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! If we understand how grounding 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, gather as much information as possible before you change anything.
Troubleshooting guru Bob Pease suggests these basics: Did it ever work right? What symptoms tell you it’s not working right? When did it start working badly or stop working? What other symptoms showed up just before, just after, or 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 grounded via its 3-prong power plug. Note any other ground 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. The tests can specifically identify:
• Common impedance coupling in unbalanced cables (vast majority of problems),
• Magnetic or electrostatic pickup by cable of nearby fields, or
• Common impedance coupling inside defective equipment.
In future issues of this newsletter we will show how to build “dummies” for troubleshooting, give a step-by-step procedure for using the “dummies” to locate the problems, and show different methods for solving the problems now identified.
Originally published by Technologies For Worship Magazine