Heathkit Model IM-17

Utility Solid-State Voltmeter

Calibration Procedure

· With the meter 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 fresh batteries matching the + markings screened on circuit board.

Ancillary Equipment Needed

AC VTVM (optional)

* * * * * * *

INITIAL CONDITIONS SETUP

Refer to Figures 1 and 2.

(    )         Set the POWER switch to the OFF position

(    )         Set the SELECTOR switch to the DC VOLTS / 1000 position.

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

(    )         Set the ZERO adjustment control to center of rotation.

(    )         Set the BIAS ADJ control (on circuit board) maximum counter-clockwise.

(    )         Set the DC CAL control (on circuit board) maximum counter-clockwise.

(    )         Set the AC CAL control (on circuit board) maximum counter-clockwise.

ELECTRICAL ZERO ADJUSTMENT

(    )         Set the POWER switch to the ON position.

(    )         Adjust the BIAS ADJ control for a zero meter indication.

(    )         Set the ZERO control fully clockwise.  This should result in an upscale meter indication.

(    )*       Adjust ZERO control for a zero meter indication.

(    )*       Alternate the POLARITY switch between the DC+ and DC- positions.

(    )         Repeat the above two steps* until no change in the meter indications.

(    )         Set the POLARITY switch to the DC- position.

(    )*       Adjust the ZERO control maximum counter-clockwise.  This should cause the meter pointer to move upscale.

(    )         Adjust the ZERO control for a zero meter indication.

DC CALIBRATION

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

(    )         Set the SELECTOR switch to the DC VOLTS / 10 position.

(    )*       Touch the tip of the DC probe (silver shielded lead with black probe) to the positive terminal (+) of the 1.5V C cell battery.

(    )*       Adjust the DC CAL control (on the circuit board) for a 1.5 V meter indication on the black scale, as shown in Figure 3.

(    )*       Remove the probe tip from the battery.

(    )*       Adjust the ZERO control for a zero meter indication.

(    )         Repeat the above four steps* until no change in the meter indications.

· If more accurate DC voltages are available, they may be used to further check the DC calibration.

AC CALIBRATION

· 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 voltmeter.

· 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 has 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 line.

(    )      Set the SELECTOR switch to the AC VOLTS / 1000 position.

(    )      Connect the AC/Ohms test leads (black and red leads) across the AC line.

· You can do this most conveniently if you first attach the alligator clip on the black lead (meter common) to the metal shank of a screwdriver.  Then insert the blade of the screwdriver into one of the slotted openings in the AC wall outlet.   If doing this using a polarized outlet, insert the screwdriver blade into the larger slot.  DO NOT TOUCH THE METAL SHANK OF THE SCREWDRIVER.

· TAKE CARE NOT TO TOUCH THE TIP OF THE RED PROBE TO THE SCREWDRIVER.

(    )         Insert the red probe into the other outlet slot.

(    )         Adjust the AC CAL control (on the circuit board) for a zero indication of 120 volts, or to whatever known AC voltage you are calibrating with.

(    )         Disconnect both test leads from the AC line.

OHMS ADJUSTMENT

(    )         Set the SELECTOR switch to Rx1M

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

(    )*       Touch the tip of the AC/Ohms test lead (the red probe) and the alligator clip together; this should cause a zero meter indication.

(    )*       Adjust the ZERO control (with the leads touching) to zero the meter.

(    )*       Separate the two leads, the meter should return to full scale (INF) reading.

(    )         Repeat the above four steps* until no change in the zero indications.

(    )         Set the SELECTOR switch to each of the other three resistance ranges.  Each range should give a zero indication with the AC/Ohms probe and the alligator clip touching; and an INF reading with the test leads apart.

(    )         Set the RANGE switch to the Rx1 position.

(    )         With the leads touching, observe that the meter will show a fraction of an ohm circuit resistance.

FINAL BIAS ADJUSTMENT

(    )         Set the SELECTOR switch to one of the DC VOLTS positions.

(    )         Set the ZERO control fully clockwise.

(    )         Adjust the BIAS ADJ control for a reading of 1 on the black 0–10 scale.

(    )         Adjust the ZERO control for a zero meter indication.

· This setting of the BIAS ADJ control should allow for future adjustments to zero out as the battery voltage decreases over a period of time.  When this zero cannot be reached by adjusting the ZERO control, the BIAS ADJ may be reset without disturbing the calibration.

(    )         Set the POWER switch to the OFF position.

(    )         Calibration procedure is complete.