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Figure 1-3. Helmet sight assembly-before and after installation
TM-9-1270-212-14-P Fire Control Subsystem Helmet Directed XM128 P/N 2277716-00 NSN 1270-00-122-9449 Manual
Figure 1-5. Electronic interface assemblies
TM 9-1270-212-14&P
1-13. Electronic Interface Assembly (EIA).
The eyepiece can be adjusted vertically and laterally to
obtain eye coincidence and is spring-loaded to retract to the
horizontal position when the detent iS released. The
eyepiece can be retracted either electrically by activating a
Except aS other  Wise stated in the discussion
solenoid in the sight assembly or manually by pressing a
below, the contigurations of the EIAs used in
button on the sight housing.
HSS XM128 and HSS XM136 are identical.
c. The electrical connector (AlPI) on the helmet sight
The EIA (fig. 1-5) contains most of the electronic
assembly connects to a plug in the helicopter.
components, except for the helicopter-mounted controls
1-12. Linkage Assemblies (Pilot and Gunner).
and displays, necessary for the operation of the HSS. The
ElA is the point of interface between the HSS and the
Each linkage assembly consists of an arm assembly, a
helicopter interface control unit and contains a power
rear support, two rails, a front support assembly, and two
supply module, a power supply circuit card, eight buffer
mounting brackets (fig. 1-4). The carriage end of each arm
amplifier modules, four boresight potentiometers that
assembly travels on the rails and contains ball bushings for
connect to an amplifier circuit card, and circuit cards for
almost frictionless movement. A stowage bracket in each
BIT capability. The EIA used in HSS XM136 also contains
front support assembly holds the arm in an overhead
an azimuth bias circuit card containing the components
position when the arm is not in use. In addition, the front
necessary to compensate for the 5.5-degree left-azimuth
support of each linkage assembly contains a magnetic
angle at which the pilot linkage is mounted in the AH-1S
fixture into which the steel-fastener end of the arm
assembly is placed for a built-in-test (BIT) of the linkage
assembly and the EIA: this fixture is called the BIT bracket.
a. The power supply module is a dc-to-dc converter that
uses 28 volts dc from the helicopter prime 28-volt dc power
a. The forward end of each tubular arm asssembly
source to produce regulated 28 volts dc for the buffer
contains a round steel fastener that attaches to the
magnetic receptacle at the top of the helmet sight assembly.
A pair of precision resolvers at the steel-fastener end of
b. The power supply circuit card functions with an
each arm and another pair at the carriage end provide for
input transformer to change 115-volt, 400-Hz ac to ac and
measuring the angular position of the helmet. Thus. with
dc vohagcs of various amplitudes.
the steel fastener attached to the helmet, the linkage
assembly can accurately determine the direction, both in
c. The buffer amplifier plug-in modules are used with
elevation and azimuth, its which the operator is looking,
the winding-compensated resolvers in each linkage assembly
i.e., the sight line to a target. Because of installation
to provide stable transformation ratios and phase shift
considerations, the pilot arm is 18 inches long and the
despite variations in temperature frequency.
gunner arm is 24 inches long. The gunner arm is bent
slightly to provide clearance within the helicopter cockpit.
d. The four boresight potentiometers provide a means
The resolvers are connected to the EIA by a coiled cable
of varying the amplitude and phase of small ac voltages for
that allows free movement of the carriage along the rails.
c o r r e c t i n g small system misalignments.  These
The pilot-arm-assembly cable connects directly to the EIA;
potentiometers are mounted on the side of the EIA used in
the gunner arm assembly requires an extension cable to
HSS XM128 and in a small housing on the end of the EIA
reach the EIA. The resolvers require precision trimming
used in HSS XM136; as mounted in the helicopters, the
resistors, which must be mounted close to the resolvers to
potentiometers are in approximately the same location.
maintain system accuracy. These resistors are mounted on a
One of these potentiometers is used for azimuth adjustment
rectangular terminal board attached to the backshell of the
of the pilot linkage; one, for elevation adjustment of the
connector (inside the connector) at the end of each coiled
pilot linkage; one, for azimuth adjustment of the gunner
linkage; and one, for elevation adjustment of the gunner
b. The two precision stainless steel rails are each 34
e. BIT circuitry  includes logic.  sequencing, and
inches long. The rear and front supports are secured to the
comparator circuits, as well as relays, reference voltage
mounting brackets, which secure the steel rails to the
helicopter overhead-canopy support members. The supports
sources, and external control switches. When externally
commanded, the circuitry tests each linkage assembly, its
are adjustable, to permit alignment of the rails during
system boresighting.
respective buffer amplifiers, and the power supply. Proper

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