b. Resistance Charts.
Resistance charts list the resistance values and the
points at which these values can be measured.
When the technician gets an
incorrect resistance measurement, he consults the schematic diagram, then makes
measurements of the individual components to isolate the faulty one.
The
resistances shown on the resistance chart represent the normal resistance measured
from the tube (or transistor) pin to ground. In figure 4-7, this is indicated in
Note 1. The ohmmeter portion of the VTVM is used, and the multiplier set to the
range that will give a center-scale reading.
(The ohmmeter scale is not linear,
and the most accurate measurement is obtained at the center of the scale.) Remove
all power from the equipment and place one probe on chassis ground and the other on
the proper tube (or transistor) pin. Any reading that is not within the tolerance
(usually 20%) is considered abnormal.
For example, if the chart gives the
resistance as 470K (470,000) ohms, the measurement should fall between 376K and
564K ohms. A zero reading obtained where there should be some resistance indicates
that there is a short circuit between the points contacted by the meter probes. A
reading at the upper extreme of the scale indicates that the circuit is open. The
circuit that presents an abnormal reading must be checked further, even down to
individual resistors, to determine the faulty resistor or to locate the short
circuit.
This is done by using the VTVM and the schematic diagram.
Locate the
proper tube and pin on both the schematic and chassis, and, working from that point
toward ground, measure each resistance in the circuit individually by placing the
probes directly across the resistor. It may be necessary to temporarily disconnect
one end of certain resistors to obtain accurate measurements.
Any defective
component must be replaced.
c. Troubleshooting Charts. The troubleshooting chart is designed to save the
technician much valuable time. This chart consists of a series of troubleshooting
shortcuts relating to the particular equipment.
The list is confined to those
troubles which experience has shown are most likely to occur. Figure 4-8 shows a
portion of a troubleshooting chart. This chart refers to a plug-in panel which is
part of a larger electronic unit.
Notice that the chart has a list of abnormal
indications (symptoms), and that for each symptom listed there is at least one
probable cause. If more than one trouble can produce the same symptoms, more than
one trouble is listed. The probable trouble guides the technician to the component
or circuit that is most likely to cause the listed symptoms. The technician then
takes the action shown in the Correction column of the chart.
The action may
further require that he also make use of the voltage and resistance charts and
schematic diagrams. Obviously, not all of the possible problems can be listed in a
troubleshooting chart.
However, the technician is supplied with a list of
potential or recurring problems and suggestions on how to correct them.
LESSON EXERCISES
In each of the following exercises, select the ONE answer that BEST completes
the statement or answers the question.
Indicate your solution by circling the
letter opposite the correct answer in the subcourse booklet.
309 L4
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