[rolger]

Have encountered a unit as above with residual hum on all of it's 5 channels in my trade as a technician.
Turned out to be a design / manufacturing problem easily rectified:
remove the earthing tags connecting the metal rear panel from both video input connector assemblies and the earth loop problem is eliminated, whisper quiet! (this original problem may worsen as the unit ages).
Since I spent the best part of 2 days traceing the fault described above without much pay, I thought I sould share my findings.
Please note:
There are other possible causes of hum that are more threatening to the long term survival of such a beautifully constructed piece of vintage audio
equipment.
They don't make them like that anymore!