surface with your free hand for at least 1 second. The bare skin contact provides a safe discharge
path for electrostatic charges accumulated while you are moving around.
While servicing equipment containing ESDS devices, do not handle or touch materials such as
plastic, vinyl, synthetic textiles, polished wood, fiber glass, or similar items that could create
static charges; or, be sure to repeat the grounding action with the bare hands after contacting these
materials. These materials are prime electrostatic generators.
If possible, avoid repairs that require soldering at the equipment level. Soldering irons must have
heater and tip assemblies grounded to ac electrical ground. Do not use ordinary plastic solder
suckers (special antistatic solder suckers are commercially available).
Ground the leads of test equipment momentarily before you energize the test equipment and
before you probe ESDS items.
Why should you avoid using ac-powered test equipment when wearing a wrist strap?
Because of the reliability of semiconductor devices, servicing techniques developed for
transistorized equipment differ from those used for electron-tube circuits. Electron tubes are usually
considered 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 without
appreciable degradation. They are often soldered into equipment in the same manner as resistors and
capacitors. Substitution of a diode or transistor known to be in good condition is a simple method of
determining the quality of a questionable semiconductor device. You should use this technique only after
voltage and resistance measurements indicate that no circuit defect exists that might damage the
substituted semiconductor device. If more than one defective semiconductor is present in the equipment
section where trouble has been localized, substitution becomes cumbersome since several semiconductors
may have to be replaced before the trouble is corrected. To determine which stages failed and which
semiconductors are not defective, you must test all of the removed semiconductors. This can be
accomplished by observing whether the equipment operates correctly as each of the removed
semiconductor devices is reinserted into the equipment.
Prior to substituting a diode, what measurements should you take to determine its condition?
Diodes, such as general-purpose germanium and silicon diodes, power silicon diodes, and
microwave 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 indication
of diode quality.
Rf Diode Test
A common type of diode test set is a combination ohmmeter-ammeter. You can make measurements
of forward resistance, back resistance, and reverse current with this equipment. You can determine the
condition of the rectifier under test by comparing its actual values with typical values obtained from test
information furnished with the test set or from the manufacturers data sheets. Comparing the diodes back
and forward resistance at a specified voltage provides you with a rough indication of the rectifying
property 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.