Heathkit Model IM-38

Vacuum Tube Voltmeter


Calibration Procedure

 

·         With the instrument off, the meter pointer should be over the zero indication on the meter face.  If it is not, turn the mechanical zero-adjust screw at the bottom of the meter until the pointer is properly aligned.

 

Ancillary Equipment Needed

 

Calibration Cable (see notes)

AC VTVM (optional)

 

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NOTES

 

·         If the instrument has a three-wire line cord, temporarily install an AC line cord adapter on the line cord plug before attempting to calibrate the instrument using the AC line voltage.

 

·         A calibration cable can be made from a polarized two wire extension cord by removing the outlet socket.  Then solder a red banana plug to the wire from the short blade (hot side), and a black banana plug to the other wire (cold side).

 

·         Modern single phase house wiring usually uses polarized outlets. If wired correctly, the taller slot is the white wire or low side of the power line, while the smaller slot is the black wire or hot side of the AC line.

 

·         An AC VTVM of known accuracy can be used as a reference instrument to calibrate the VTVM.

 

·         If a more accurate AC voltage source is available than that from the AC lines, it should be used.

 

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CALIBRATION USING THE AC LINE VOLTAGE

 

·         Always connect any test cables to the instrument being calibrated first, and then to any high voltage source.  And always disconnect from the high voltage source first.

 

·         Use extreme care when measuring the AC line voltage.  Personal shock or instrument damage could result from carelessness.

 

Calibrating with a Calibration Cable

 

(    )         Set the RANGE switch to the OFF position.


(    )         Connect the calibration cable’s banana plugs to the input terminals.

 

(    )         Set the RANGE switch to the 300 V position. Allow the tubes to come up to operating temperature.

 

(    )         Connect the calibration cable’s outlet plug to the AC line voltage.

 

(    )         Adjust the calibration control C until the meter reads the AC line voltage.

 

(    )         Disconnect the calibration cable’s outlet plug from the AC line voltage.

 

(    )         Set the RANGE switch to the OFF position.

 

(    )         Disconnect the calibration cable’s banana plugs from the input terminals.

 

(    )         Calibration procedure is completed.


Calibrating with Test Probes

 

(    )         Set the RANGE switch to the OFF position.


(    )         Connect a pair of test probes to the input terminals.

 

(    )         Set the RANGE switch to the 300 V position. Allow the tubes to come up to operating temperature.

 

(    )         Connect the test probes to the AC line voltage.

 

·         If a test probe has an alligator clip on the end, this can be done most conveniently by attaching the alligator clip first to the metal shank of screwdriver.  Then insert the blade of the screwdriver into the slotted opening on the AC outlet.  DO NOT TOUCH THE METAL SHANK OF THE SCREWDRIVER.

 

·         Do not allow the test probes to touch while connected to the AC line voltage.

 

(    )         Adjust the calibration control C until the meter reads the AC line voltage.

 

(    )         Disconnect the test probes from the AC line voltage.

 

(    )         Set the RANGE switch to the OFF position.

 

(    )         Disconnect the test probes from the input terminals.

 

(    )         Calibration procedure is completed.

 

CALIBRATION USING AN ACCURATE AC SOURCE VOLTAGE

 

(    )         Set the RANGE switch to the OFF position.

 

(    )         Connect a pair of test probes to the input terminals.

 

(    )         Set the RANGE switch to match the AC source voltage. Allow the tubes to come up to operating temperature.

 

(    )         Connect the test probes to the AC source voltage.

 

(    )         Adjust the calibration control C until the meter reads the AC source voltage.

 

(    )         Disconnect the test probes from the AC source voltage.

 

(    )         Set the RANGE switch to the OFF position.

 

(    )         Disconnect the test probes from the input terminals.

 

(    )         Calibration procedure is completed.