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- Never start troubleshooting a circuit with test equipment. Start with the power of observation. A technician’s senses and thinking can help to isolate a problem quickly.
- Many problems have simple solutions. When troubleshooting, do not immediately assume a problem is complex. It could be a bad connection or a blown fuse. Remember that problems with the most impact usually have the simplest solutions. Difficult problems to solve are those that are intermittent or hard to reproduce.
- If a problem is difficult to solve, step away from it for while. Sometimes, just thinking about the problem will lead to a different approach or solution. Never work on a problem when tired because it is both dangerous and nonproductive.
- Whenever working on equipment, make notes and diagrams or take photographs for use when it is time to reassemble the unit. Most connectors are keyed against incorrect insertion or interchange of cables, but not all. Apparently identical screws may be of differing lengths or may have slightly different thread types. Little parts may fit in more than one place or orientation. Pill bottles, film canisters, egg cartons, and plastic ice cube trays provide a handy place for sorting and storing screws and other small parts during disassembly.
- The workspace should be open, well lit, ESD protected, and situated so that dropped parts can be easily found. The best location should be dust free and allow suspending troubleshooting without having to place everything in a box for storage.
- Take the precautions of touching a ground point first to prevent electrostatic discharge into a component. Components such as MOSFETs and ICs are vulnerable to ESD.
- When troubleshooting, always use a system point of view. Don’t ignore other equipment and/or software that could be affecting performance, and always check the obvious.
- If the unit is ac-operated, disconnect it from the line before taking it apart.
- Use service literature and the proper tools.
- Sort and save all fasteners, knobs, and other small parts.
- Make a thorough visual inspection of the interior of the equipment.
- Try to determine why a component failed before turning on the power.
- Check for overheating.
- Verify all power-supply voltages.
- Lack of amplifier output may not be in the amplifier itself. There could be a defective output device or no input signal.
- A multistage amplifier can be viewed as a signal chain.
- Signal injection begins at the load end of the chain.
- Signal tracing begins at the input end of the chain.
- Voltage analysis is generally used to limit the possibilities to one defective component.
- Some circuit defects cannot be found by dc voltage analysis. These defects are usually the result of an open device or coupling component.
- Low output from an amplifier may be due to low input.
- A dummy load resistor is often substituted for the output device when amplifier performance is measured.
- Both signal tracing and signal injection can give misleading results when troubleshooting for the
- low-gain stage.
- Voltage analysis will lead to some causes of low gain.
- A capacitor suspected of being open can be checked by bridging it with a new one.
- A linear amplifier is not supposed to change anything but the amplitude of the signal.
- Noise may be originating in the power supply.
- Hum refers to a 60-Hz or a 120-Hz signal in the output.
- Hum may be caused by a defective power supply, an open shield, or a poor ground.
- Operate all controls to see if the noise occurs before or after the control.
- Motorboating noise means the amplifier is oscillating.
- Distortion can be caused by bias error, defective transistors, or an input signal that is too large.
- Thermal intermittents may show up after the equipment is turned on for some time.
- Use heat or cold to localize thermal problems.
- Vibration intermittents can be isolated by careful tapping with an insulated tool.
- Failure rates are directly related to device temperature and complexity.
- Transients can and often do damage solid-state devices.
- An amplifier with negative feedback may have latch-up if it is driven into saturation.
- Boundary scan is an automated testing procedure that can be applied to any phase of the life cycle of a product.
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