Garmin GTX 320 ATC Maintenance Manual page 21

3.3.3.17 Front Panel Test - Reply Light
1. Rotate the Mode Select Switch from TST to ALT and with the unit not replying.
Increase the RF level on the ATC-1400A above MTL so that the transponder has a reply rate above 90%.
The triangle-shaped reply light shall turn on.
2. Rotate and hold the mode select switch in the TST position. The reply light shall remain on while in
the TST position.
3. Release the switch so that it returns to the ALT position. The reply light shall remain on.
3.3.3.18 Numerical Reply Code Switches
1. Rotate the XPDR MODE switch on the ATC-1400A to position A.
2. With the transponder mode switch in the ON position, incrementally rotate the four reply code
switches through each position. Verify that the unit causes the ATC-1400A to display the same number
on the XPDR CODE digital display.
3. While the code switches are in a numbered detent position, apply a light force against the knob from
all directions. Verify that the digit displayed on the ATC-1400A test set does not include any intermit-
tent numbers.