About Optoisolator IC In Power Section
If you are repairing a power supply that do not use optoisolator IC as a feedback that means if all the secondary output voltages read zero volt then i can say that the primary section have problem. It could be a dry joints, ciruit track broken, bad FET and power IC, leaky or shorted diode and transistor, cap that have high ESR or even an open circuit in resistor (start up resistor). However if the power supply use optoisolator IC then there is chances the secondary side components that corresponding to the optoisolator may have problem. If you concentrate only on the primary section, i can say you may not find out the culprit. A shorted optoisolator IC and TL431 IC could cause no power symptom (zero output voltage) and you may thought that it was the primary section fault.
Understanding the power supply design is one of the key to successful power supply repair. Practice, practice and practice on different type of power supply repair (don’t just repair-you have to understand how it works) will make you better in solving any type of power supply problems.