Chasing 60 hz hum

Does the 7150 have a two-prong cable? If it has a non- polarized plug, take a voltage reading from the chassis to the screw that holds the wall-outlet cover in place. Reverse the plug and take a reading again. The plug orientation with the lower voltage reading should be the quieter of the two.
2) The filter capacitors in these components power supplies are supposed to filter out 60 cycle hum after the line voltage has been rectified."Leaky" or worn out caps will not filter the ripple, leading to increased hum. In older equipment replacing the caps might be worth looking into.

Joenies....glad you got rid of the hum, but cheaters degrade the sound of your system....instead... remove the plug from the wall and make the necessary wire changes behind the wall.

If you have to use cheaters to get rid of hum, then the problem is that the equipment itself is incorrectly wired.

The problem is that the chassis ground and the circuit ground are the same thing, which means the equipment is wired with a ground loop.

IMO, you should send the equipment back to Mac and have them fix it, as other than using ground cheaters, or breaking ground pins off of the AC cords (neither of which is recommended) there will be nothing else that is effective.

Resist the temptation to install an external ground outside the house or other such nonsense- you don't need it! That will simply add to the ground loop problem, which can really only be solved by having the equipment itself wired correctly.

Will study all these suggestions. Many thanks! I was getting the hum (just realized) with TV input selected only and will look at maybe a missing ground where the coax comes into the house.

11-09-11: Broimp
Will study all these suggestions. Many thanks! I was getting the hum (just realized) with TV input selected only and will look at maybe a missing ground where the coax comes into the house.

Ground loop issues involving connections between cable tv and audio systems are common. A good solution is often a ground isolator such as this one.

1-09-11: Stringreen
Joenies....glad you got rid of the hum, but cheaters degrade the sound of your system....instead... remove the plug from the wall and make the necessary wire changes behind the wall.

Besides being a code violation, intentionally miswiring a wall outlet compounds the risks that would result from using a cheater due to the fact that the miswiring may be forgotten in the future. If a component is acquired in the future that is old and/or in questionable condition, and it is plugged into that outlet, the fire and shock protections that properly wired 3-prong outlets are intended to provide may be necessary but not present.

11-08-11: Joenies
I totally disconnected the ground wires that comes from my breaker box and installed a real Isolated ground to my outlets. I ran a dedicated ground from the outlets to a water pipe in my house.

Hopefully a very low resistance path exists between the water pipe ground and the ground at your electrical service entrance. Otherwise fire, shock, and lightning hazards may exist. Those risks are presumably small but cannot be said to be zero. See section 1.2 of this paper, especially the part of that section on page 8.

11-09-11: Atmasphere
If you have to use cheaters to get rid of hum, then the problem is that the equipment itself is incorrectly wired. The problem is that the chassis ground and the circuit ground are the same thing, which means the equipment is wired with a ground loop. IMO, you should send the equipment back to Mac and have them fix it,

Ralph, isn't that very commonly done intentionally, notwithstanding the fact that it creates the ground loop issue you are describing? And if so, wouldn't a fix (a)be likely to be hard to implement, and (b)be likely to degrade the integrity of the internal grounding scheme that was intended in the design, thereby affecting sonics?

Regards,
-- Al