Diagnosis of Malfunctions in Complex Electronic Assemblies. Final Report. Page: 60 of 66
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Special Troubleshooting Procedures
Preliminary Work. -The results of a study of troubleshooting methods used
in the factory showed that one type of electrical fault could not be isolated by
conventional troubleshooting methods. When short circuits occurred inside
or under a flatpack and on a line common to many packs, such as a B-supply
line to ground, the short could not be isolated because the only isolation
between the sealed packs was the low resistance of the printed circuit board
The short isolation technique was developed on a printed circuit board assem-
bly which had failed a production electrical test because of a short circuit
on the 28-volt supply line. The 28-volt path passed adjacent to ground paths
underneath six separate hybrid circuits and also entered these hybrid circuits
to become part of the internal circuitry. There were many possible locations
for the short. The only isolation between these possible locations was the
resistance of the copper circuit paths between these points. The obvious
choice seemed to be current detection, since workable current can be passed
through low resistance circuits with negligible power dissipation.
The method chosen for isolating the short circuit was to apply a low voltage
to the 28-volt line and try to identify the paths carrying current; then, by
strategically moving the current application points and observing those paths
that carried current, to plot a physical location of the short circuit. Voltage
and resistance measurements were not considered desirable because of the
obvious difficulties encountered in measuring voltage drops or resistance
in short lengths of copper circuit paths. Noncontacting current probes are
commercially available, but design changes would be required to adapt such
probes to printed circuit board paths. Since no totally-adequate probe was
readily available and redesigning a probe would be time consuming and ex-
pensive, alternate methods of detection were investigated.
X-ray techniques were considered, but were not chosen as a primary tech-
nique principally because of analysis time and possible additional equipment
needs. Thermal indicators and liquid crystal techniques were then con-
sidered. Because speculation about the location of the short could be replaced
with exact knowledge of the location of the short, several hybrid components
worth about $250 each were saved.
Short-Circuit Detection With Liquid Crystals. By using the liquid-crystal
technique, hidden short-circuit faults (inside flatpacks, hybrid circuits, and
under large components) may be isolated by identifying current-carrying
circuit paths with a liquid-crystal temperature indicator. The liquid crystals
are mixed to display color through a temperature range of approximately 3 C.
Below and above this range, the crystals are transparent. The mixture is
adjusted so the lower end of the 3 C range is at ambient room temperature.
The color at the lower end of the range is red and at the higher end it is blue.
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Losure, J. A. Diagnosis of Malfunctions in Complex Electronic Assemblies. Final Report., report, January 1, 1971; Kansas City, Missouri. (https://digital.library.unt.edu/ark:/67531/metadc1031893/m1/60/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.