Section II. DESCRIPTION AND LEADING PARTICULARS

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. Change 5