LDM

This video first shows the teardown of a vintage EGT indicator manufactured end of 60's by General Electric CO. In the second part the schematic diagram is presented. The video ends by some tests performed on this instrument.
This instrument uses a simple technique. The signal which comes from the thermocouple (around 40mV at 900°C) is compared to the signal coming from the feedback potentiometer of the servo control loop. The servo control loop uses an AC motor, therefore the DC error signal is fed to a chopper in order to convert this DC error signal into an AC error signal. This error signal is amplified and fed to the AC motor. The motor runs until the error signal becomes null. The track of the feedback potentiometer is engraved in order to have a non-linear operation which corrects the natural non-linear behavior of the thermocouple. A temperature sensor made with a special diode is used for cold junction compensation.
Connector pinout (not exhaustive):
Pins 3-4: 115VAC 400Hz
Pin 6: thermocouple positive input
Pin 5: thermocouple positive input
Pins 10-11: solenoid, 28VDC

[@mattwietlispach7606]

Excellent teardown, as always. This is from a Boeing 747-100/200/300 aircraft. GE made instruments in this same annoying stacked board design for the DC-10 and L-1011 as well. The case is so thin that it is easy to dent and makes removing the case from the chassis extremely difficult when there is a dent. The red needle is a temperature exceedance needle that is pushed past the red dial mark if it should pas that point. The red needle can be sent back to the red marker on the dial via a discrete input pin that vibrates the red needle back wards to that stop. Other 747 engine instruments made by GE are all assembled the same way. N1, N2, and EGT. Eldec appears to have made the Fuel Flow indicators which are driven by a remote LRU out of the cockpit. The very worst case scenario for this design is the DC-10 N1 indicator that is 32 cm (12.5") deep. A good size dent towards the front of the indicator makes the case almost impossible to remove without cutting. Smiths eventually came out with their own 747 Classic analog engine indicators that typically were selected when Rolls Royce engines were chosen for the aircraft. The Smiths indicators are FAR better designed both electrically and mechanically. Their cases are hard to dent and the circuit boards inside are not stacked up like crackers in a can. The mechanics are far better designed and far easier to work on. I've had to deal with these GE indicators from the 747 and DC-10 and share your observations about their design.