TM  1-1520-240-10 3-4-2            CONTROL/ INDICATOR FUNCTION A Enables transponder to reply to code A interroga- tions. B Enables transponder to reply to code B interroga- tions. ZERO Clears (zeroizes mode 4 code settings in trans- ponder computer. MODE 4 AUDIO LIGHT OUT Switch AT AUDIO, the REPLY indicator light will illumi- nate when the trans- ponder replies to valid mode 4 interrogations. A pulse tone will be heard in the headset when a code A interrogation is received but the CODE control switch is in B position and vice versa. At LIGHT, only the light will illuminate when the transponder replies to mode 4 interrogations. At OUT, the audio and RE- PLY light monitoring is disabled. MODE 4 TEST-ON-OUT Switch At ON, the transponder replies to valid mode 4 interrogations. At OUT, mode is disabled. At TEST, mode 4 is tested for proper operation. If test is valid, TEST GO. Mode 4 REPLY Light Illuminates to indicate valid mode 4 replies when MODE 4 AUDIO LIGHT switch is set to ei- ther AUDIO or LIGHT. TEST GO Light Illuminates when trans- ponder responds proper- ly to TEST position of M-1, M-2, M-3A, M-C, or MODE 4 TEST-ON-OUT swithces. TEST/MON NO GO Light The light illuminates when any of the following occurs: a. MASTER switch is on STBY. b. Transponder does not respond to interrogation. CONTROL/ INDICATOR FUNCTION c. Transponder does not respond to TEST position of mode switches. d. Transponder malfunc- tions. RAD TEST Switch At RAD TEST, the trans- ponder replies to interro- gations from external test equipment. At OUT the RAD TEST feature is in- operative. IDENT-MIC Switch AT IDENT, the set will transmit a coded identifi- cation pulse for about 20 seconds to all interrogat- ing stations on modes 1, 2, and 3/A to identify the helicopters position. OUT position turns off the identification pulse. MIC feature is not used in this installation. STATUS Indicators The indicator lights are part of built-in-test equip- ment. A lit ALT indicator indicates trouble in The encoding altimeter. A lit KIT indicator indicates trouble in the computer or the computer is not installed. A lit ANT indi- cator indicates antenna trouble. ANT Switch Three-position antenna diversity switch labeled TOP, DIV, and BOT. Nor- mal position is DIV. When jamming is heavy, TOP or BOT antenna may be selected. IFF Fail Light (Located on the center instrument panel) The light will illuminate in MODE 4 when any of the following occurs: a.   The computer is installed without a code. b.   No reply or improper reply is made to a valid interrogation. c.    A malfunction occurs in the transponder.