Heathkit Model IM-13

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.

· Install a fresh C cell battery before proceeding.

ZERO ADJUSTMENT

(    )         Set the FUNCTION switch to the DC+ position.

(    )         Set the POWER switch to the ON position. Allow the tubes to come up to operating temperature.

(    )         Adjust the ZERO ADJ control for a zero meter reading.

(    )         Adjust the ZERO ADJ control clockwise.  This should result in an upscale meter reading.

(    )         Readjust the ZERO ADJ control for a zero indication on the meter.

(    )         Alternate the POLARITY switch between the DC+ and the DC- positions.  If the meter pointer was set exactly on zero, it should remain on zero in either switch position.

DC CALIBRATION

(    )         Insert the test lead phone plug into the input jack.

(    )         Set the probe tip to the DC position.

(    )         Set the FUNCTION switch to the DC+ position.

(    )         Set the RANGE switch to the 1.5V position.

(    )         Touch the tip of the probe to lug 1 on the rear wafer of the RANGE switch (blue wire).

(    )         Adjust the DC CAL control [on the circuit board] until the meter needle falls directly over the small red dot on the meter face.

OHMS ADJUSTMENT

(    )         Set the FUNCTION switch to the OHMS position.

(    )         Set the OHMS ADJ control for full scale (infinity).

(    )         Set the probe tip to the AC / OHMS position (opposite side of the DC marking).

(    )         Adjust the OHMS control for a full scale (INF) reading on the top scale.

(    )         Short the probe tip and the alligator clip test lead together; this should cause a zero meter reading.  If it does not, adjust the ZERO ADJ control (with the leads shorted) to zero the meter pointer.  Then separate the leads and repeat the preceding step and this step until no change in the meter readings.

(    )         Turn the SELECTOR switch to each of the other six resistance range positions.  Each range should give a zero indication with the test leads touching each other, and an INF reading with the test leads apart.

AC CALIBRATION USING THE AC LINE VOLTAGE

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

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

· If a more accurate AC voltage is available than the AC line, it should be used for the AC calibration.

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

(    )         Disconnect the test lead phone plug from the input jack.

(    )         Set the FUNCTION switch to the AC position.

(    )         Set the RANGE switch to the 1.5V position.

(    )         Adjust the AC BALANCE control [on the circuit board] until no meter movement is detected when switching from AC through DC- to DC+ positions.

(    )         Set the RANGE switch to the 150V position if unit is wired for 120 VAC or to the 500V position if wired for 240 VAC.

(    )         Set the probe tip to the AC / OHMS position.

(    )         Insert the test lead phone plug into the input jack.

(    )         Touch the test probe to one side of the AC line and then the other.  Note the lowest reading and remove the probe from the AC line.

· You can do this most conveniently if you first attach the probe’s alligator clip to the metal shank of a 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.

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AC Line Calibrating with a Line Cord Adapter

(    )         Connect the common alligator clip lead to the side of the AC line with the lowest reading.

(    )         Connect the test probe to the high side of the line.

(    )         Adjust the AC CAL control [on the circuit board] until the meter reads the AC line voltage.

(    )         Disconnect the both test leads from the AC line.

(    )         Set the probe tip to the DC position.

(    )         Unplug the test lead phone plug from the input jack.

(    )         Set the RANGE switch to the 1500V position.

(    )         Set the FUNCTION switch to the OFF position.

(    )         Calibration procedure is complete.

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AC Line Calibrating without a Line Cord Adapter

· When using the AC line for calibration without a line cord adapter installed DO NOT use the common alligator clip lead.  It is connected to the earth ground and will short out one side of the AC line and can possibly cause damage to both the instrument and the probe.  Use the test probe only when calibrating using the AC line.

(    )         Connect the test probe to the highest reading side of the AC line.

(    )         Adjust the AC CAL control [on the circuit board] until the meter reads the AC line voltage.

(    )         Disconnect the test probe from the AC line.

(    )         Set the probe tip to the DC position.

(    )         Unplug the test lead phone plug from the input jack.

(    )         Set the RANGE switch to the 1500V position.

(    )         Set the FUNCTION switch to the OFF position.

(    )         Calibration procedure is complete.