Beginners usually lack troubleshooting skills because troubleshooting, by its nature, takes quite a bit of time to learn. No one becomes completely proficient overnight. That said, excellent troubleshooting skills will make you stand out as an engineer; it’s a skill that translates into dollars at some point down the road.
Let’s begin by stating one of the most important (and seemingly obvious) aspects of troubleshooting: Before you can fix a device you must be absolutely sure that it indeed needs to be fixed.
What does that mean? Basically, you need to know how a device operates before you can make the judgement that it isn’t functioning correctly. Knowing how a device functions is a prerequisite for knowing when a problem is fixed.
IT’S NOT SO SIMPLE, SOMETIMES
Many of you have heard that troubleshooting is the simple process of deduction. In other words, when you’ve eliminated all the items that are not causing the problem, whatever remains must be the root of the problem.
It sounds simple, but it rarely works out that easily; often there will be several candidates for the troublemaker, each of which could be the root cause.
Still, the notion of simple deduction is true conceptually. You will need to drill down and down until you find the root cause, and that is basically the real art of troubleshooting. (It probably goes without saying that the better one knows a system or a piece of gear, the faster troubleshooting will be.)
Another important aspect in the art of troubleshooting is in knowing the difference between symptom and cause.
Let’s say you go to the transmitter site because one of the transmitters is dead and off-air. Upon arrival, you find a tripped circuit breaker and a blown fuse. You reset the breaker and change the fuse, and the transmitter comes back on-air. You fixed it, right?
Well, 30 minutes later, the transmitter does the same thing, and goes off the air again. You return and find the breaker tripped and the fuse blown. At that point, it is safe to assume that the circuit breaker and fuses are symptomatic of the problem, not causing the problem.
Here’s another example. You go out to an AM transmitter site because the transmitter keeps tripping off-air. When you get there, you notice that high reflected power is indicated, and you also notice that phase and ratios are out of their normal tolerance windows.
The concept of Occam’s Razor is simple and important in troubleshooting. Basically, if there are multiple possibilities for the root of a problem, it’s likely that the most simple one is the cause of the trouble.
This implies two things: You must identify all of the possibilities, and you should start the process by investigating the simplest first. If that isn’t the root cause, move on to the next simplest possibility. This methodology will usually speed up the troubleshooting process.
APPLYING THESE PRINCIPLES
Now let’s apply some of these principles to a hypothetical transmitter problem.
Novices will not start off by fixing older transmitters; however, that is something that you will eventually need to do. If you can’t fix (or otherwise handle) transmitter problems at you might not advance much farther in your career.
You arrive at the transmitter site and find the main transmitter dead — unable to stay on-air. When you hit plate ON, you note a plate overload light come on. That,s not the reason the transmitter won’t come o,n but it is a critical symptom.
The transmitter has four basic building blocks: power supply, logic/control, excitation and power amplifier. You’ll need to know if each is OK, and you check them in turn.
You do have a dummy load on site, and you connect the transmitter to it. Hitting plate ON you notice the same symptom. Thus, strike the power amplifier’s output load as a possibility — it’s not the antenna, in other words. You note that the plate voltage supply looks normal for the brief time that it is on. Check other voltages by comparing to previous log readings or factory test data: You note they look OK. For the moment, strike the power supply as a possibility.
In this particular case, the logic and control seem to be operating normally. Something is causing a plate overload, meaning too much current is being drawn through the power supply. The transmitter is shutting itself off, just like it is supposed to. For the time being, strike logic/control as a possibility.
You also notice that the excitation appears OK because the correct amount of drive is noted by the reading of grid current; you also notice no reflected power back to the exciter. For the moment, strike the possibility that excitation is a problem.
SUBSTITUTE KNOWN GOOD COMPONENT FOR SUSPECT COMPONENT
Now we come down to the idea that one of several things could be wrong, and we need to drill down to see which one it is. This is the same process you would use in any troubleshooting: When you have your suspects, find a known good component (or perhaps circuit board) and substitute it for the suspect. This is applicable whether or not you are troubleshooting a transmitter, a console or computer hardware.
For this transmitter problem, something in the final amplifier compartment, or the final amplifier tube itself, can cause the plate overload problem. In this case, you would completely de-energize the transmitter, using the shorting stick as a final measure to be sure the high voltage is zero. Only then can you safely open up the final. Look for obvious problems — burned and/or broken components. Also look for problems brought on by dirt.
If you find nothing obvious from a physical inspection, it’s time to substitute a good vacuum tube for the one that has been in the transmitter. I don’t necessarily mean a new tube — in fact, as I’ve previously written, a tube that is known to tune up well in this transmitter is usually a better choice in troubleshooting.
Why? Because sometimes new tubes, or newly rebuilt ones, come back with issues that don’t show up in QC. It’s an unfortunate reality. If you introduce a tube with another problem in to your troubleshooting process, you can waste a lot of time. Having a known-good spare is the way to go.
Let’s say now that after you substitute the known-good tube, and the transmitter comes right up, and just needs a minor tune up. Problem solved. The tube was bad.
Troubleshooting is the process of elimination, aided by the use of substitution when determining the actual suspect out of several. Use the concept of Occam’s Razor to guide you to which of the suspects should be swapped first.