Marked "393" and "(ST)GZ439" or "6Z439" under conformal coat.
Siemens DC/DC PSU section, near a UC3843B current-mode PWM controller, HF transformer, etc.
Checked the ST TS393ID datasheet, that is a micropower dual CMOS voltage comparator. Micropower doesn't make much sense in this application, and if it were that it should be marked "S393I". Checked TI's LM393D, nothing they make is marked with only "393" either.
But the other side of the two caps below it do connect to pin 4, so this seems fitting for a 393. Is this a custom marking, custom device, or?