Heathkit Model AW-1

Audio Wattmeter

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

VTVM

Calibration Cable (see notes)

Audio Generator (optional)

Audio VTVM (optional)

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NOTES

· If the instrument has been rewired with 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 two wire extension cord by removing the outlet socket and soldering a pair of banana plugs to the ends of the wires.

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

· The LOAD switch should never be rotated while any voltage is applied to the input terminals.

· The internal load resistors are rated at 24 watts continuous dissipation.

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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.

· 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.

(    )         Set the POWER switch to the OFF position.

(    )         Set the RANGE switch to the 50W position.

(    )         Set the LOAD switch to the 600 ohms INT. LOAD position.

· Verify that the internal 600 ohm load is across the meter terminals with an ohmmeter.

· Using any INT. LOAD setting other than 600 ohms when calibrating using the AC line voltage or any other high voltage source will result in damage to the instrument.

Calibrating with a Calibration Cable

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

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

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

· Use an insulated screwdriver when making this adjustment.

(    )         Adjust the input calibration control D (see Figure 2) until the meter pointer is over the “a” in “cal” on the meter face.

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

(    )         Set the POWER switch to the OFF position.

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

(    )         Set the LOAD switch to the OFF position.

(    )         Calibration procedure is completed.

Calibrating using Test Probes

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

(    )         Set the POWER switch to the ON 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 EXPOSED METAL SHANKS OF THE SCREWDRIVERS.

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

· Use an insulated screwdriver when making this adjustment.

(    )         Adjust the input calibration control D (see Figure 2) until the meter pointer is over the “a” in “cal” on the meter face.

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

(    )         Set the POWER switch to the OFF position.

(    )         Disconnect the test probes from the input terminals.

(    )         Set the LOAD switch to the OFF position.

(    )         Calibration procedure is completed.

CALIBRATION USING AN AUDIO GENERATOR

(    )         Set the POWER switch to the OFF position.

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

(    )         Set the RANGE switch to the 50mW position.

(    )         Set the LOAD switch to the 600 ohms INT. LOAD position.

· Verify that the internal 600 ohm load is across the meter terminals with an ohmmeter.

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

(    )         Set the audio generator’s frequency to 1000 Hz.

(    )         Set the audio generator output voltage to 5.47 VAC using an Audio VTVM.

(    )         Connect the test leads to the audio generator output.

· Use an insulated screwdriver when making this adjustment.

(    )         Adjust the calibration control D (see Figure 2) until the meter reads 50 mW.

(    )         Disconnect the test leads from the generator output.

(    )         Set the POWER switch to the OFF position.

(    )         Set the RANGE switch to the 50W position.

(    )         Disconnect the test leads from the input terminals.

(    )         Calibration procedure is completed.