Figure 1-39B.Zeroing differential synchros by the voltmeter method.
3. Energize the circuit, unclamp the differential's stator, and turn it until the meter reads minimum.
The differential is now approximately on electrical zero. Deenergize the circuit and reconnect it
as shown in view B.
4. Reenergize the circuit. Start with a high scale on the meter and work down to the 0- to 5-volt
scale to protect the meter movement. At the same time, turn the differential's stator until you
obtain a zero or minimum voltage reading. Clamp the differential stator in position, ensuring the
voltage reading does not change. This is the fine electrical zero position of the differential.
Zeroing a Control Transformer (Voltmeter Method)
Two conditions must exist for a control transformer (CT) to be on electrical zero. First, its rotor
voltage must be minimum when electrical zero voltages (S2 = 52 volts; S1 and S3 = 26 volts) are applied
to its stator. Second, turning the shaft of the CT slightly counterclockwise should produce a voltage
across its rotor in phase with the rotor voltage of the CX or TX supplying excitation to its stator. To zero
a CT (using 78 volts from its transmitter) by the voltmeter method, use the following procedure:
1. Set the mechanism that drives the CT rotor to zero or to its reference position. Also, set the
transmitter (CX or TX) that is connected to the CT to zero or its reference position.
2. Check to ensure there is zero volts between S1 and S3 and 78 volts between S2 and S3. If you
cannot obtain these voltages, you will need to rezero the transmitter. NOTE: If you cannot use the
78 volts from the transmitter circuit and, an auto transformer is not available, you may use a 115-
volt source. If you use 115 volts instead of 78 volts, do not energize the CT for more than 2
minutes because it will overheat and may become permanently damaged.
3. Deenergize the circuit and connect the circuit as shown in view A of figure 1-40. To obtain the 78
volts required to zero the CT, leave the S1 lead on, disconnect the S3 lead on the CT, and put the
S2 lead (from the CX) on S3. This is necessary since 78 volts exists only between S1 and S2 or
S2 and S3 on a properly zeroed CX. Now energize the circuit and turn the stator of the CT to
obtain a minimum reading on the 250-volt scale. This is the coarse or approximate zero setting of