Technician's Bench
ST-360 Altitude Selector/Alerter Quick Bench Test
by Butch Nimmo, Tech Rep
The S-TEC ST-360 altitude selector/alerter is a complex yet reliable unit, which is not field repairable. Despite its complexity, you can perform a limited bench test in the field to diagnose a few installation or operational problems. In this article I will discuss two of the most common problems that occur with the unit: 1) Altitude Encoder Code Translation and 2) the autopilot not flying the selected vertical speed.
To diagnose these problems, you need to evaluate the encoder code line recognition by the altitude selector/alerter and test for the proper vertical-speed output voltage. You can perform a quick bench test in the field with the following procedure and parts list.
First you will need an S-TEC breakout box, P/N 9524; a 0-28 VDC power supply; a DVM; an assortment of pin plugs, pin jacks, and alligator clips; bulk 22-gauge wire; and a 25- to 50-pin adapter cable (this is described in Volume 3, Issue 3, page 7 of The S-TEC Approach). See Table 1 for all of the parts necessary to fabricate the adapter cable. You can purchase these from S-TEC by contacting our sales department.
Table 1
25- to 50-pin adapter cable parts list
P/N Description Qty
3411 50-PIN "D" Recpt. 01
3423 25-PIN "D" Recpt. 01
5208 "D" Crimp Socket 50
5504 50-Pin Cover 01
5519 25-PIN Cover 01
6127 25-PIN Lock Lever 01
37082209 22-gauge wire A/R
Here is the quick bench test procedure for:
Altitude Encoder Code Translation
1. Connect the 50-pin plug on the 9524 breakout box to the 50-pin receptacle of the adapter cable.
2. Connect the 25-pin receptacle of the adapter cable to the altitude selector/alerter.
3. Using pin plugs, pin jacks, and bulk 22-gauge wire, fabricate a "Daisy Chain" grounding wire as shown in Figure 1.
4. Insert the six pin plugs into the following breakout box locations using Figure 2 for reference: 3, 4, 11, 12, 13, and 19. This simulates an encoded altitude of 9,200 ft.
5. Apply A+ (14 or 28 VDC) to pin 18. Apply ground to the pin jack of the "Daisy Chain" grounding wire.
6. Strap either 6 to 7 for 14V operation or 7 to 14 for 28V operation.
7. Jumper 16 to A+.
8. Turn on the power supply and observe the UUT (Unit Under Test). Backlighting should illuminate, and all segments and annunciators should illuminate After approximately five seconds, the display should have the following segments and annunciators remaining: ENT, 29.9, and BARO flashing.
9. Press DTA once and ALT twice. ALT should display 9.2. Turn off supply voltage.
10. Repeat steps 4 through 8 except in step 4 remove the "Daisy Chain" grounding wire from locations 3, 4, 11, 12, 13, and 19 and re-install into 5, 11, 22, 23, 24 and 25. This simulates an altitude of 31,600 ft.
11. Press DTA once and ALT once. ALT should display 31.6. These two altitudes test the altitude selector/alerter's ability to decode encoded altitude information.
Autopilot Vertical Speed
12. Press VS. Using the knob on the altitude selector/alerter, cycle through all 100 ft. increments from -16 to +16 and verify output using a DVM connector to pin 11 (ground) and pin 17 (VS command). See Table 2 for the proper output.
13. The audio output can be monitored with an oscilloscope at pin 8. The output is an 8-10 volt P-P.1-kilohertz signal capable of driving a 500 ohm load.
Figure 1
Figure 2
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Table 2
Displayed VS 0utput (Pin 17)
+16 +7.040 +/-.060VDC
+15 +6.912 "
+14 +6.785 "
+13 +6.657 "
+12 +6.530 "
+11 +6.403 "
+10 +6.275 "
+ 9 +6.147 "
+ 8 +6.020 "
+ 7 +5.892 "
+ 6 +5.765 "
+ 5 +5.637 "
+ 4 +5.510 "
+ 3 +5.382 "
+ 2 +5.255 "
+ 1 +5.127 "
- 1 +4.873 "
- 2 +4.745 "
- 3 +4.618 "
- 4 +4.490 "
- 5 +4.363 "
- 6 +4.235 "
- 7 +4.108 "
- 8 +3.980 "
- 9 +3.853 "
-10 +3.725 "
-11 +3.597 "
-12 +3.470 "
-13 +3.343 "
-14 +3.215 "
-15 +3.088 "
-16 +2.960 "
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Troubleshooting hints:
- When VS selected does not match the VSI indication, take a stopwatch and time a one-minute climb. Often, the VSI is out of calibration.
- If the display remains in the self-test configuration, check for an open master disconnect line to pin 16.
- If you select an altitude to climb or descend to and the altitude selector/alerter displays VS which contradicts your input (i.e. -VS when you input a climb or +VS when you input a descent) look for one or more open code lines.
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