Overloaded resistors will be discolored and give off
relay test facility (fig. 7-25) consists of two relay test
a burnt odor and are often caused by shorted capacitors
sockets, two indicating lamps, and a test switch. Test
in the circuit. If you find an overloaded resistor in a
suspected defective relays by plugging into the proper
circuit, check all the capacitors in the circuit as well.
socket and operating the test switch. When a crystal
can-type relay is tested, both lamps should illuminate in
CAPACITORS.-- A shorted or leaking coupling
both test switch positions. When a power-type relay is
capacitor or an open cathode bypass capacitor will cause
tested, both lamps should illuminate only in test switch
an amplifier to hum and reduce amplification. An open
coupling capacitor will block amplification and no
sound will be produced by the amplifier.
TUBES.-- Defective or weak tubes cannot easily be
detected when checking operation of the power
Y o u can use an ohmmeter to check for
amplifiers with the meter test switch and test meter. The
short-circuited or leaky capacitors with the power off. A
system is designed to operate despite partial tube
good capacitor will give you an ohmmeter reading of
failures. The actual failure of one tube per stage will not
very high resistance or infinity. However, the same is
true for an open capacitor. Therefore, if you get a good
noticeably affect the meter reading of that stage or the
reading and still suspect the capacitor of being faulty,
output signal if the remaining tube or tubes are good.
shunt a new capacitor across it and note the effect.
A tube with internally shorted electrodes may affect
TRANSFORMERS AND CHOKES.-- In most
operation of its stage and give an abnormal meter
cases, a shorted transformer or choke will cause the
reading, and a tube with an open filament will have no
fuses in the supply circuit to blow. An ohmmeter can be
glow. Tubes that are suspected of being weak or
used to check the dc resistance of transformers and
defective will have to be checked with a tube tester.
chokes. A voltmeter can be used to check power
RESISTORS.-- You can use an ohmmeter to check
transformers and chokes. To check a suspected
the value of resistors in a circuit. Make sure the power
transformer with a voltmeter, apply primary power to
is turned off and the circuit discharged before putting
the input terminals; then use the voltmeter to read the
the meter across the resistor. If other components are in
output of the secondary windings. If one or more of the
parallel with the resistor, one end of the resistor will
secondary windings show no voltage ouput, the
have to be disconnected to get a correct reading.
transformer is defective.
Figure 7-25.--Relay test facility.