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c. DISCOLORED OR LEAKING TRANSFORMER—This is a sure sign that there is a short
somewhere. Locate it. If the equipment has a fuse, find out why the fuse did not blow; too
large a size may have been installed, or there may be a short across the fuse holder.
d. LOOSE, BROKEN, OR CORRODED CONNECTIONS—Any connection that is not in good
condition is a trouble spot If it is not causing problems now, it probably will in the future. Fix
it.
e. DAMAGED RESISTORS OR CAPACITORS—A resistor that is discolored or charred has
been subjected to an overload. An electrolytic capacitor will show a whitish deposit at the
seal around the terminals. Check for a short whenever you notice a damaged resistor or
capacitor. If there is no short, the trouble may be that the power supply has been overloaded
in some way. Make a note to replace the part after signal tracing. There is no sense in risking
a new part until you have located the trouble.
2. PLUG IN THE POWER SUPPLY AND LOOK FOR:
a. SMOKING PARTS—If any part smokes or if you hear any boiling or sputtering sounds, pull
the plug immediately. There is a short circuit somewhere that you have missed in your first
inspection. Use an ohmmeter to check the part again; begin in neighborhood of the smoking
part.
b. COLD TUBES—After allowing the equipment about two minutes for warm-up, touch all the
tubes. If a tube is cold, it is either burned out or there is a break in the heater connections and
the tube is not receiving proper heater voltage. Remove the tube and connect an ohmmeter
across the heater terminals to see if the filament is open (reads almost infinite resistance). If
the filament reads open, it is burned out. Replace the bad tube with a good one. If the
filament reads a low resistance, this indicates that the filament is all right. Use an ac
voltmeter to find the break between the filament and the output of the transformer.
c. SPARKING—Tap or shake the chassis. If you see or hear sparking, you have located a loose
connection or a short. Check and repair the problem.
If you locate and repair any of the defects listed under the visual check, make a note of what you find
and what you do to correct it. It is quite probable you have found the trouble. However, a good technician
takes nothing for granted. You must prove to yourself that the equipment is operating properly and that no
other troubles exist.
If you find none of the defects listed under the visual check, go ahead with the signal tracing
procedure. The trouble is probably of such a nature that you cannot see it directly with your eye-you must
see it through the eye of the oscilloscope.
The second type of testing is signal tracing. Tracing the ac signal through the equipment is the most
rapid method of locating a trouble that you cannot find by a visual check. It also serves as a check on any
repairs you may have made. The idea is to trace the ac voltage from the transformer, to see it change to
pulsating dc at the rectifier tube filament, and then to see the pulsations smoothed out by the filter. The
point where the signal stops or becomes distorted is the place to look for the trouble.
Before you begin signal tracing, it is a good idea to measure the dc voltage. The dc output voltage
should be in the neighborhood of 340 volts. If you have no dc output voltage, you should look for an open
or a short in your signal tracing. If you have a low dc voltage, you should look for a defective part and
keep your eyes open for the place where the signal becomes distorted.
