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Lesson 74 - Troubleshooting

Troubleshooting is the art of finding a problem, isolating it down to the source, so that you can replace the faulty component and restore proper operation of a circuit.

Whether you are designing a new piece of electronics equipment, or repairing that old tube radio you found at a garage sale, you will need to use some basic troubleshooting concepts.

The basics of troubleshooting is to create an organized plan to aid you in repairing faulty electronic equipment more rapidly and reliably. The time tested method I discuss here has been used for hundreds of years on all sorts of mechanical and electronic equipment, and been found to be the best method for determining and resovling a problem. No matter what piece of equipment you have, there are certain methods that work to aid you in finding a fault faster and more reliably. Those methods can be broken down into 7 steps

A) Determine Symptom(s)
B) Sectionalize
C) Localize
D) Isolate
E) Repair
F) Bench Test

A) Determining Symptom. Look at the equipment. What is it doing that it shouldn't be? What isn't it doing that it should be? In order to determine what needs fixed, you must first determine what is wrong. This means you need a basic understanding of what the equipment is supposed to do and how it works under normal conditions. Lets assume for a this discussion that we are repairing a radio receiver. You turn it on, and the lights come on, but there is no sound coming out of the speaker. You turn the tuning knob, switch radio bands, nothing. The symptom in this case is: Power but not audio. Write that down.

The symptom lets you know what is wrong with the equipment, so you can determine what you need to check. You have to analyze the symptom as to how it affects the proper operation of the equipment. The symptom will always provide clues that show you where to look (and where not to look) for the trouble, saving precious time.

B) Sectionalize. Once you recognize the symptom,the next step is to break the equipment down into sections in your mind. In the case of our radio, we have a Power Supply section, a tuner section, a mixer section, a local oscillator section, and an audio section. Sectionalizing means determining which section the problem is in. This requires some knowledge of how the equipment works, at least at a block diagram level. In this case, the lights come on, indicating that the power supply is at least partially functional. You may want to break out a multimeter at this time and make sure the power supply is putting out all the proper voltages, as a faulty power supply can cause any number of problems, but if the meter shows that it has the right voltages coming out, then you have at least eliminated that section.

This is how troubleshooting works. You use a divide and conquor method to solve the problem. Split the equipment up in your mind. Is the problem in the power supply, or somewhere after it? Power is good, we move on. Now we break out some other piece of test equipment. An oscilloscope works wonders to find out if the RF, IF, and Local Oscillators are working properly. Usually, if you get a signal at all out of them, they are good. Granted - if the symptom was "receiving off frequency" you might want to check out these areas in more detail, making sure the L.O. is on proper frequency, etc, but in this case, they are all working, and we move on to the Audio section. We have successfully Sectionalized the problem.

C. Localize. After you have located the faulty section (in this case the Audio section), the next step is to localize the problem to a particular circuit or stage. Check the input with our scope. Do we have audio going into the first amplifier? Do we have audio coming out of the last amplifier? Is the audio coming out larger than the audio going in? If the answer is no, then we examine the circuit closer. Each transistor is an amplifying stage - with multiple components in the circuit required to make the transistor work. It has noise suppression filter capacitors and biasing resistors. If one resistor is bad, the whole amplifier doesn't work. So by localizing the problem to a given transistor, we can then move on to isolating it to the exact component (resistor for instance) that is causing the transistor not to be biased, and therefore not to amplify. Oddly we find that our final amplifier has all kinds of sound coming out of it. The outgoing signal on the oscilloscope is much larger in amplitude than the incoming signal. So why do we hear no sound?

D. Isolate. We have localized the problem. We know that there is audio coming out of the final amplifier, but still no sound at the speaker. It is time to isolate the problem to the exact component that is causing the headaches.

Often, this can be done by a simple visual inspection. Is the capacitor bloated with the heat shrink melted down? Is there a burned up resistor. Use all your senses. A burned up transformer has a very distinct smell. Sometimes you can detect a bad IC by feeling it. Turn off the lights and check for arcing. Many times I have found problems in extremely high tech equipment using only the test equipment I was provided at birth - Eyes, Ears, Nose, etc. In this case, I'll need something more.

I use my scope and find that there is no audio at the speaker. I do have audio at the amplifier. The problem must be somewhere inbetween. I follow the wires and find that there is a headphone jack between the two. The jack is old, and there is some debris/corrosion on it. That corrosion is causing the connectors to not make proper contact, and the jack thinks I have my headphones plugged in, which of course turns off the speaker.

E Repair: I can either clean the jack, or replace it. Each has its good points and bad points. Replacing costs more, but is more reliable in the long run.

F Check for proper operation. After soldering in the jack, I put headphones in, and they work fine. I remove them, and the speaker works. I check the volume control in several positions, and change frequencies. I go completely through the equipment to see if there may be other problems in it that were not reported.    

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Hi, I`m Sostenes, Electrical Technician and PLC`S Programmer.
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