When I turn my stereo I get a low level hum/buzz. How do I get rid of it? My system is a few year old Pioneer rack system. Nothing special.

mine did that when my ground on the turntable was screwed up...after that ?????

Ground loop problem. Search for ground loop on google.

Ground loops can be a pain to trace. There are two most common problems that create the hum besides ground loops. The first is wall warts and other power supplies. Don't run your power cables bunched up with your line level (hopefully shielded) cables. Second is make sure you are not using unshielded cable for your component interconnect. E.G. use shielded between your CD player and tuner / amp. Make sure you only use speaker cable for speakers.

I had a capacitor go bad on my BGW 750-A rack mount amp and it developed a hum,

replaced it and it's now clear as a bell.