Funny that you should say this - I bought a used 502A also from an Audiogon member BUT I have hum from the speakers (& not the chassis)! I wonder if this is normal??!!
Brad Day,
He's got hum from the chassis & not the speakers!
Might suggest tightening all the screws related to the 3 xformers. Transformers DO hum when electric current & hence a magnetic field is created within them. This is normal. However, if the xformer is correctly designed, the hum is of negligble volume.