T.O. 33A1-3-358-11/TM 11-6625-842-15
Section IV
NAVAIR 16-30APM239-2
Paragraphs 4-11 to 4-18
4-11. MODE C ENCODER SIMULATOR. The Mode C encoder simulator consists of the TEST CONDITION
switch, S13, and 11 toggle switches, S2 through S12. S13 is a two-position wafer switch which in the VOLTS
position applies the two test voltages from the reference regulator to the 11 toggle switches, and in the OHMS
position allows electrical connection between the 11 toggle switches and the 22 test resistors, R4 through R25,
located on terminal board TB1. Each of the 11 toggle switches is labeled to represent a single bit of the Mode
C reply code, and applies either a turn-on (ON position) or turn-off (OFF position) test condition to MODE C
ENCODER connector J5. When jumper plug P5 is connected to J5, the simulated Mode C reply code lines are
routed through the test set harness to TRANSPONDER connector J2 as the Mode C inputs to the receiver-
transmitter.
4-12. MODE 4 REPLY CIRCUIT. The Mode 4 reply circuit consists basically of a limiter, which produces
a +5-volt output pulse when the received Mode 4 reply exceeds +5 volts, and which allows the Mode 4 reply
signal to pass reduced in amplitude by 0.5 volt when it is less than or equal to +5 volts. The Mode 4 reply
signal is applied via S17 (in the REPLY position) to the cathode of diode CR7. The anode of CR7 is biased on
by resistors R27, R28, and R29 and zener diode VR2. This network produces a level of +6.5 volts at the junc-
tion of R28 and VR2, which is required to compensate for the diode voltage drops to provide +5 volts limiting.
In this manner, when the Mode 4 reply signal is less than or equal to +5-volts, diode CR7 conducts, applying
the Mode 4 reply to the base of transistor Q1, Also, if the Mode 4 reply exceeds +5 volts, diode CR7 opens
and the +5-volt bias is applied to the base of transistor Q1. Transistor Q1 is an emitter follower which drives
the Mode 4 reply output to the receiver-transmitter under test.
4-13. CONTROLS, INDICATORS AND CONNECTORS. The balance of the circuitry of the test set consists of
the internal intraconnecting wiring harness and certain controls and indicators connected to the harness which
are used to simulate or monitor functions of the transponder set under test. The names, functions, and loca-
tions of all test set controls, indicators and connectors are provided in figures 3-1 and 3-2. The internal wir-
ing harness and the cables, supplied as accessories with the test set, provide interconnection between the test
set, the transponder set control, and the receiver-transmitter. In addition, functions which are associated
with Transponder Computer KIT-1A/TSEC, Transponder Set Test Set TS-1843/APX, and the altitude encoder
are made available at connectors J4 (MODE 4 COMPUTER), J3 (TRANSPONDER TEST SET) and J5 (MODE C
ENCODER), to facilitate interconnection of these units.
4-14. Nine BNC connectors, J6 through J14, are provided on the test set front panel to allow access through
the internal wiring harness to various video functions of the receiver-transmitter. These connectors permit
the use of external test equipment or simulators to introduce certain signals, normally found in the aircraft
system, to the equipment under test, and also provide a means of monitoring other receiver-transmitter func-
tions.
4-15. TEST EQUIPMENT REQUIRED.
4-16. The following test equipment or equivalent is required for test set maintenance:
4-17. CHECKOUT PROCEDURE.
4-18. The following procedure should be performed to determine that the test set is operating properly.
a. Place the REPLY-INTERRUPT switch in the REPLY position. Energize the test set in accordance
with the procedure of paragraph 3-31.
b.
Connect the simulator test set to MODE 4 COMPUTER connector J4 and operate to produce a
Mode 4 reply pulse train to the test set. Vary the magnitude of the pulse train from +2 to +35 volts while ob-
serving the, limited Mode 4 reply pulse train as directed in c.
c.
Connect cable W1 to TRANSPONDER connector J2. Connect an oscilloscope to pin 47 of cable
W1, terminated in 92 ohms, and observe the amplitude of the limited Mode 4 reply pulse train. The amplitude
Change 2 4-3
