1-1
TM 55-4920-401-13&P
Chapter 1
INTRODUCTION
Section I. GENERAL INFORMATION
1-1. SCOPE. The purpose of the BH112JB-53 and
BH112JB-79 Exhaust Gas Temperature Tester (fig. l-l) is
to provide ground support for gas turbine aircraft engines.
When the appropriate cables and accessories are used with
the tester, it can perform any of the following functions
a. Test Engine thermocouple harness for error.
b. Test continuity of thermocouples in parallel engine har-
ness.
c. Test aircraft temperature indicator.
d. Test resistance of aircraft temperature circuit.
e. Test insulation of aircraft temperature circuit.
f. Verify accuracy of aircraft rpm system.
g. Verify accuracy of aircraft temperature indicating sys-
h.
i.
j.
Monitor temperature during engine trimming.
Monitor rpm during engine trimming.
Check cockpit instruments directly by using portable
trim module in cockpit.
k. Automatically correct temperature and rpm to standard
day conditions.
l. Test temperature spread.
m. Test individual thermocouples on bench.
n. Test thermal switches on aircraft or bench.
1-2 FORMS AND RECORDS. Maintenance forms,
records and reports which are to be used by maintenance
personnel at all maintenance levels are listed in and pre-
scribed by DA PAM 738-751.
Section II. DESCRIPTION AND LEADING PARTICULARS
1-3. PHYSICAL DESCRIPTION. The BH112JB-53 and
BH112JB-79 Exhaust Gas Temperature Tester consists of
an instrument ease assembly (2 fig. 1-1) and an accessory
case assembly (1, fig. 1-1). The instrument case latches to
the lop of the accessory case and contains two assemblies-
a portable trimmer assembly (3, fig. 1-1) and a probe con-
troller assembly. Two wheels, a support post, and a handle
provide mobility for the tester. Cables and accessories
needed to operate the tester are housed in the accessory
1-4. ELECTRICAL DESCRIPTION.
a. General Theory of Operation.
(1) Block Diagram. Figure 1-2 is a block diagram of
the tester. All circuitry is located in the probe controller and
trimmer. Input power distribution is shown with two lines.
AU other power and signal connections are shown with sin-
gle lines for simplicity. Function switching is omitted for
the same reason. Board and cable connectors reference des-
ignations are shown in parentheses.
(2) Digital Indicators. The trimmer contains two solid state
indicators (analog-to -digital converters) for measuring tem-
perature and engine speed (% rpm). Each indicator circuit is
on a printed circuit board (temperature board or tachometer
board) that plugs into the mother board. Displays for the
indicators are located on the display and counter board
which also plugs into the mother board. Brightness of the
displays is adjusted with the DISPLAY potentiometer. Two
calibration signals for the indicators are generated by the
calibrator board. One is an equivalent 600ºC chrome-
lalumel signal and the other is a crystal-controlled 70 Hz
signal. External calibration adjustments of an indicator are
made with its respective CAL ADJ potentiometer.
(3) Standard Day Measurements. For the Purpose of com-
paring an engin’s performance against the manufacturer’s
specifications, engine temperature and speed measurements
are corrected to standard day conditions. During the engine
trimming procedure, egt and % rpm measurements, cor-
rected to standard day condition, arc obtained by turning
the AMB TEMP potentiometer to display ambient tempera-
ture and the STD DAY switch on ON.
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