5.3 Calibration Check Procedure
With suitable calibrating equipment (paragraph 5.1) and
taking necessary precautions (paragraph 5.2), we can now
check calibration accuracy. Each range should be checked
at a voltage representing 90% of full scale value. If these
check points agree within 2% of the standard, it is recom-
mended that no adjustment be made. If the check points
deviate by more than 5% from the standard, it is recommended
that a trouble shooting procedure be followed as outlined in
the standard, the calibration adjustment procedure as outlined
in paragraph 5.5 should be followed.
a. Case Removal
The instrument may be removed from its case after first
removing the two screws located at the back of the case
(near the bottom) and the four screws located on the
sides of the case near the front.
The power supply may be removed from the case after
first removing the four screws located on the sides of
the case near the rear.
b. Tube Replacement
If the tube VI should become weak or noisy, it should
be replaced with Boonton Electronics Part Number 525001.
A commercial low noise 12AX7 which has low microphonism
and meets the requirements of paragraph 3.2c may be used
if a BEC 525001 is not available. The Telefunken ECC83/
12AX7 has been found to meet these requirements consist-
ently. After replacing this or other defective tubes,
re-check calibration as outlined in paragraph 5.3 and
adjust if necessary as described in paragraph 5.5
c. Chopper Replacement
If satisfactory operation is not obtained by replacing
AN/URM-145
a
