Tips on finding "hum" problems

There have been quite a few posts lately on "hum" problems. For what it is worth, I will try to explain, in as simple terms as possible, how I have learned to solve/find the sources of hum over the years. The techniques are as follows.
Those were pretty brief tips you had there : ) Sean
When I have a call to investigate hum, I first

check to see if any equipment on site is the source
of the problem. This is done by turning the branch circuits off one at a time to see if it makes a difference. Air conditioning, heat pumps, refrigerators, washers, dryers, freezers, dimmers, etc.....even if they are turned off, there may be a component inside that is drawing power and causing a problem. If this is the case, the offending equipment may need repair. Or it may be normal for the equipment to cause this noise. You'll have to decide if it should be replaced or turn it off when you listen. The second area to check is the physical AC power system. Starting at the branch circuit device (recepticle) and working my way back to the main AC distributon panel or service entrance. I take a recepticle tester and check the wiring of the device. I also check for blade tension. If you take a volt meter, you should read the same voltage hot to ground and hot to neutral. If you don't, there is a problem somewhere in the wiring of the branch circuit or the service panel. The most common cause is poor terminations. Many devices have "push" in terminations. The old saying was "push or screw it's up to you". If this is the case, all of the devices on that branch circuit should be reterminated with wire nuts and a tail to the screw on the device. Grounds should be twisted and crimped with a tail to the device and be bonded to the box if it's metal. Once I get back to the service entrance panel, I check the ground and neutral conductors. At the main service entrance all grounds and neutrals terminate on the same bar. The bar is also bonded to the panel with a screw or jumper. If a sub panel is run out of the main panel, the grounds and neutrals a kept seperate. The grounds connect to a bar that is bonded to the panel, the neutrals are on a bar that is insulated from the panel. The only place that the grounds and neutrals are together, is the MAIN SERVICE ENTRANCE PANEL. From there on out, they are to never touch! If you wish to run an isolated ground circuit, that ground runs back to the MAIN SERVICE ENTRANCE PANEL NEUTRAL. Not to a seperate ground. This will cause a difference of potential on the system. All grounding electrodes connect at the MAIN SERVICE PANEL. Grounding electrodes are bonded to the same point in the main service panel as the neutrals and ground wires. Ground rods (driven electrodes) are actually considered to be supplementary, but are required. Grounding electrode examples are, street side of the water meter, concrete encased rebar that is wired together, verticle steel posts or columns that are in contact with earth and driven ground rods. All of the examples that are on site, need to be bonded to the MAIN AC neutral/ground bar. Here in the states, our residential power runs on a grounded system. The neutral (grounded conductor) actually does carry current. Thats why it's important to keep seperation out from the main service. When neutrals and grounds are tied together out in the branch wiring, current is placed on the grounds and that will cause hum. Think of it as a single point that the neutrals and grounds tie together. If they tie together at another point, you have a loop. (ground loop) Now that I have checked out the wiring, next is the equipment. I rarely find bad gear, but it does happen. You need to check chassi voltage to ground. On equipment that has a grounded plug, you should get close to zero volts. On 2 prong polarized plugs you might get a low reading. On 2 prong none polarized plugs you will have to check both ways and plug it in the way that reads the lowest. People do make wiring mistakes at the factory. The bad thing is that most gear will work with the neutral and ground swapped. (its called a one wire hook up, two wire screw up) This can cause hum. Single ended input connectors can also be a problem. If they are soldered on to a board, many times the ground pin breaks off the board from stress. Stress caused by the weight of the cable, or just from use. This will cause hum and is easy to fix, once you find it. Cables can also be the cause of hum. Not just from being run to close to one another. Just like the input jacks, cable connectors can fail also. You need to check with a continuity meter the outer shield connections. If you find a broken one, it may or may not be easy to repair. It will depend on the location of the break and if you can get to it. Also check the cables if they are directional. I'm not saying that they will hum if connected wrong, but you never know. Cable TV lines can cause a lot of hum problems. Cable should enter at the same location as your AC service. It should connect to a ground block as soon as it enters the home. The block is then bonded to the Main AC grounding electrode. This eliminates difference of potential between the cable shield and your other grounds in the home. If after all of this examination, the problem still exists, I power the equipment up from a generator. This tells me if it's actually something wrong in the gear or that it may be a utility problem. On the utility side, transformers do go bad and underground cables do break down. Plus you may not be the only customer on the supply side of the transformer. I just prsonally experienced this. On the day that a set of speakers was to arrive, my tube amp started to hum. (figures eh) This amp never hummed before. I couldn't find the problem if my life depended on it. The next day when I came home from work, the local utility was out digging up the neighbors underground service. Of course I walked over to see what was up. The lineman explained that there was an open neutral that needed repaired on their service. I share the same transformer secondary with several other neighbors. When the utility was done, the tube amp hum was gone. Feel free to comment. I'm sure that I may have left something out. If you are wondering what I do for a living, I have well over 20 years in the electrical trade. Specializing in power and grounding for the telephone industry. Unfortunately, I'm not able to troubleshoot down to the componet level.
Sorry Sean, it got a bit long......
Of course, dedicated isolated circuits are your best bet for your equipment power. Try to stay away from multi wire branch circuits. This is when opposite hot legs share the same neutral on a pair of branch circuits. I forgot to mention this.
On a very practical note from personal experience... I had a hum problem whenever I attached the cable tv coax cable to the system. If I removed the cable tv connection, the hum was gone. I was able to eliminate the problem by wrapping bare copper wire around the outer conductor of the "F" connector plug from the cable tv coax cable and grounding it to the center screw that holds the cover plate onto the AC wall outlet where my gear was plugged in. This eliminated or greatly reduced a ground-loop problem and the hum was gone.

Also on a practical note... if you can plug all your gear into ONE wall outlet, you're better off than plugging it into different outlets. Plugging into one outlet ensures one point of ground for your gear and minimizes ground-loops which can cause hum.