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3)  At each altitude, the difference between ATC test set display and altimeter reading does not exceed ± 125 feet.
NOTE: 1.  A test altitude above 30,800 feet is required for check of D4 pulse.
2.  The altimeter reading may be increased or decreased to see if 125-ft tolerance can be met PROVIDED the ATC test set continues to display the designated altitude.

(g)  Repeat step (f) decreasing altitude to field elevation.
(5)  
Test alternate ATC system as outlined in steps (3)(i) thru (4)(g) (if installed/required).

(6)  
Place ATC control module function switch to OFF or STBY as appropriate.

(7)  
Open ATC and altitude source circuit breakers.

(8)  
If no longer required, remove external power from airplane.

5.  System Test - ATC System - Using Instrument Flight Research ATC - 600A
A.  General
(1)  The system test is a complete system checkout of the ATC. The first part of the system test consists of the performance of all tests possible using only the built-in test capability of the equipment installed in the airplane (operational test). The balance of the system test requires the use of an ATC ramp test set.
B.  Equipment and Materials
(1)  
Transponder/DME Test Set, IFR Model ATC-600A

(2)  
Portable pressure equipment to simulate 0 to 35,000 feet altitude and 0 to 400 knots airspeed.

C.  Referenced Procedures
(1) 
 34-11-0, Adjustment/Test

(2) 
 34-12-0, Adjustment/Test

(3)  
34-53-0, Adjustment/Test (Operational Test)

D.  Prepare to test ATC system
NOTE:  The test set is a source of interference for radio and L-band radar equipment operating on airplanes located in the vicinity of the test set. Turn test set off as soon as the test is completed or when other radio checks are being performed on the airplane.
(1)  
Position remote test set antenna approximately 21 inches from ATC antenna for system under test. Test set antenna should be approximately same height as the airplane antenna.

CAUTION:  NEVER PLACE REMOTE TEST SET ANTENNA CLOSER THAN 15 INCHES TO THE AIRPLANE ANTENNA WITH TEST SET "ON". DAMAGE TO THE TEST SET WILL RESULT.

(2)  
Route and connect coax from remote test set antenna to test set.

(3)  
Position controls on test set as follows:

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PWR Switch OFF
Mode Switch A/C Code
SLS Switch Center Position
Framing Control 0
INTERR Control 0
XPDR SIG Control FULLY CCW
XMTR FREQ Control 0

CAUTION:  IN THE FOLLOWING TESTS DO NOT SELECT CODES 3100, 7500, 7600, OR 7700. THESE ARE EMERGENCY CODES.
NOTE:  In the following tests the reply light on the ATC control panel shall illuminate whenever the ATC transponder transmits a reply to the test set, unless otherwise stated. Also the % reply meter on the test set shall indicate 100% when the transponder is replying (unless specified otherwise).
(4)  Close ATC System and Altitude Source (ALTIMETER or CADC) circuit breakers.
E.  Test ATC System
(1)  
Activate ATC system to be tested in Mode A with ALT RPTG off and code 0000 selected.

(2)  
Perform ATC system OPERATIONAL TEST par. 2, 34-53-0, ATC System
Adjustment/Test.

(3)  
Place PWR switch on test pet to AC or BAT as appropriate. Test set display shall indicate 0000.

NOTE:  For information regarding battery testing, timing and recharging refer to operation section of ATC-600A test set operators manual.

(4)  
Test receiver sensitivity as follows:

(a)  
On test set, verify % REPLY meter indicates 100.

(b)  
Slowly rotate XPDR SIG control clockwise until % REPLY meter indicates 90. The XPDR SIG control shall be between 69 and 77 (Note control reading. This is transponder system minimum triggering level). Return XPDR SIG fully counterclockwise 1.

(c)  
Select ALT RPTG on ATC control panel and A/C ALT on test set.

(d)  
Repeat step (b). XPDR SIG control reading shall not differ from reading in step (b) by more than 1.

(e)  
Return XPDR SIG control fully counterclockwise. Position test set mode switch to A/C CODE.

(5)  
　
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