First of all semiconductors connected to it, capacitors as well. I would start with ZD604, C613, D603. Just note that you should not power it on without ZD604 as something else may blow up due to possible overvoltage, or temporarily replace it with say 12V or similar zener diode and power on. You could also remove R607 to disconnect a large part of the circuit (to exclude it as a cause), just do that with IC601 still absent from the circuit.