Nakamichi PA-7 Hum

I don't think lack of line filtering would have anything to do with the problem. I would focus on the following possibilities:

-- Interconnect cables that might have an open shield connection somewhere, or that are excessively long, or that are low quality (specifically, that have excessive resistance across the length of the shield).

-- Oxidized plugs or jacks. Use contact cleaner if you have it, or if not just remove and reinsert the plugs a couple of times; the scraping action will often help.

-- Interconnect cable plugs that fit loosely, so that the ground connection is not being made securely.

-- Try plugging the ac power cords from all of the components into the SAME power strip or surge suppressor or outlet receptacle, if that is not already the case.

-- If any of the components have two-prong non-polarized power plugs, try reversing their polarity.

Hope that helps,
-- Al

Only thing to add to Al's response is to see if it only happens when your CATV coax is connected to something in the chain. If so, you most likely need a coax ground isolation transformer, which you can buy at Radio Shack for $10 or get the Mondial MAGIC for $100 or something in between from Jensen or others.

I agree with Swampwalker. Good chance you have a ground loop hum caused by the CATV coax cable. Disconnect the coax temporary and see if that doesn't stop the hum.