2. Service
Assembly/Module Diagnostic Trees (Diagnostic Section 3)
124
3.9.1.7 A19 Signal-Frequency Test
1 Connect the output of the semi-rigid cable to the frequency counter
with a 50 input terminator.
2 Observe the A19 frequency with an accuracy of 1Hz. (The counter timebase
must be calibrated to an accuracy of better than 110
-7
.)
3 Calculate the nominal output frequency of the A19 from the EFC control
percentage using the equation:
Freq_nom = 10.0 MHz + 0.0331*(%Ctrl) Hz.
4 Verify that the measured frequency agrees with the calculated frequency to
2 Hz.
QUESTION: Does the measured frequency agree with the calculated
frequency to 2 Hz?
If Yes: The A19 reference-oscillator chain has checked-out correctly.
If No: Go to, 3.9.1.8.
3.9.1.8 A19 VCXO Set-Point Adjustment
The following procedure attempts to adjust the VCXO set-point.
It should be carried out if the A19 appears to have aged out of tolerance, or if
a new A19 has been installed.
1 Remove the cover plate from the left side of the instrument as seen from the
front.
2 Observe the frequency output of A19 as described in procedure 3.9.1.7.
3 Using a non-conducting screwdriver, adjust the VCXO mechanical trimmer
accessible through a hole in the left-hand chassis side-rail, to bring the
frequency within 1 Hz of its calculated nominal value.
4 Carefully withdraw the screwdriver.
QUESTION: Can the frequency be correctly adjusted?
If Yes: The procedure is complete and the A19 VCXO-chain operation
has been verified.
If No: Remove power, replace A19, and retest.
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