Under fortuitous conditions, systems may be acceptably quiet in spite of poor techniques. But physics will ultimately rule and noises may appear for no apparent reason! Once we understand how earthing systems and interfaces actually work and how noises couple into signals, finding and fixing problems becomes simple and logical.
Perhaps the most important aspect of troubleshooting is how (or if) you think about the problem. Without a methodical approach, chasing noise problems can be both frustrating and time-consuming. For example, don't fall into the trap of thinking something can't be the problem just because you've always done it that way. Remember, things that "can't go wrong" do! Further, problems that go away by themselves also tend to reappear by themselves.
Don't start by changing things. Because many problems reveal themselves if we just gather enough clues, get as much information as possible before changing anything.
Ask questions Did it ever work right? Under what conditions does the noise appear? Do any other symptoms appear at the same time?
Be alert to clues from the equipment itself. Operation of the equipment's controls, along with some simple logic, can provide very valuable clues. For example, if the noise is unaffected by the setting of a volume control or selector, logic dictates that it must be entering the signal path after that control. If the noise can be eliminated by turning the volume down or selecting another input, it must be entering the signal path before that control.
Write everything down. Less than perfect memory can waste a lot of time.
Sketch a block diagram of the system. Show all signal interconnecting cables, including digital and RF, and indicate their approximate length. Mark any balanced inputs or outputs. Generally, stereo pairs can be indicated with a single line. Note any equipment that's earthed via its 3-prong power plug. Note any other earth connections such as cable TV or DSS dishes.
Work through the system BACKWARDS. As a general rule, and unless clues suggest another starting point, always begin at the inputs to the power amplifiers (for audio systems) or the input to the monitor (for video systems) and sequentially test interfaces backward toward the signal sources. Easily constructed test adapters or "dummies" allow the system to test itself and pinpoint the exact entry point of noise or interference. By temporarily placing the dummies at strategic locations in the interface, precise information about the nature of the problem is also revealed.
Figure 1: Think Before You Act
FINDING PROBLEM UNBALANCED INTERFACES
For unbalanced audio interfaces, the dummy tests can specifically identify:
Common-impedance coupling in cables, or
Pickup of nearby magnetic or electric fields, or
Common-impedance coupling inside defective equipment, known as the "pin 1 problem."
Figure 2: Dummy for Unbalanced Interfaces
The dummies are made from standard connectors wired as shown in figure 2. THEY DO NOT PASS SIGNAL, so make sure they're clearly marked and don't accidentally get permanently installed in a system! Be very careful not to damage speakers or ears!
The surrogate schematic details are a little unfamiliar. Are those coax feedthroughs that are electrically isolated from the box on boththe center conductor and the shield?? Can you draw it differently? Is the 75 ohm resistor connected to the surrogate box? Is the box metal ? I assume so?
Are you going to use a signal analyzer that will actually identify the noise frequencies and power spectrum? Or just monitors?
I like the hummer idea.
What is common impedance coupling? Same as common mode noise?
The assumption below is that a properly designed device will have very low resistance between device chassis and ground contacts for each input or output, right?
4. Connect one hummer lead to the device chassis and touch the other lead to the ground contact of each input or output connector. If the device is properly designed, there will be no output hum or change in the noise floor.