This video documents the troubleshooting and repair of an ILS (Instrument Landing System) test generator that was exhibiting signal saturation in its glide path mode. The investigation begins with an analysis of the schematic, focusing on the modulation depth adjustment circuit. After determining that a simple trimmer adjustment was insufficient, a comparison with the correctly functioning "localiser" circuit revealed a significant discrepancy in a key resistor value. The fault was traced to an overly high resistance (150 kΩ) in the glide path circuit. The problem was resolved by adding a resistor in parallel to reduce the total resistance to approximately 68.5 kΩ, matching the design of the localiser circuit and eliminating the saturation. The successful repair is then verified by connecting the generator to an aircraft glide slope receiver and observing the correct response of a flight path deviation indicator to various DDM (Difference in Depth of Modulation) settings.
Troubleshooting and Repairing an ILS Glide Path Generator
0:00 - Problem Identification: The ILS test generator shows a saturation of the modulation signal when set to glide path mode.
0:18 - Initial Investigation: A review of the schematic points to the modulation depth circuit, specifically trimmer RV502 and its associated resistors. Initial adjustments to the trimmer fail to resolve the saturation issue completely.
2:24 - Root Cause Discovery: By comparing the glide path circuit to the localiser circuit, a key difference is found. The localiser circuit uses a 68 kΩ resistor for modulation, while the faulty glide path circuit uses a much higher 150 kΩ resistor, causing the signal to saturate.
2:54 - Implementing the Fix: The issue is corrected by adding a 127 kΩ resistor in parallel with the existing 150 kΩ resistor. This modification brings the total resistance down to approximately 68.5 kΩ, which successfully resolves the signal saturation.
3:30 - Verification with Aircraft Equipment: To confirm the repair, the test generator is connected to a real aircraft glide slope receiver, with the output displayed on a flight path deviation indicator.
5:51 - Successful Test and Demonstration: The test confirms the fix is successful. The indicator's flag disappears, signaling a valid input. The horizontal glide slope bar remains centered when the DDM (Difference in Depth of Modulation) is zero and moves up and down correctly as the DDM is adjusted, validating the generator's proper operation.