Heathkit Model IM-58

Harmonic Distortion Meter

Calibration Procedure

· With the power supply 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.

· Disconnect all external cables, components, and wires from the input and output terminals.

Ancillary Equipment Needed

AC Voltage Source

AC VTVM

Audio Generator

Frequency Counter

* * * * * * *

INITIAL SETUP

Refer to Pictorial 2

Construction Manual page 21

(    )   Disconnect the unit’s line cord from the AC power lines.

(    )   Set the CALIBRATE control H [chassis – nearest the meter] to the center of rotation.

(    )   Set the COARSE BALANCE control G [chassis – by the power switch] to the center of rotation.

(    )   Set the HUM BALANCE control F [chassis – rear center] to full counterclockwise.

(    )   Set the RANGE switch [front panel] to the SET LEVEL position.

(    )   Set the SENSITIVITY switch [front panel] to the 100% position

(    )   Connect the unit’s line cord to the AC power line.

(    )   Set the POWER switch [front panel] to the ON position

         (Allow the unit to warm up for 30 minutes)

* * * * * * *

VOLTMETER CALIBRATION

Using AC Filament Voltage

(    )   Set the SENSITIVITY switch to the 30V position

(    )   Connect a test lead to the red INPUT binding post.

(    )   Touch the test lead to each one of the pilot light terminals separately.

(    )   Connect the test lead to the terminal that gives the highest meter indication.

(    )   Set the SENSITIVITY switch to the 10V position.

(    )   Adjust the CALIBRATE control H until the meter reads 6.3 volts.

(    )   Disconnect the test lead from the INPUT binding post and the pilot lamp terminal.

* * * * * * *

VOLTMETER CALIBRATION

Using a Known AC Source Voltage

(    )   Select an AC source voltage close to a full scale meter indication.

(    )   Verify the source voltage using an AC VTVM.

(    )   Set the SENSITIVITY switch scale position to match the source voltage.

(    )   Connect the source voltage to the INPUT binding posts.

(    )   Adjust the CALIBRATE control H until the meter indication matches the source voltage.

(    )   Disconnect the source voltage from the INPUT binding post.

* * * * * * *

BALANCE ADJUSTMENT

Using an Audio Generator

(    )   Set the Audio Generator’s output to 2.0 kHz using a frequency counter.

(    )   Set the Audio Generator’s output to 0.5 volts using an AC VTVM.

· Repeat the whole balance adjustment section twice, making adjustments only if necessary the second time.

(    )  (    )    Set the SENSITIVITY switch to the 100% position

(    )  (    )    Set the RANGE switch to either one of the SET LEVEL positions.

(    )  (    )    Connect the Audio Generator’s output to the Distortion Meter’s input terminals.

(    )  (    )    Advance the LEVEL control [front panel] to give a full scale meter indication.

(    )  (    )    Set the RANGE switch to the 2000–20,000 position.

(    )  (    )    Set the BALANCE control [front panel] to the center of its rotation.

(    )* (    )*  Adjust the TUNING control [front panel] for a minimum meter indication.

(    )* (    )*  Adjust the COARSE BALANCE control G for minimum meter indication.

(    )  (    )    Repeat the above two steps * until no further improvement is obtained.

· It may be necessary to turn the SENSITIVITY switch to a lower percentage value to show the minimum more clearly.

(    )  (    )    Set the RANGE switch to the 200–2000 position.

(    )  (    )    Set the SENSITIVITY switch to the 100% position

(    )* (    )*  Adjust the TUNING control for a minimum meter indication.

(    )* (    )*  Adjust the Tuning trimmer capacitor for a minimum meter indication.

                  (Through the top hole in the rear chassis shield enclosure [see Figure 15, page 16])

(    )  (    )    Repeat the above two steps * until no further improvement is obtained.

· It may be necessary to turn the SENSITIVITY switch to a lower percentage value to show the minimum more clearly.

(    )  (    )    Disconnect the Audio Generator from the INPUT binding posts.

(    )  (    )    Carefully loosen the setscrew of the insulating coupling [see Figure 10, page 13].

(    )  (    )    Adjust the knob pointer to indicate the correct input frequency.

                  (Make sure the insulated coupling does not turn)

(    )  (    )    Carefully retighten the setscrew before continuing.

* * * * * * *

HUM BALANCE ADJUSTMENT

(    )   Disconnect all external cables, components, and wires from the input and output terminals.

(    )   Disconnect the line cord from the AC power line.

(    )   Install the instrument into its case.

(    )   Re-connect the line cord to the AC power line.

         (POWER switch should still be in the ON position)

(    )   Set the RANGE switch to the 20–200 position.

(    )   Set the SENSITIVITY switch to the 1% position.

(    )   Set the TUNING control to show about 60 Hz on the frequency scale.

(    )   Adjust the HUM BALANCE control F to give a minimum indication on the meter.

· The minimum reading should fall around 1/10 of full scale and usually depends upon the 5879 tube’s heater to cathode leakage. Readings above 1/10 full scale may drop below that value after a few hours of continuous operation.

 (    )  Set the POWER switch to the OFF position.

 (    )  Disconnect the line cord from the AC power line.

 (    )  Calibration procedure is complete.