This video features a detailed teardown and reverse engineering of a vintage Airesearch Interstage Temperature Indicator (ITT), an avionics instrument designed to measure inter-turbine temperature in aircraft engines. The presenter disassembles the unit, revealing a complex electromechanical servo system and multi-board analog circuitry. Initial inspection uncovers a unique friction-coupler mechanism in the servo, which required repair due to degraded O-rings. Through extensive reverse engineering (nearly 10 hours), the circuitry for thermocouple input, cold junction compensation, and servo control is mapped. The primary functional fault is then diagnosed as an open thermistor within the cold junction compensation circuit. The video concludes by demonstrating the indicator's restored functionality using a resistance decade box to simulate the thermistor, confirming the diagnosis and the viability of repair.
Vintage Avionics Indicator Teardown and Repair
0:00 Introduction to the Indicator: The video begins with an introduction to a vintage Airesearch Interstage Temperature Indicator (ITT) obtained as part of a four-instrument lot for $50.
0:09 Instrument Function: This specific indicator measures the temperature between the low and high-pressure turbines of an aircraft engine, with a range of 100 to 1000 degrees. It operates on a 28V DC power supply.
1:21 Internal Components Revealed: Upon opening, the instrument reveals a DC motor-driven servo mechanism with a feedback potentiometer and a gear train. The thermocouple input connects directly to specific pins (yellow and red wires for Chromel and Alumel).
2:00 Cold Junction Compensation: The internal circuitry includes a temperature sensor (thermistor, R27) glued to the PCB for cold junction compensation, which accounts for the ambient temperature at the thermocouple connection point.
3:09 Multi-Board Electronics: The device comprises a front-end board for thermocouple signal processing (featuring op-amps like UA725 and LM1987) and a separate servo control board with multiple LM1987 op-amps and power transistors.
3:41 Initial Mechanical Issue: An early problem identified was a non-rotating output despite the servo motor running. This was traced to a friction coupler (not a conventional gear train) where internal O-rings had degraded, requiring a mechanical repair using a heat shrink tube.
5:04 Detailed Reverse Engineering: The presenter spent nearly 10 hours reverse engineering the entire circuit. Key findings include a differential amplifier with a gain of 60.5 for the thermocouple input, a cold junction compensation circuit using a positive coefficient thermistor (likely Pt500), and a servo amplifier with a gain of 249.
8:10 Functional Test Setup: After mechanical repair, the indicator was connected to a thermocouple simulator (set to 200 degrees) and a 28V power supply for functional testing.
8:45 Diagnosis of Primary Electrical Fault: Despite the servo repair, the indicator remained non-functional. Measurements showed the output of the cold junction compensation amplifier was abnormally low and constant (1.5V instead of an expected 6V decreasing with temperature). The root cause was definitively identified as an open thermistor (R27).
9:24 Confirmed Functionality with Replacement: The video demonstrates replacing the faulty thermistor with a resistor decade box. Adjusting the resistance immediately causes the indicator needle to move, confirming that the open thermistor was the critical fault preventing proper operation, and the rest of the unit's electronics and mechanics are now functional.