33. NO A-RT KEYING (EPC STEP 31).
a. Situation 19.
This indicates the keying circuit (radio/intercom
interface) is bad. This symptom is caused by an open circuit. Trace the
keying circuit and identify the following possible bad items of equipment:
C-2298 (CDR), CX-4723 (CDR), AM-1780, CX-4723 (from the AM-1780 J501 to J-
3513/U J7), J-3513/U (Figure 100. Issued as adjunctive material.).
b. General test procedures.
You must determine the type of test and
the location of the test points to identify the actual bad item of
equipment. Troubleshooting techniques for this type of defect are voltage
trace, followed by continuity measurements.
c. Test procedure 74.
The first test point is at the commander's C-
2298.
Remove the CX-4723 from J801.
Measure voltage on the CX-4723, pin D to pin A.
If 22-30 volts D.C. is measured, the commander's C-2298 is bad.
If volts is measured, remove the CX-4723 from the AM-1780 J504
(unbolt AM-1780).
Measure voltage on the AM-1780 pin D to pin A J504.
If 22-30 volts DC is measured, the CX-4723 is bad.
If volts DC is measured, turn the RT and AM-1780 OFF.
Remove the CX-4723 from the AM-1780 J501.
Measure continuity from J501 pin S to J504
pin
D.
If the
measurement is infinity, the AM-1780 is bad.
If zero ohms is measured, remove the RT from the MT-1029.
Disassemble the MT-1029.
Remove the CX-4723 from the J-3513/U J7.
Measure continuity on the CX-4723, end-to-end, on pin S.
If infinity is measured, the CX-4723 is bad.
If zero ohms is measured, the J-3513/U is bad.
Figure 100.
Radio/intercom interface keying circuit.
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