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Problem In Optoisolator IC
The most common fault for an Optoisolator IC is the short circuit between the collector and the emitter pin and you can test it by using an analog meter set to X 10 K ohm and it should not show two readings. If there are two readings then it is either consider shorted or leaky. The LED diode inside the Optoisolator IC rarely have problem and you can test it using analog meter set to X 1 Ohm and it should have one reading.
In power supply, if the Optoisolator have problem it will cause many different types of symptom like no power, low power or even power shutdown. Since this IC is not difficult to test, i suggest that if you come across problem with the power supply with the symptoms that i have mention above, you may check on the Optoisolator IC first. What about Optoisolator IC that breakdown when under load? Yes in my repairing career i do came across Optoisolator IC that breaks down when under full operating voltage but it was rare too. If you have test all the components in the power supply section and still could not find out the fault, i guess there is no harm in replacing the IC and retest it again. Who knows the power supply may come back to life again.