pfc ic, using hot ground check the vcc voltage on pin8 of ic909, it should be around +20vdc
The PFC circuit is not being turned on. If it was, the voltage on the main cap would icrease. I suspect you are not getting 20v VCC, this VCC voltage also supplies ic902 pin6.
The 10 volts on IC909 pin 8 is supplied by it's internal startup cell. Check the voltage on Q903, The emitter should have ~25vdc, the base is controlled by optocoupler IC905. When IC905 is ON, pin4 will be low turning on Q903 which supplies the +20dc for IC909 and IC902 VCC
Is this the voltage on the emitter? This voltage needs to be higher, check that C912 is not bad, also ZD910 and ZD903 could be bad and zenering at too low a voltage
If ic901 was bad you would not have any +5v standby voltage. You can't check the zener point of zener diodes with a meter. Have you tried replacing C912? Have you checked if R918 is ok?
If your +5v standby is ok. disconnect thge s/b line from 5vstb, now check the voltage across IC906 pins 3&4. There should be some voltage there. Now connect the S/B line to +5v and see if that voltage goes to 0 volts. It should if the optocoupler is working.
Check the voltage across IC906 pins 1&2, that will tell you if the internal led is being turned on or not. Use the same method for testing by disconnecting and reconnecting S/B to +5v
You can also monitor the voltage across C912 and try shorting IC906, pins 3&4 to see if the voltage changes and goes up to around +20v
There could be a fault wiith Q908 not turning on the optocoupler, when S/B is connected to +5v, Q908 drain should go to 0 volts, the same with Q907
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