I've attached a photo of the top and bottom of the PCB of a SMPS.

I found the TL431 but I can't seem to locate the voltage divider connected to the reference pin.

I would really appreciate if someone could take a look!

Thanks!

 

hi bigjon,
Are sure that it is a TL431 with that 4 pin arrangement??
E

 

hi bigjon,
Are sure that it is a TL431 with that 4 pin arrangement??
E

Only 3 pins to my eye, but I can't see a divider there either. Are you certain that is the 431?

 

hi boost,
I can see that middle 'stub' on the IC.
E

 

R31 and R34||R42.
It's a three-pin TL431, which means the pin on its own is 0V. The 0V track goes underneath the zero-ohm link and across to the Class Y capacitor.
C7 is the feedback capacitor to keep it stable
R24 is the opto current limit resistor
R25 is the opto bypass resistor
What C8 is doing there is a mystery. It would put a pole in the feedback loop and most likely make it unstable.

 

It would look like Q4 could be a TL431, I agree with @Ian0, R31, 34 & 42 are precision resistors. 34 & 42 appear to be in parallel. R31 completes the divider. (my opinion)

 

obvious!