Tools of the Trade

Toos fo the Trade By Jim Sparks April 1999 Avisit to a well-equipped avionics shop can often be awe-inspiring. All of the various test equipment used to calibrate, repair, and certify a wide array of indicators and computers will leave most...

The television type screen or Cathode Ray Tube (CRT) on the face of this tool will enable a technician to view the electrical characteristics of the components in question. A scope draws a graph by moving an electron beam across the CRT, which results in a glow reflecting the characteristics of the electrical circuit being monitored. A grid of horizontal and vertical lines is etched on the display and serves as the reference for gauging circuit parameters. Anytime a disturbance occurs in a medium, the result is a wave. Waveforms are graphic representations of an electrical wave. Wave shapes will tell a great deal about the electrical signal. Anytime a change occurs in the vertical range, it means the amplitude or voltage has changed. Anytime there is a flat horizontal line, there is no change in voltage for that period of time. Sharp angles observed on a scope mean sudden change. Common controls for a scope display include focus and signal intensity. Focus is used to maintain an optimum trace on the grid and is accomplished by concentrating the trace beam to the maximum definition. An intensity adjustment will control the brightness of the trace beam. This is needed when using the scope in various ambient light conditions.

Trace Rotation is another adjustment needed prior to scope use. This is a means of calibration and is accomplished by placing the trace beam in the proper position relative to the grid. Some oscilloscope models also include a Beam Finder. This is a function that will allow the display range to change to accommodate the signal being monitored. Electrical noise is a frequent cause of weak radios or erratic flight deck displays. By properly installing an O' Scope, the source of the noise can be identified and corrected. These devices are very effective in troubleshooting inductive and capacitive systems. Selecting the proper test probe to use with the scope is determined by the function to be performed and the voltage levels to be encountered. Probes are classified as either voltage sensing or current sensing and proper termination of the cable ends is important to avoid unwanted signals finding their way into the probe cable.

Using a device of this sophistication will require some preparation. In fact, prior instruction for a first time Oscilloscope user is encouraged. There are introductory manuals available through most electronics suppliers or scope manufacturers as well as basic electronic courses through many community colleges.

The ability to make electrical circuit measurements of voltage, current, and resistance will help in understanding proper circuit operation and identifying trouble spots. A Volt Ohm Meter (VOM) or Multimeter will always earn its keep in the hands of the well-versed electrical technician. Two types of meters are being used to make electrical measurements.

The first has a mechanically operated pointer that deflects along a calibrated scale to provide the user a specific value when viewed and is called an analog meter. A digital meter has no sweep arm; instead a digital read out proves a group of numbers to display circuit values. Each one of these has special benefits. The meter movement in an analog device works as a small electrical motor. To run the motor, an electrical potential and current flow are required. This means the meter is an additional load on the circuit being tested. A digital meter will have much less of an effect on the system as the current draw is insignificant.

Regardless of meter type, circuit measurements for voltage or resistance are taken with the meter in parallel to the component being tested and in series with the component for amperage tests. When in doubt of circuit's characteristics, it is always best to start with the meter set to the highest range available and then work down. Many newer Digital Volt OhmMeters (DVM) have a feature called "Auto Range," which automatically adjusts the measuring circuits to the correct voltage, current, or resistance range of the circuit being tested. In most models, this starts on the lowest range and works its way up. In certain situations, depending on circuit tendencies, the auto range may cause shifting to occur. This can cause what are frequently interpreted as circuit malfunctions if the range is not noted during each measurement.

In some meters, an audio tone will be heard if the signal being measured is out of the selected range. Another common feature on a DVM is a "HOLD" selector. Using hold will enable a technician working in a tight environment to take a measurement, then remove the test leads, and then read the result.

A Diode Test is available on many multi-testers and will allow the full potential of the meter's internal battery to initiate a current flow through a semi conductor device in a forward bias condition. This feature will allow a reasonably accurate means of checking the conducting as well as blocking functions of a diode. However, the best way to check a diode or most other electrical components is under actual electrical loads. Therefore, since the diode is a current-controlled device, the best way to check its operation is with an amp meter installed in series. In many cases, when meter leads are touched together and DVM is set to measure continuity or resistance, an audible tone will be heard. This is a real benefit when the technician is in a cramped area and is concentrating on the pins of an electrical connector. The resulting tone will advise the technician of a complete circuit or a short depending on the installation of the meter.

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