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Learjet: Master Caution Box & Aircraft-Mounted Transistors

November 2012

The Learjet 35/36 model and some Lear 20s are wired in such a manner that there is potential to cause problems with the Master Caution Box (MCB), P/N 2610855-29 and 2618055-30, which may result in incorrect troubleshooting of the MCB and Glare Shield. It could also 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 light intensity of the Pilot and Copilot Glare Shield assembly. Transistor Q211 is for a 3.4Vdc regulated voltage Power Supply for the Master Caution Box; that supplies power to the Electronic Integrated Circuit (IC) inside the MCB. The ICs are voltage sensitive with a maximum rating of 15 Volts direct current (Vdc). If transistor Q211 fails, the voltage will be driven to 28 Vdc, nearly twice the maximum voltage that the ICs can handle. This can result in the failure ICs in the MCB.

Care should be taken when installing the new MCB to ensure that unregulated voltage (above 5Vdc) is not applied to the MCB (failed Q211); causing a complete failure of the new MCB. Prior to replacing the MCB; Q211, 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 aircraft ground. This is to insure Master Caution Box has proper grounding. Problems caused by grounding issues can be numerous; from completely inoperative and intermittent to erroneous caution trips. Troubleshooting this problem can be a daunting task but will save time and money in the long run.

2. Verify TRANSISTOR Q211 is working properly and replace 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 “pin outs,” connectors, meter readings, etc.; contact Duncan Aviation’s Accessory Electrical Team Leader Tim Smith.