Figure 7-25.--Relay test facility.

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

position 2.

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.

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