A slightly sick SG505


I am hoping the wonderful minds here can help me with a sick SG505. The main output is working and the distortion is good. They Sync output is not working correctly. I have checked U1300 and Q1300 and 1301 off the board and they seem to be behaving like they should. For U 1300 the resistance between pins 4 and 5 goes down when power is applied to pin 1 and 2 with a bench power supply. I can see signal at the base of Q1301 but the signal seems to die in the optoisolater probing 5 or 4 with a scope shows dc only. Q1301 and 1300 both pass a diode test off the board as transistors of their respective types. I suspect U1300 but what is odd it seemed to work off the board. Not sure what I am missing.

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