Diagnosis of Malfunctions in Complex Electronic Assemblies. Final Report. Page: 61 of 66
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Blue indicates that the land which carries the current has a temperature about
1. 5 to 3 C above room ambient. Green is the mid-color of the spectrum,
and indicates a partial temperature rise. Figure 14 is a partial sketch of a
printed circuit board subassembly that failed electrical testing on the pro-
duction line. Preliminary troubleshooting showed the 28-volt line to be
shorted to ground. The short circuit had a resistance of less than 100 milli-
ohms, and its location was within or underneath one of the hybrid packs. The
short was so near the circuit path resistance level that it could not be iso-
lated by conventional voltage or resistance measurements without breaking
the electrical circuit. The sealed hybrid circuit components are welded to
the mother board, making it impossible to remove the components without
degrading the pack. The circuits cost approximately $250 each, making iden-
tity of the faulty pack highly desirable. On the pc board tested (Figure 14),
a current source limited to 8U mA was applied between the 28-volt input
terminal and the ground input terminal. Temperature-checks with liquid cry-
stals proved the top of pack-B to be at a higher temperature than the other
packs. With the current source applied between land-1 aid ground, land-2
and the circuit path from land-1 to land-2 gave a blue crystal indication.
Land-5 and the associated circuit paths remained cool. Application of the
current source to the 28-volt input land and taping the circuit path from
land-5 to the 28-volt input indicated that this path was carrying current, thus
verifying a short to ground between lands-2 and -5.
After the short circuit has been located, the circuit pack can be removed and
the cause of the short can be corrected. In some instances, where metal
flakes under the circuit are causing the short, sufficient flexibility in the
terminal leads may allow removal of the short. Care must be taken not to
The following items of test equipment are needed:
" A dc power supply (0 to 6A, 0 to 20V) and meters to monitor terminal
voltage and current;
" A limiting resistor (1 ohm, 5 W);
" A roll of Mylar tape ( 3 inches wide, 1/5 mill thick, dark tint); and
" A kit of liquid crystals (Vari-Light Model 101).
Mix R and L components 50/50. Add one drop of broadner for each 20 drops
of R and L component mixture. The predominate color of the crystal mixture
at 72 F ambient temperature is red. If the temperature condition is such
that this mixture does not yield a predominate red, adjust by adding more
R component. The section of the pc board to be tested is covered with a strip
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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/m1/61/: accessed March 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.