Contrary to some previous comments, the Fisher 400 and 500C receivers should be absolutely dead quiet with the volume full down and turned to an Aux or Phono setting (this to prevent any bleed though from the high output of the FM tuner section) -- to the point that it would be hard to tell if the receiver is even on by listening to high sensitivity speakers in a dead quiet room with your ear at the speaker board.
If a 400 receiver is producing hum under this condition, then one strong likelihood is that a tech installed (typically) 10Ω cathode current sensing resistors somewhere along the way, but did not account for one of the heater winding balancing resistors that also used an (originally) grounded cathode terminal as a grounding point for that heater winding balancing resistor as well. If the resistor is left connected to the cathode terminal when cathode sensing resistors are installed, then the channel the resistor is installed in will hum -- even with the volume full down. This particular problem does not exist in the 500C, as the heater balancing resistors are installed over in the power supply area in that (as well as the 800C) model.
As for the 500C, you didn't specifically indicate if the problem is in one channel or the other, but implied it is both. We also need to know where the volume control is when you are hearing the hum. Besides that, to help in tracking it down, try to determine if it is 100/120 Hz in nature, or 50/60 Hz (one octave lower). If it is the former, then power supply caps or un-matched output tubes are likely suspects assuming all else is as it should be. If it's the latter, then wiring in the volume control should be inspected -- particularly is the AC power switch has been examined, and the rear cover for the switch is now missing.
As for interconnects -- I'm no fan of expensive boutique interconnects, but I can tell you that inexpensive stereo ones purchased at big box stores like Home Depot are simply junk for use with vintage vacuum tube equipment -- they don't use shielded cable!!! For SS gear that has a very low output impedance, they may work OK, but for high sensitivity use such as for connecting a turn table or connecting two pieces of vacuum tube equipment together, then hum will be a virtually guaranteed result. Anymore then, make sure that the interconnects you're using actually employ shielded cable. They don't need to be boutique pieces, but they do need to be shielded!
Dave
