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Technician's Bench
System 55 Quick Bench Test

by Scott Howard, Tech Rep

When power is applied to a System 55 a brief self-test is activated. During this test all annunciations on the controller (and on the remote annunciator, if installed) will illuminate. At this time the system monitors the presence of and the condition of certain internal circuits and functions, most notable the 10-volt regulator and accelerometer pitch limiting.

After the self-test, the controller annunciations extinguish and leave one of the following conditions:

1) The LCD screen displays "RDY". This indicates that the unit has passed the self-test, that the rate gyro is up to speed, and that the autopilot is ready for roll mode engagement. (There is a 15-20 second time delay after applying power until the pitch functions, ALT, VS, or CWS, can be engaged.)

2) The LCD screen is blank. This usually indicates a successful completion of the self-test, but further indicates there is no presence of a valid gyro tach signal (approx. 9 vdc) from the turn coordinator to pin 5 of the P1 connector.

3) The LCD screen displays "FAIL". This indicates failure of the self-test, usually due to a blown fuse on the 10v regulator or accelerometer offset.

Since certain conditions in the aircraft (i.e. a faulty rack connector seating or loading of the AP excitation circuit by external equipment such as heading system) can cause failure of the self-test, it is advantageous to be able to verify this type of problem on the bench. Even if complete test equipment is not available, a quick check may be performed utilizing the 9524 breakout box with the following troubleshooting procedure:

Connect the 50-pin receptacle on the breakout box to the P1 connector on the System 55. Apply the proper 14v or 28v voltage from the power supply to pin 34 and ground pin 35. Observe the display panel.

If the "FAIL" light remains on, check the following test points:
Pin 33 = 10vdc with respect to ground (Fuse may be open)
Pin 27 = 5 vdc with respect to ground
Pin 47 = 0 +/-10mv with respect to pin 27 (Accelerometer Centering)

If the display screen goes blank after test, jumper pin 33 to pin 5 to provide simulated gyro tach. "RDY" should illuminate. At this point, autopilot A+ applied to pin 17 will enable mode selection by pressing the buttons on the mode controller.

This simple test, performed with common shop equipment allows verification and the location of a source of suspected failure.

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