Problems In Isolator IC
There are few problems that can be found in Optoisolator IC (4N35, PC817 and etc). The most common problem is the internal transistor shorted or turn leaky. The diode LED could have open circuit but rarely shorted. Apart from that, optoisolator IC could also breakdown when under load (remember, it has transistor and diode LED inside). It could also cause intermittent problem. If all the components in the power supply tested good and the power supply still don’t work, then i suggest that you directly replace the Optoisolator IC and retest the power supply.