Nakamichi PA-7 Hum

With time the internal power supply of the amp may become noisy due to the lack of DC filtering. If this is the case you'll need to inspect PCB for the loose soldering joints and replace power supply electrolytic capacitors.

Very true, but in this case he said that the problem does not exist when he selects a cd player with his preamp, while it does exist when he selects other line-level inputs.

Which tells me the problem is unlikely to be internal to his power amp, or to his preamp for that matter. I think it's more likely to be an open return path (shield connection) in an interconnect, or one of the other things I mentioned above.

Or, as others have said, a ground loop with the catv system IF the problem only occurs when there is a connection to something within that system (which I suspect is not the case based on the wording of his original post).

Regards,
-- Al

With time the internal power supply of the amp may become noisy due to the lack of DC filtering. If this is the case you'll need to inspect PCB for the loose soldering joints and replace power supply electrolytic capacitors.

Yep, Hifigy is right. Its an easy way to check to see if that is the problem, since most of us have dozens of those little buggers lying around in a drawer somewhere. I then went and got the $10 RS isolation tranny which of course looks a lot neater.

I had the same problem with my PA-7 & CA-7. The hum was intoduced by the audio cables coming from anything connected to my cable tv system. This was the quick and cheap way to create an isolation transformer. Take 2 75ohm/300ohm transformers that come with vcrs or tvs. Hopefully you'll have one with pigtails (300ohm) and one with screw teminals. Connect the pigtails to the teminals. You'll now have two "f" connectors at either end. Connect one end to your cable feed and one to your video system. This was essentially what the old "Magic" box was from a few years back but, instead of $99, you've spent maybe $5.

Happy holidays

You many have introduced a ground loop into the system by connecting the audio system to a Home Theater system, Cable TV box, and or powered subwoofer. The cable is ususally guilty followed by the Sub which usually gets pluged into a different outlet. I installed two isolation transformers from Jensen transformers (not cheap about $ 150+ each )My background is black as night now.

Unplug the above items to see if they are at fault when you plug them back in

Dear Almarg,Swampwalker,Jea48 and Thorman,

Thank you all for your quick responses and great thoughts on the problem. First thing in the morning I will run down all of the suggested possibilities. Thanks for helping to make my traditional viewing of a Christmas Carol "Hum-Free"
Have a great holiday and to rob a line from a favorite movie of mine - May the best of your past be the worst of your future.
I'll post the results.
Thanks again.

Try pulling the plug disconnecting the television from the circuit..It sounds like interference from having the stereo and television cable on the same circuit..You may need to seperate ( different dedicated lines ) the two..............

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.

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 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