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troubles that you should keep in mind. The meaning of each trouble will be clear by the time you end
your study of vacuum tubes, even though you may not quite understand them now.
1. The filament, after long service, may be unable to emit as many electrons as are required for
proper operation.
2. The filament may burn out.
3. A tube element-the plate, for instance-may break its connection with the tube base pin.
4. Two elements, such as filament and plate, may short together.
5. The tube may become gassy.
The symptoms you will come across in signal tracing will be many and varied. You will need to
combine your "know-how" of the circuit and your knowledge of these five possible tube troubles to
determine if the tube could in some way be causing the symptoms. If you suspect the tube of causing
trouble, either try another tube in its place or check it on a tube tester. But remember, the final check of
whether or not the old tube was bad is whether or not the equipment works properly when a good tube is
put in its place. Therefore, putting in a good tube and then trying out the equipment is the best check.
Transformer and Choke Troubles
As you should know by now, the transformer and choke are quite similar in construction. Therefore,
it is no coincidence that the basic troubles they can develop are the same.
1. A winding can open.
2. Two or more turns of one winding can short together.
3. A winding can short to the casing, which is usually grounded.
4. Two windings can short together. This trouble is possible, of course, only in transformers.
As with the tube, the symptoms of these troubles will vary with the type of circuit. However, when
you have decided that one of these four possible troubles could be causing the symptoms, there are
definite steps to take. If you surmise that there is an open winding or windings shorted together or to
ground, an ohmmeter continuity check will locate the trouble. If the turns of a winding are shorted
together, you may not be able to detect a difference in winding resistance. Therefore, you need to connect
a good transformer in the place of the old one and see if the symptoms are eliminated; but keep in mind
that transformers are difficult to replace. Make absolutely sure that the trouble is not elsewhere in the
circuit before you change the transformer.
Occasionally, shorts will appear only when operating voltages are applied to the transformer. In this
case you might find the trouble with a megger—an instrument that applies a high voltage as it reads
resistance.
