My Harmon Kardon receiver recently started to hum. It does it regardless of
which input is selected. The hum varies with the volume control. I opened
the unit and tapped on various components with a wood stick. This had no
affect on the hum.

This is an old unit that I have owned new since early 70s. Perhaps this
might not be worth getting serviced by a shop? I'm good with tools but lack
electronics skills. No scope either. What could be the cause of the hum? Is
this something that is fairly easy to replace/repair?

Here is a pic of the circuit board:

http://home.everestkc.net/upperquad/...rmonKardon.jpg

Thanks
John