Smps output fluctuation

[kobliha]

Hi, I have a problem, that the power supply from the coffee machine is not working. From the beginning, the source did not give any input voltage, after replacing the electrolytic capacitors (the small ones at the IC controller, at the optocoupler and at the output terminals), the source started up and gives 5.5 volts and 26.5 volts. however, it should be at 25 volts. After connecting the load, the voltage of the source drops by about 0.5-1 volt and the voltage constantly varies between 26.5-25.3 volts. When I changed the value of the resistor for the TL431 so that the output was 28 volts, everything ran as it should and the source was stable. So I want to ask where should I look for the error? Or is this normal behavior.
the power supply has a Viper22A and lb598 control circuit.
Photos of board

SMPS - Google Drive

drive.google.com

 

[D.A.(Tony)Stewart]

So it’s ok now?

 

[kobliha]

it's okey when i replaced that resistor and the source had 28 volt output. but I need the source to give 25V (originally) which, when I return the value of the resistor from the tl431, the output is not stable and I don't know why.