Calibration Check Procedure

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

paragraph 5.4. If the check points fall between 2% and 5% of

the standard, the calibration adjustment procedure as outlined

in paragraph 5.5 should be followed.

5.4 Trouble Shootinq Procedure

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