is zero voltage between the bridge (points A and B). A
difference between the bus voltage and the oncoming
voltage causes a voltage to exist across the bridge.
Connected between points A and B of the bridge is
the emitter and base of transistor Q5. The collector of
Q5 is connected to the base of Q1. The circuit is
completed from the emitter of Q1 to the emitter Q5. If
the voltage between A and B (across the bridge) is zero,
You can use signal tracers (such as dual trace
Q5 cannot be biased on. Therefore, the base to emitter
oscilloscopes) on transistor circuits if you observe the
of Q1 is not shorted out. If a voltage does appear across
precautions concerning the power supplies. Many
points A and B of the bridge, which can be caused by as
signal tracers use transformerless power supplies. To
little as a 5 percent voltage difference between the bus
prevent damage to the transistor, use an isolation
and the oncoming generator, Q5 will be biased on and
transformer.
short out the base to emitter of Q1. This will turn off
Q1 (fig. 4-21) and prevent energizing of relay K1.
Multimeters used for voltage measurements in
Resistor R19 prevents small momentary changes in
transistor circuits should have a high ohms/volt
voltage differences from turning on Q5 once relay K1
sensitivity to ensure an accurate reading. This should
has picked up.
beat least 20,000 ohms/volt.
Ohmmeter circuits that pass a current of more than
1 milliampere through the circuit under test cannot be
SERVICING TECHNIQUES FOR
used safely in testing transistor circuits. Before using
TRANSISTORIZED CIRCUITS
an ohmmeter on a transistor circuit, check how much
current it passes on all range settings. Do not use any
There are many differences between transistorized
range that passes more than 1 milliampere.
and electron tube circuits from the standpoint of
When used in the closely confined areas of
servicing. For instance, you rely on your senses of sight,
transistor circuits,
touch, and smell in the visual inspection of electron tube
circuits. This is not as feasible in transistor circuits.
test prods are often the cause of accidental shorts
Many transistors develop so little heat that you can learn
between adjacent terminals. In electron tube circuits the
nothing by feeling them. High-frequency transistors
momentary short caused by test prods rarely results in
hardly get warm. Usually, if a transistor (except a
damage. However, in transistor circuits this short can
high-powered transistor) is hot enough to be noticeable,
destroy a transistor. Also, since transistors are very
it has been damaged beyond use.
sensitive to improper bias voltages, you must avoid the
practice of troubleshooting by shorting various points to
In electron tube circuits, you often make a quick test
ground and listening for a click. When you test
by the tube substitution method. You replace the tube
transistor circuits, remember the vulnerability of a
suspected of being bad with one you know to be good.
transistor to surge currents.
In solid state circuits, the transistors are frequently
soldered in. This makes the substitution method
impractical.
Furthermore, you should avoid
SUMMARY
indiscriminate substitution of transistors and other
In this chapter, you have learned about voltage and
semiconductors. You should test transistors with an
frequency regulation. Within this area, you have
approved transistor test set.
learned about types I, II, and III power, the principles of
Most good quality test equipment used for electron
ac voltage control, the various types of voltage
tube testing can also be used for transistor circuit testing.
regulators, closely regulated power supplies, and
You can use signal generators, both RF and AF, if the
synchronizing monitors. You have also learned about
power supply in the equipment is isolated from the
the various techniques used to service transistorized
power line by a transformer.
circuits.
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