On some ATX power supplies from 2000s, a chip called FAN4800IN was used for PFC+PWM control.
On at least one supply, I had to replace the high voltage electrolytic capacitor and maybe some other components, before I got the whole power supply to work again.
The capacitors from the “capacitor plague” time leaked electrolyte, or the electrolyte just evaporated.
One of the indicators that the FAN4800IN has also burned down is pin 14 to pin 10 voltage - it should be the reference voltage (7.5V), if it's not, it's probably bad.
I ordered this from Ebay, replaced the FAN4800IN and managed to make a 600W ATX power supply working again.
FAN4800AS/CS can be used to replace FAN4800IN, but some additional components need to be changed also. Maybe ML4800 and CM6800 can replace it also, this needs further investigation (datasheets need to be compared, etc).