About 15 years ago I worked for a HV power supply company. A 723 was used to regulate the HV output (15KV). Using a new batch of 723's the HV output started going high & unregulated. The problem was traced to the 723, The old one had 2 of the pins open while which were being used for voltage feedback sensing. The new 723 replacement had these two pins shorted internally. Just wanted to mention this for something to think about while troubleshooting. Sorry I can't remember which pins. But if your power supply is very old, and you have replaced the IC......Something to verify.

About 15 years ago I worked for a HV power supply company. A 723 was used to regulate the HV output (15KV). Using a new batch of 723's the HV output started going high & unregulated. 

The problem was traced to the 723. The old one had 2 of the pins open while which were being used for voltage feedback sensing. The new 723 replacement had these two pins shorted internally. 

Just wanted to mention this for something to think about while troubleshooting. Sorry I can't remember which pins. But if your power supply is very old, and you have replaced the IC. Something to verify.