TM 55-4920-429-13
SECTION III TECHNICAL PRINCIPLES OF OPERATION
1-12. Block Diagram.
(See fig. 1-2.) The test set consists of several independent test circuits and common digital multimeter,
analog meter, and power supplies. Through these power supplies, the test set provides drive voltages to
operate external valves and actuators and excitation voltages for external position and feedback
transducers. Signals from position transducers are used to monitor position of cockpit controls. Signals
from gyro synchros are used to monitor the output of various helicopter gyros. The test set contains the
following circuit cards:
a. Discrete Signal Monitor CCA A3. Contains three servo loop amplifiers and demodulators. These
control closed loop operation of pitch, roll, and yaw ILCA. They also normalize signals through a
program function-select logic of discrete-signal inputs from various helicopter systems.
b. Digital Multimeter CCA A2. Processes and converts information for the digital display and
analog meter. It first establishes the value of measured variables. It then converts those values to equi-
valent BCD codes and analog signals.
c. Digital Display CCA A1. Decodes BCD data from CCA A2 and displays its numerical value on
dmm. This card also contains auto-ranging circuits that dynamically adjust the measuring circuits for
increasing/decreasing values.
1-13. Discrete Signal Monitor Circuit.
(See FO-2 and FO-8.) The discrete signal monitor enables selective monitoring of various helicopter
functions through position or feedback signals routed to the test set. A selected signal is first nor-
malized to the range of threshold detector circuits. Following that, it is tested to determine whether ac-
tive or inactive states are acceptable. Normalizing is accomplished through the program function-select
logic of discrete signal monitor circuit card A3. The logic consists of a matrix of nand gates and in-
verters. These selectively drive various independent sections of electronic switches U11, U12, and U14
in response to an active low (–8.66 volt dc) input signal on one of the program input lines. (Refer to ta-
ble 1-2.) Each of the electronic switch sections controls the open/closed condition of a specific signal
path in the normalizing circuits.
a. Normalizing of discrete signals includes several processes. It includes conversion of ac signals to
dc equivalents. It includes amplification of signals for appropriate scaling. It includes inverting and
offsetting signals where required. it includes selecting a logic ground (O volt) or a +7.5-volt dc level.
The combination of normalizing functions required for an input signal is selected by the SIGNAL SEL
1 or the SIGNAL SEL 2 switch. The actual value of any discrete signal selected for input to the discrete
signal monitor can be monitored on the dmm.
b. The helicopter test signals selected by the discrete signal monitor switches are applied to pins 51
and 53 of card A3. Resistors R57, 63, 64, and 66 establish the gain of the input differential amplifier at
-0.215. Diodes CR5 thru CR8 provide input protection. If the signal at U2-7 is dc, it is selected through
contacts of electronic switch U11-3, 4. If the signal at U2-7 is ac, the demodulated signal is selected.
From U2-7 the ac signal is demodulated. The demodulator rectifies, integrates, and provides a signal
gain of 1.1. The output of the demod at U2-8 is the dc equivalent of the ac rms value. This signal is
selected through contacts 1 and 2 of switch U11 whenever the input signal is ac.
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