Master Caution Box Troubleshooting: Two Recommended Procedures

December 2023

Great care needs to be taken when troubleshooting the wiring of the MCB (Master Caution Box), PNs 2610855-29 and 2618055-30, on the Bombardier Learjet 35 and 36 aircraft, as well as some Bombardier Learjet 20 aircraft. Incorrect troubleshooting of the MCB and Glare Shield may result in complete failure of the MCB.

For example, three transistors, Q210, Q211, and Q212, are mounted in the nose of the aircraft. Transistors Q210 and Q212 control the light intensity of the pilot and copilot Glare Shield assembly. Transistor Q211 is for a 3.4Vdc (Volts direct current) regulated voltage power supply for the MCB that supplies power to the Electronic IC (Integrated Circuit) inside the MCB.

The ICs are voltage-sensitive with a maximum rating of 15Vdc. If transistor Q211 fails, the voltage will be driven to 28Vdc, nearly twice the maximum voltage that the ICs can handle. This can result in the ICs in the MCB failing.

Care should be taken when installing the new MCB to ensure that unregulated voltage (above 5Vdc) is not applied to the MCB (failed Q211), potentially causing a complete failure of the new MCB. Prior to replacing the MCB, the Q211 transistor, insulator, and hardware should be checked or replaced.

If in doubt, these transistors are easy to access and cheap enough to warrant changing them at regular intervals.

Recommended procedures when replacing or troubleshooting the MCB

1. Check the ground leads from the master caution connectors to the aircraft ground. This is to ensure the Master Caution Box has proper grounding. Problems caused by grounding issues can be numerous, from completely inoperative and intermittent to erroneous caution trips. Troubleshooting a grounding issue can be a daunting task. Taking steps to ensure the MCB is properly grounded will save time and money in the long run.
2. Verify Transistor Q211 is working properly and replace it if needed. A failure of this transistor will allow the unregulated voltage to be applied to the MCB, leading to a complete failure when power is applied to the aircraft.

For exact details, including pinouts, connectors, meter readings, etc, contact me anytime.

Learn more about Duncan Aviation's component repair and overhaul capabilities.