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Installation Secrets
Time Saving Test and Troubleshooting Devices

by Scott Howard, Tech Rep

When trouble shooting an installation, even uncomplicated tests can be time-consuming without the proper equipment. Simple test devices which are easy and inexpensive to build can save you a lot of time and aggravation. Here are a few examples of devices which will help in the installation of S-TEC Autopilot systems.

Servo Drive Box
Anyone who has ever installed an S-TEC Autopilot knows that the servos require a clutch adjustment before installation and sometimes require a reversal in the direction of rotation.

Figure 1 shows the schematic of a device which will apply voltage to the solenoid and provide motor drive in both directions for clutch set-up on the bench. Two LED's verify the trim switch's actuation on the pitch servo. This unit can be used in the aircraft during the servo burn-in procedure. Test jacks on the motor pins allow for the monitoring of the servo starting voltage when using a variable external supply, or for simply reading motor resistance. (Re: The Technician's Bench, S-TEC Approach Volume 1, Issue 2, March 1996)

Figure 1

Altitude Deviation Signal Source
During the ground test of an autopilot system, it is necessary to verify that the servo motors respond to autopilot inputs. Since S-TEC System Thirty, ThirtyALT & 50 autopilots have altitude hold without pitch command, the only way to cause the pitch servo to drive is by connecting a static pump to the transducer static source.

An easier method to confirm that the computer is responding to the transducer signal, producing a servo drive in the proper direction is to use the device shown in Figure 2. Engage "ALT" with the switch in the center position. The switch allows a simulated altitude signal change of + or - 50 feet, creating a servo response to correct the error. This also verifies the proper direction of rotation and trim sensor switch actuation in the pitch servo. Test point jacks allow you to check for 10 VDC and ground from the computer. Note: With the transducer unplugged and the autopilot on, signal line pin "1" should be 5 VDC respective to pin "2" ground.

Figure 2

Turn Coordinator Tach Generator
S-TEC Systems 40, 50, 55, 60, & 65 all require a valid gyro tach signal from the turn coordinator (among other things) to enable the "RDY" light to illuminate, prior to mode engagement. The tach signal is initially a logic low signal (approximately 1 VDC) until the turn coordinator reaches almost its full rated speed, at which point it transitions to high (8-10 VDC). The tach signal can be monitored with a Digital Volt Meter at the respective pin on the autopilot controller, but the device shown in Figure 3 provides a quick and easy way to substitute for the turn coordinator during ground testing. A 14/28V selectable voltage divider between A+ (pin A) and GND (Pin B) provides a valid tach voltage of 8-9 volts @ Pin "E" for autopilot operation.



Figure 3

These devices are so inexpensive and simple to build, if you only use them once or twice a year, they are worth the effort.

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