TM 11-6625-2616-14
mark on the printed circuit board at one end
(c) Replace integrated circuits, readout
tubes, or modules from spares stock.
of the socket.
(2) Substitution may also be employed with
(4) Integrated circuit with bent lead not
resistors, capacitors, diodes, and transistors, but
mating with socket on all contacts. It may be
this involves soldering and should be avoided. If
necessary to unplug the integrated circuit to see
possible, use voltage and resistance tests first to
this.
pinpoint a defective part which is soldered in.
(5) Integrated circuit not plugged into
right socket.
c. Signal Tracing. Most circuits have an input
vs output relationship which can be measured.
(6) Fractured diodes.
Sometimes a volt-ohmmeter (VOM ) can be used
(7) Broken readout tubes; a crack in the
and sometimes an oscilloscope. However, some-
glass will cause a failure eventually. Look closely
times neither will work because of circuit load-
at the base and tip for flaws.
ing or induced oscillation, or because the signal
(8) Readout tube with bent leads or with
characteristic of interest cannot be detected.
internal pieces loose. It may be necessary to
Whenever applicable, this is a very powerful
remove the tube to see this.
troubleshooting technique. Compare measure-
ments taken on an operating instrument with
(9) Foreign objects, such as nuts, screws,
the measurements on the faulty one.
pieces of wire, solder, etc. may cause intermit-
tent shorts.
(10) Dirt or grime will degrade insulation
semiconductors can be quickly detected by using
resistance causing problems, especially in high
the R 1K range of a VOM. The use of other
impedance circuits.
scales should be avoided because of excessive
voltage or current which may damage a good
(11) Cold solder joints. Look for holes in
semiconductor.
solder joint, crystalline appearance of solder or
improper wetting of solder to parts joined.
e. Mechanical Stress. When electrical connec-
(12) Hairline cracks in printed circuit lines
tions are suspected, it is useful to apply mechani-
or bridges between circuits.
cal stress to the area to see if the performance
(13) Charred or discolored printed circuit
of the instrument is effected by the stress. This
can be accomplished by operating the instru-
board, indicating an overheating component,
ment and simultaneously pushing, twisting, pry-
(14) Faulty pin and socket connection. A
ing, etc. the suspected component or assembly.
0.040-inch diameter pin is used on the plug-in
The repairman should use caution while doing
modules, on the connector between the two
this so as not to injure himself or damage the
boards, and on the switches. Use the shank of a
instrument.
new #60 drill to check the individual sockets
for good contact gripping action. Check the pins
f. Heating or Cooling. If the fault is inter-
and sockets for wear, or for foreign matter ad-
mittent or occurs during temperature extremes,
hering to the mating parts and interfering with
it can sometimes be located by the use of a heat
the electrical contact between them.
gun or an aerosol can of carbon dioxide (CO)
or Freon to heat or cool a suspected component
b. Substitution.
or assembly while observing the performance of
(1) Many components and assemblies in the
the instrument. Care should be taken to not ex-
AN/GSM-64A plug into the printed circuit
ceed the operating temperature rating of the
board and, in fact, the entire top board plugs in-
instrument (122 F.) during this procedure.
to the bottom board. The repairman should make
use of this feature to the fullest extent possible.
g. Cleaning. Sometimes when all else fails, this
There are three options available:
approach may fix the problem. Cleaning the sus-
(a) Interchange like numbered integrat-
petted area with detergent and water and then
ed circuits or readout tubes within the voltmeter.
drying the instrument in an oven set at low tem-
If the symptoms change, replace the suspect part.
perature won't damage the instrument and may
(b) Interchange like numbered integrat-
solve the problem. Even though the area looks
ed circuits, readout tubes or modules between
clean, it may not be with resultant poor insula-
the faulty instrument and a properly operating
one.
tion resistance.
6-2
