Chapter 1

Introduction to Troubleshooting.

using this book.

I can't force you to take my advice. All I can do is give it and hope. If you run into trouble as you work your way through this book you may find that you need to stop and study "Electronics for Physicists". There is nothing wrong with parallel study of the two books but if you skim through the other book you may occasionally find you have missed some point and need to go back for more study. In the final analysis it's up to you. However you decide to do it I wish you luck.

You may feel that there is an organization problem with this book. I tend to agree but I am in a bind because much of the material is circular. If I explain troubleshooting first you won't know how to apply them to specific circuits unless I explain them in that chapter. If I explain circuits first you won't know how to go about trouble shooting them. So I will explain both in both places. This will result in some repetition of material but I think that's better than asking you to flip back and forth between two chapters. If I fail in this I apologize in advance.

Troubleshooting.

Electronic troubleshooting in the modern world has moved from consumer electronics to other fields. these are:

1. Maintaining laboratory equipment used in industrial research and development laboratories.

2. Maintaining automated production equipment.

3. Working for a manufacturer to perform field service of laboratory or industrial equipment manufactured by the company.

4. Troubleshooting prototype equipment in an R and D lab.

5. Troubleshooting production equipment at the end of an assembly line.

6. Troubleshooting research and teaching laboratory equipment at a university, college, or secondary school system.

7. Individuals, who are most likely to be retired, who restore and repair antique electronic equipment for themselves and others.

8. DIYers and hams who build their own equipment from kits or scratch.

Number 6 was essentially half of the job I did for the university. The other half was teaching.

Engineering technologists are not necessarily overqualified for these positions.

Finding troubles in an electronic circuit is rather like figuring out "Who done it?" You are Sherlock Holmes. You have a number of seemingly unrelated clues and you have to figure out which part is defective.

In every defective circuit there are symptoms and causes. The most obvious symptom is "it doesn't work". When you open up the box, you may see a burned resistor. "Aha!" you will say, and you replace the resistor. When you turn the power on, the newly replaced resistor begins to get hot, smoke and burn up. The burned resistor was just another symptom, not a cause. There is another defect which caused the resistor to burn out.

A burned out transistor may be a cause or a symptom. The odds are in favor of it being a symptom.

Often the clues are more subtle than a scorched resistor. Nine times out of ten, there are no visible defects. When this happens it is necessary to make tests on the circuit.

At first, every part is suspect. As you make more tests, some parts are eliminated as suspects. As you gather more clues, the number of suspects is narrowed down until only one is left.

Holmes' first rule: When all other suspects have been eliminated, the one which remains, no matter how improbable, is the correct one. When you apply this rule, be sure that you have eliminated all other suspects.

Probability plays a very large role in troubleshooting. If you have narrowed down the suspects to two parts, and they are equally difficult to replace, replace the one which has the highest probability of being defective.

Ease of replacement will override the probability rule. If in the above situation one of the two parts is difficult to replace, and the other is easy, then replace the easiest one first. If that one is the trouble, you have saved yourself a lot of work. If the easy one is not the trouble, you haven't wasted very much work and you now know for sure that the "hard to replace" one is bad.

The first law of troubleshooting is check the easy things first. It is possible to gain a reputation as a miracle worker by always using this law.

You may be called in to "look at" a major piece of equipment for which there is no documentation. The thing to do is to check the power supply. If you are lucky, the supply will be a simple circuit consisting of a transformer, a rectifier and a filter capacitor. If the fault is in the supply, you can "do the impossible" and fix it.

The difficulty of any given troubleshooting job depends on the amount of documentation you have for the particular piece of equipment. The less documentation you have, the more difficult the task.

Troubleshooting a piece of equipment which has no documentation can be a formidable task at best. There are times when you can "give it a try" and times when it is best to just walk away. For example, the six-transistor radio uses a somewhat standard circuit. Regardless of the manufacturer or even the country of origin, the circuits used vary only slightly. If you are familiar with this circuit, don't hesitate to "have a go at it". The same applies to the standard 5 tube radio. On the other hand, if someone asks you to work on a video cassette recorder or a computer, check the line cord, the fuses and the power supply. If the simple things are all right, forget it! For cases in between these two extremes you will have to weigh the complexity of the device against your skills and make your own decision.

A situation which is frequently encountered is to have a detailed schematic diagram but no normal voltages or wave-forms. In this case you must deduce what the normal voltages will be, based on your knowledge of electronics.

Those problems which seemed so theoretical in the early courses will now come down to earth and take on reality. You will use Ohm's law, Kirchhoff's laws and all you have learned about transistors and operational amplifiers to figure out what normal voltages are.

Another way to get a reputation as a miracle worker is to overestimate the amount of time required to solve the problem. That one comes from Star Trek's Scotty.

There are two distinct kinds of troubleshooting. One is in a piece of equipment that has worked but suddenly stopped working. The other is less familiar and is in a piece of equipment that has never worked. This could be something produced in a research and development laboratory or something you built yourself. It is definitely harder to troubleshoot something that has never worked. The reason is that all the parts may be good but a design flaw is keeping it from performing its intended operation. In such a case you may or may not have the cooperation of the design engineers. They have egos which are wrapped up in the design. They are likely to be unwilling to admit that they have made a mistake.