Whether switching power supply or linear (whichever it is) try this experiment. Disconnect all inputs to the receiver & retest. Also try turning the AC plug around & test once more. Also try it with the tuner playing. If you get a buzz no matter what, then the receiver's power supply needs repairing.
Need Help
My dad has an older Sony reciever.There is a LOUD buzz in both channels.The only source hooked to it is a CD changer.I had a ground loop hum in my system a while ago.This is not the same noise.I checked all the connections,everything is hooked up right.The outlet is two prong,so he is using a power strip with a cheater plug.I tried plugging in the reciever directly into the wall,same results.Any help would be appreciated.Thank You. John