Discharging capacitors is rule #1 when working on power supplies or voltage circuits. I've been in Electronics since "shep was a pup", worked on high voltage tube devices including transmitters and have held a general class ham radio license and a 2nd class FCC radio telephone license. I've also worked as a guitar amp tech in Nashville.
As far as working on power supplies, I have a friend that is a retired broadcast engineer (and has an EE degree). He says some of the components in a power supply are specialized and replacements can't be purchased. He has a son that does electrical power supply design work and they both agree that a PC power supply is not realistically a "repairable" device.