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 TM 55-1905-220-14-11
5-117. SHIP'S COURSE INDICATOR - MAINTENANCE INSTRUCTIONS (Continued).
3
Push-pull transistor Q1 and Q2, mounted on heat sinks, provide power to control winding of the Type 15
servomotor. Transformer T1 couples the input signal to the transistor bases. Output signal flows directly
from the collectors to the control field of the servomotor. Transformer T2 and diodes CR1 and CR2 provide
bias for the common-emitter amplifier circuit, resistor R2 forward biases the transistors.
4
Reliability of the transistor amplifier is enhanced in several ways. In the emitter circuit, R2 minimizes current
spikes. The collector circuit, C1 and C2 prevent voltage spikes. In addition, zener diode CR3 limits excess
voltage spikes that might enter the amplifier from the power line.
5
Several circuit elements contribute to the stability of the servo loop of which the amplifier is a part. Capaci-
tors C1 and C2 serve the additional function of converting the reactive load of the motor control field to a
resistive load across the transistors. Capacitors C3 and C4 shift the phase of the current through the fixed
field winding of the servomotor to be in quadrature with the current through the control winding. These
capacitors also serve to decrease the voltage across the fixed field to about 38 rms volts so that the motor
can be damped magnetically. Diode CR8, rheostat R6, and the associated L-C filter provide an adjustable
direct current through the fixed field. This current serves to magnetically damp the servomotor for stable
operation. Rheostat R6 is adjusted at the factory and normally needs no further adjustment unless the
indicator is extensively overhauled.
6
As described under anti-stickoff voltage, two identical synchros provide input signals to the amplifier. In the
vicinity of the one true null position the more sensitive 36X data is the dominant signal by virtue of the anti-
stickoff voltage described above. Away from the null the 1X data is the dominant signal. The mixing circuit
selects the more dominant signal.
7
Near null both signals are small. The 36X signal is applied across diodes CR4 and CR5 which present a
high impedance at low voltage. The 36X signal thus passes through input transformer T1. The 1X signal is
applied in series with another pair of diodes CR6 and CR7; these diodes also present a high impedance at
low voltage. The 1X signal is thus blocked.
5-1313
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