2-16surface with your free hand for at least 1 second. The bare skin contact provides a safe dischargepath for electrostatic charges accumulated while you are moving around.9.While servicing equipment containing ESDS devices, do not handle or touch materials such asplastic, vinyl, synthetic textiles, polished wood, fiber glass, or similar items that could createstatic charges; or, be sure to repeat the grounding action with the bare hands after contacting thesematerials. These materials are prime electrostatic generators.10.If possible, avoid repairs that require soldering at the equipment level. Soldering irons must haveheater and tip assemblies grounded to ac electrical ground. Do not use ordinary plastic soldersuckers (special antistatic solder suckers are commercially available).11.Ground the leads of test equipment momentarily before you energize the test equipment andbefore you probe ESDS items.Q-12.Why should you avoid using ac-powered test equipment when wearing a wrist strap?DIODE TESTINGBecause of the reliability of semiconductor devices, servicing techniques developed fortransistorized equipment differ from those used for electron-tube circuits. Electron tubes are usuallyconsidered to be the circuit component most susceptible to failure and are normally the first to be tested.Transistors, however, are capable of operating in excess of 30,000 hours at maximum ratings withoutappreciable degradation. They are often soldered into equipment in the same manner as resistors andcapacitors. Substitution of a diode or transistor known to be in good condition is a simple method ofdetermining the quality of a questionable semiconductor device. You should use this technique only aftervoltage and resistance measurements indicate that no circuit defect exists that might damage thesubstituted semiconductor device. If more than one defective semiconductor is present in the equipmentsection where trouble has been localized, substitution becomes cumbersome since several semiconductorsmay have to be replaced before the trouble is corrected. To determine which stages failed and whichsemiconductors are not defective, you must test all of the removed semiconductors. This can beaccomplished by observing whether the equipment operates correctly as each of the removedsemiconductor devices is reinserted into the equipment.Q-13.Prior to substituting a diode, what measurements should you take to determine its condition?DIODE TESTERSDiodes, such as general-purpose germanium and silicon diodes, power silicon diodes, andmicrowave silicon diodes, are most effectively tested under actual operating conditions. However,rectifier testers are available for you to determine direct-current characteristics that provide an indicationof diode quality.Rf Diode TestA common type of diode test set is a combination ohmmeter-ammeter. You can make measurementsof forward resistance, back resistance, and reverse current with this equipment. You can determine thecondition of the rectifier under test by comparing its actual values with typical values obtained from testinformation furnished with the test set or from the manufacturer’s data sheets. Comparing the diode’s backand forward resistance at a specified voltage provides you with a rough indication of the rectifyingproperty of a diode. A typical back-to-forward resistance ratio is on the order of 10 to 1, and a forward-resistance value of 50 to 80 ohms is common.
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