Another good way to isolate the trouble in an operating synchro system is to use known operating
voltages as references for faulty operation. Since the proper operation of a system is indicated by specific
rotor and stator voltages, an ac voltmeter can be used to locate the trouble. When an ac voltmeter is
connected between any two stator leads, the voltage should vary from 0 to 90 volts (0 to 11.8 volts for 26-
volt systems) as the transmitter rotates. The zero and maximum voltage values should occur at the
S1 and S2
S2 and S3
S1 and S3
The rotor voltage should remain constant at all times, either 115 volts or 26 volts. In a system where
the units are close enough to permit checking, the voltage between the R1 and R2 terminal of any unit
energized by the primary ac source and the corresponding R1 or R2 terminal of any other unit energized
by the primary ac source should be zero. When the excitation voltage (115 volts or 26 volts) is above or
below the nominal value, the maximum stator voltages will also be above or below normal.
Synchro testers, as stated earlier, are used primarily for quickly locating a defective synchro. These
testers are capable of functioning as either transmitter or receiver.
When a transmitter is suspected of being defective, a synchro tester is usually substituted in its place
to simulate its actions. When the tester is used in this manner, a braking arrangement on the tester applies
the necessary friction to hold its shaft in different positions so you can determine whether the transmitter
is good or bad. When using the tester as a transmitter, it is usually a good idea to use only one receiver so
as not to overload the tester. If the tester is connected in place of a TR or used to check the output of a
transmitter, the brake is released, allowing the rotor to turn and indicate the transmitter's position. By
observing the tester's response to the transmitted signal, you can determine if the TR is defective or if the
transmitter's output is incorrect.
Q-71. What should you do with a synchro that has a bad set of bearings?
Q-72. Name two types of trouble you would expect to find in a newly installed synchro system.
Q-73. What type of indicator is usually placed in the stator circuit of a torque synchro system?
Q-74. What is the most probable cause of trouble in a synchro system that has all of its receivers
Q-75. If an ac voltmeter is connected between the S2 and S3 windings on a TX, at what two rotor
positions should the voltmeter read maximum voltage?
Q-76. What precaution should you take when substituting a synchro tester in a circuit for a transmitter?