a hum i could blame on ground loop. a crackle i would lean towards switches, dials, transistors, and/or resistors.
the time factor makes this sound more temperature related, maybe mosture? when you cleaned the dials, did you disassemble them and used an alcohol solution? did you damage any wires by tugging or bending too much? or did you run them back and forth several times?
I didn't disassemble, just took the cover off and used the switch cleaning lubricant Servisol to spray a little in each side of each dial - volume, balance, treble, bass, and selector (all of which were slightly malfuntioning and have been corrected). I also cleaned inside the phones jack (all following this: http://www.tomshardware.co.uk/forum/57120-6-cleaning-controls-jacks-info
). This stopped all the crackling resulting from moving the dials.
I just made this video of the problem: https://www.dropbox.com/s/fz2y10iys3pwsbh/3.mp4
Even with the volume at zero it still happens. Shortly after this it stopped crackling after turning it on, when I turn the volume up there is a definite hum. After leaving it off for 5 min I got an initial crackle for a few secs but it stopped. It comes and goes. By the way there is also a ground connector at the back, might that help or make it worse?
This has been sitting in a garage for maybe the last 5 years, so it may have been exposed to low levels of moisture. Maybe there is corrosion somewhere that I can't see. The reason I stopped using it was because it developed sound issues like L/R channel cutouts (found it was the selector which I just fixed), but I don't think it was anywhere near this bad back then.