Procontrol Power supply hi frequency sound

I have a ProControl that has been working for 10 years. I powered it up recently and it has a hi frequency sound coming from the power supply. It has a sound similar to a fax machine. It is probably related to a capacitor that is going out. I would like any information anyone might have as well as any schematic or service manual that would help. Thanks for anyone that might point me in the right direction. My only plan now is to replace all the caps in the power supply. Thanks again. Gary