Voltage and Resistance Tests

tubes, a replacement chopper should be tried. If the new

chopper does not clear the difficulty, place the original

chopper back in socket. The calibration accuracy must be

If the chopper is replaced, the drive coil

re-checked.

connections should be reversed (twisted pair from chopper

top cap to meter terminal board) to discover which

polarity gives lowest residual meter deflection (para. 3.2c)

The leads should then be resoldered in this polarity.

Voltage and Resistance Tests

After determining that the trouble cannot be cured by re-

placement of tubes or chopper it is advisable to make a

systematic check of AC voltage, DC voltage, and resistance

at each socket pin. Table I shows the nominal AC and DC

voltages to ground from each socket pin number. Table 2

shows the nominal resistance values expected for each point.

Large or erratic deviations from the listed values of

voltage or resistance will serve as a clue in tracking

down a faulty component. Once the trouble has been found

and corrected, the calibration must be re-checked and re-

adjusted as outlined in paragraph 5.3 and paragraph 5.5

RF Probe Replacement:

Probes that have been damaged by overload or have become

inoperative or insensitive for any other reason should

be replaced rather than repaired. After replacement

calibration must be rechecked and adjusted if necessary.

The Probe serial number should agree with that of the

instrument; if exchanged, recalibration may be necessary.

5.5 Calibration Adjustment Procedure

Before making any calibration adjustments, it is essential

to provide the necessary reference standards as described

in paragraph 5.1 and take all precautions as outlined in

paragraph 5.2. The calibration adjustment procedure shall

then be made as outlined in Table 3.

AN/URM-145