2. Service
Assembly/Module Diagnostic Trees (Diagnostic Section 3)
119
3.8.4.0 A18J4 Signal Input Test
1 Disconnect cable from A18J4 and reconnect that cable to the input of a 2-
way 3-dB. power splitter.
2 Connect one of the power-splitter outputs to A18J4 and the other to a
spectrum analyzer.
3 Verify the frequency of the signal on the spectrum analyzer to an accuracy of
100 kHz.
QUESTION: Is frequency 87.3 MHz? Yes, Go to, 3.8.4.1.
Is frequency 87.4 MHz? Yes, Go to, 3.8.4.2.
Is frequency between 87.3 and 87.4 MHz? Yes, Go to, 3.8.4.3.
3.8.4.1 A18 Mon Voltage Test
1 Measure dc voltage on the A18 mon terminal with a scope.
2 Verify that the average voltage is less than +0.05 Vdc?
QUESTION: Is the average voltage less than +0.05 Vdc?
If Yes: Go to, A15 diagnostic tree.
If No: Remove power, replace A18, and retest.
3.8.4.2 A18 Average Mon Voltage Test
1 Measure dc voltage on the A18 mon terminal with a scope.
2 Verify that the average voltage is more than +0.45 Vdc.
QUESTION: Is the average voltage more than +0.45 Vdc?
If Yes: Go to, 3.6.1.0 (A15 diagnostic tree).
If No: Remove power, replace A18, and retest.
3.8.4.3 Mon Signal AC-Content Test
1 Examine voltage on the A18 mon terminal with a scope.
2 Observe the signal and measure its ac content.
QUESTION: Is the ac content more than 10 mV?
If Yes: A18 is probably at fault, (although A15 is suspect).
Remove power, replace A18, and retest.
If No: A18 is nominally operational.
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