Text-only reference. Published from the recorded official FAA General Chapter 12 PDF. Diagrams, photographs, and figure artwork are not reproduced here; use the official FAA PDF for those materials.
12-78 CRT base Anode ScreenMating connector Horizontal deflection plates (H)Preaccelerating anode (A1) Cathode (K) & heater Control grid Vertical deflection plates (V) Accelerating anode (A3) Focus anode (A2) Electron gun e e e e e e e e e e e e e e e e e e e e R Current coil M Potential coil Load the fixed saw-tooth sweep rate. In a typical oscilloscope, this triggering signal comes from the input signal itself at a selected point during the signal’s cycle. The horizontal signal goes through one sweep, retraces back to the left side and waits there until it is triggered again by the input signal to start another sweep.
Flat Panel Color Displays for Oscilloscopes While the standard CRT design of oscilloscope is still in service, the technology of display and control has evolved into use of the flat panel monitors. Furthermore, the newer oscilloscopes can even be integrated with the common personal computer (PC). [Figure 12-168] Some of the features of this technology include easy data capture, data transfer, documentation, and data analysis. Hand-held oscilloscopes are now available that can perform the functions of larger bench type equipment but are mobile and great tools for trouble shooting. [Figure 12-169] Digital Multimeter Traditionally, the meters that technicians have used have been the analog voltmeter, ammeter, and the ohmmeter. These have usually been combined into the same instrument and called a multimeter or a VOM (volt-ohm-milliammeter). This approach has been both convenient and economical. Digital multimeters (DMM) and digital voltmeters (DVM) are more common due to their ease of use. These meters are easier to read and provide greater accuracy when compared to the older analog units with needle movement. The multimeter’s single-coil movement requires a number of scales, which are not always easy to read accurately. In addition, the loading characteristics due to the internal resistance sometimes affect the circuit and the measurements. Not only does the DVM offer greater accuracy and less ambiguity, but also higher input resistance, which has less of a loading effect and influence on a circuit.
Basic Circuit Analysis & Troubleshooting
Troubleshooting is the systematic process of recognizing the symptoms of a problem, identifying the possible cause, and locating the failed component or conductor in the circuit. To be proficient at troubleshooting, the technician must understand how the circuit operates and know how to properly use the test equipment. There are many ways in which a system can fail and to cover all of the possibilities is beyond the scope of this handbook. However, there are some basic concepts that enable the technician to handle many of the common faults encountered in the aircraft. Before starting a discussion on basic circuits and troubleshooting, the following definitions are given.
• Short circuit—an unintentional low resistance path between two components in a circuit or between a 12-79 Applied(0) volts (+) volts Horizontal deflection plates (H) Vertical deflection plates (V) (0) volts The horizontal beam position will depend on the applied saw-tooth voltage Voltage starts at a maximum negative value and increases to a maximum positive value, tracing the beam from the left to the right (−) volts Vertical deflection plates (v) Horizontal deflection plates (h)Screen Beam on center (0) volts (−) volts Electrons repelled down (−) volts Electrons repelled to left (0) volts (0) volts(0) volts (0) volts (+) volts Electrons attracted up (+) volts Electrons attracted to right (0) volts component/conductor and ground. It usually creates high current flow, which burns out or causes damage to the circuit conductor or components.
• Open circuit—a circuit that is not a complete or continuous path. An open circuit represents an infinitely large resistance. Switches are common devices used to open and close a circuit. Sometimes a circuit opens due to a component failure, such as a light bulb or a burned out resistor. • Continuity—the state of being continuous, uninterrupted or connected together; the opposite of a circuit that is not broken or does not have an open. • Discontinuity—the opposite of continuity, indicating that a circuit is broken or not continuous. Voltage Measurement V oltage is measured across a component with a voltmeter or the voltmeter position on a multimeter. Usually, there is a DC and an AC selection on the meter. Before the meter is used for measurements, make sure that the meter is selected for the correct type of voltage. When placing the probes across a component to take a measurement, take care to ensure that the polarity is correct. [Figure 12-170] Standard practice is for the red meter lead to be installed in the positive (+) jack and the black meter lead to be installed in the negative meter jack (−). Then when placing the probes across or in parallel with a component to measure the voltage, the leads should match the polarity of the component. The red lead is on the positive 12-80 Applied (0) volts (+) volts Horizontal deflection plates (h) (−) volts Vertical deflection plates (v) (0) volts (–) volts (+) volts (0) volts Input signal side of the component and the black is on the negative side, which prevents damage to the meter or incorrect readings.
All meters have some resistance and will shunt some of the current. This has the effect of changing the characteristic of the circuit because of this change in current. This is typically more of a concern with older analog type meters. If there are any questions about the magnitude of the voltage across a component, then the meter should be set to measure on the highest voltage range. This prevents the meter from “pegging” and possible damage. The range should then be selected to low values until the measured voltage is read at the mid-scale deflection. Readings taken at mid-scale are the most accurate.
Current Measurement Current is measured with the ammeter connected in the current path by opening or breaking the circuit and inserting the meter in series. [Figure 12-170] Standard practice is for the red meter lead to be installed in the positive (+) jack and the black meter lead to be installed in the negative meter jack (−). The positive side of the meter is connected towards the positive voltage source. Ideally, the meter should not alter the current and influence the circuit and the measurements. However, the meter does have some effect because of its internal resistance that is connected with the rest of the circuit in series. The resistance is rather small and for most practical purposes, this can be neglected.
Checking Resistance in a Circuit The ohmmeter is used to measure the resistance. In its more basic form, the ohmmeter consists of a variable resistor in series with a meter movement and a voltage source. The meter must first be adjusted before use. Refer to Figure 12-171 for meter configurations during adjustments. When the meter leads are not connected (open), the needle points to the full left-hand position, indicating infinite resistance or and open circuit. With the lead placed together, the circuit is shorted as shown with the meter needle to the full right-hand position. When a connection is made, the internal battery is allowed to produce a current through the movement coil, causing a deflection of the