Heathkit Model IM-48

Audio Analyzer


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 VTVM

Audio Generator

Frequency Counter

1 – 4.7 kW 1% resistor

2 – 10 kW 1% resistors

 

* * * * * * *

INITIAL SETUP

Refer to Pictorial 2

Construction Manual page 23

 

(    )         Connect the line cord to the AC power lines.

 

(    )         Rotate the I. M. ANALYZER INPUT / POWER SWITCH knob clockwise, until the switch just clicks ON and the pilot lamp lights. (Allow the unit to warm up for 30 minutes)

 

(    )         Adjust the VTVM CAL control Q [chassis – right front] to maximum clockwise.

 

(    )         Adjust the VTVM LIN control R [chassis – right rear] to maximum counter-clockwise.

 

(    )         Set the LOAD SELECTOR switch [front panel] to the HI-Z position.

 

(    )         Set the FUNCTION switch [front panel] to the VTVM position.

 

·         The VTVM calibration can be performed one of two ways as described below.

 

* * * * * * *

VTVM CALIBRATION

Using the AC line voltage

Refer to Pictorial 1

Construction Manual page 10

 

(    )         Set the RANGE switch [front panel] to the 300 volts position.

 

(    )         Connect a test lead from the red IN binding post to terminal strip X3 [chassis – underside rear center]. CAUTION: The AC supply line is dangerous.

 

(    )         Adjust the CALIBRATE control Q until the meter indicates the AC line voltage on the 300 volt scale.

 

(    )         Disconnect the jumper wire from both terminals.

 

* * * * * * *

VTVM CALIBRATION

Using a known AC voltage source

 

(    )         Set the RANGE switch [front panel] to match the AC calibration voltage.

 

·         Both the range scale and the calibration voltage should be as close as possible to a full scale meter indication.

 

(    )         Connect the AC calibration voltage source to the IN binding posts.

 

(    )         Adjust the CALIBRATE control Q until the meter indicates the AC voltage on the selected voltage scale.

 

(    )         Disconnect the AC voltage from the IN binding posts.

 

* * * * * * *

METER LINEARITY ADJUSTMENT

 

(    )         Set the FUNCTION switch to the LF HF TEST position.

 

(    )         Set the TEST switch [front panel] to the LF TEST position.

 

·         Repeat the linearity adjustment section twice, making adjustments only as necessary the second time.

 

(    )  (    )    Set the RANGE switch to either the 3 or 10 volt position.

 

(    )  (    )    Adjust the LF HF GENERATOR OUTPUT control [front panel] for an indication near or at full scale.

 

(    )  (    )    Record the meter reading on the correct scale, and do not change the LF HF GENERATOR OUTPUT control setting until adjustments are completed.

 

(    )  (    )    Set the RANGE switch one position clockwise.

 

(    )  (    )    Record the meter trading on the correct scale, and do not change the LF HF GENERATOR OUTPUT control setting until adjustments are completed.

 

·         The two meter readings should be exactly the same.

 

(    )  (    )    If the two readings are the same, skip the next 3 steps.

 

(    )  (    )    Set the RANGE switch to the original 3 or 10 volt position.

                  (Use the switch position that gives an indication at or near full scale.)

 

(    )  (    )    Adjust the VTVM LIN control R until the meter pointer moves downscale slightly.

 

(    )  (    )    Adjust the CALIBRATE control Q until the meter indicates the original reading.

 

* * * * * * *

WATTMETER CALIBRATION

 

·         No Wattmeter calibration is necessary since the AC VTVM is automatically corrected for power readings by the use of precision resistors.

 

* * * * * * *

MINIMUM HUM ADJUSTMENT

Refer to Pictorial 2

Construction Manual page 23

 

·         It is unlikely that a complete null will be obtainable. However, the small amount of hum pickup remaining after the chokes are tuned will not upset the accuracy of the I. M. Analyzer.

 

(    )         Set the LOAD switch to the HI-Z position.

 

(    )         Set the FUNCTION switch to the VTVM position.

 

(    )         Set the RANGE switch to the .3 volt position.

 

(    )         Remove the 12AX7 tube from socket C [chassis].

 

(    )         Adjust the HIGH FREQUENCY LEVEL control [front panel] to maximum counter-clockwise.

 

(    )         Adjust the LF HF GENERATOR OUTPUT control to maximum counter-clockwise.

 

* * * * * * *

Refer to Pictorial 1

Construction Manual page 10

 

(    )         Connect a test lead from the red IN binding post to terminal strip M1 [chassis – underside right center].

 

(    )         Set the RANGE switch until a satisfactory meter indication is obtained.

 

(    )         Rotate choke N [chassis – underside right front] until a minimum meter indication is obtained.

 

(    )         Disconnect the test lead from terminal M1.

 

(    )         Connect the test lead from the red IN binding post to terminal strip H1 [chassis – underside center, next to control Z].

 

(    )         Connect a test lead from the black IN binding post to terminal strip H3.

 

(    )         Set the RANGE switch until a satisfactory meter indication is obtained.

 

(    )         Rotate choke P [chassis – underside right center] until a minimum meter indication is obtained.

 

(    )         Disconnect both test leads from terminal H.

 

(    )         Disconnect both test leads from the IN binding posts.

 

(    )         Insert the 12AX7 tube back into socket C and allow it to warm up.

 

* * * * * * *

SIGNAL MIXER SETUP

Refer to Figure 15

Construction Manual page 25

 

(    )         Construct a resistor divider network as shown in Figure 15.

 

(    )         Connect the free end of the 4.7 kW resistor network to ground.

 

(    )         Connect the free end of one of the 10 kW resistors to the red OUT binding post of the Audio Analyzer.

 

(    )         Connect the free end of the other 10 kW resistor to the OUTPUT binding post of the Audio Generator.

 

·         The resistor network can be assembled in a small mini-box. Binding posts are used for the three instrument connections. A BNC jack is wired across the audio input for a frequency counter. A set of three DPDT toggle switches are used to replace the dash line wiring. A separate switch is wired for each of the numbered setups.

 

* * * * * * *

I. M. ANALYZER CALIBRATION

Refer to Figure 15

Construction Manual page 25


(    )         Set the Audio Generator to sine wave output.

 

(    )         Set the Audio Generator MULTIPLIER switch to the X1000 position.

 

(    )         Adjust the Audio Generator output level to 10 volts using an AC VTVM.

 

(    )         Set the TEST switch to the HF TEST position.

 

(    )         Set the FUNCTION switch to the VTVM position.

 

(    )         Set the LOAD switch to the HI-Z position.

 

(    )         Adjust the I. M. ANALYZER INPUT control to minimum counter-clockwise.

               (DO NOT turn the unit OFF.)

 

(    )         Adjust the GENERATOR OUTPUT control to maximum counter-clockwise.

 

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

 

(    )         Adjust the HIGH FREQUENCY LEVEL control to maximum clockwise.

 

(   )          Adjust the LF HF GENERATOR OUTPUT control to maximum clockwise.

 

* * * * * * *

ZERO BEAT ADJUSTMENT

[Circuit # 3]

 

(    )3        Connect a test lead from the red IN binding post to the central solder junction of the three resistors.

 

(    )         Connect a Frequency Counter across the Audio Generator’s output terminals.

 

(    )         Adjust the Audio Generator for a zero beat meter indication at 6.0 kHz.

 

·         Adjusting the Audio Generator through the zero beat frequency will cause the meter indicator to oscillate with the beat.

 

(    )3        Disconnect the test lead at the central solder junction of the three resistors.

 

* * * * * * *

EQUALIZATION ADJUSTMENT

[Circuit # 1]

 

(    )         After the zero beat has been obtained, adjust the Audio Generator to either 60 – 100 Hz higher or lower than the zero beat frequency.

 

(    )         Set the RANGE switch to the 30 volts position.

 

(    )1        Connect a shorting jumper across the 4.7 kW resistor from the central solder junction of the three resistors to ground.

 

(    )1        Connect the test lead from the red IN binding post to the red OUT binding post.

 

(    )*       Adjust the HIGH FREQUENCT LEVEL control for a suitable meter indication.

 

(    )*       Adjust the GENERATOR OUTPUT control for a suitable meter indication.

 

(    )         Repeat the above two steps * for the maximum meter indication on the 30 volt range.

 

(    )         Make a note of the maximum meter indication.

 

(    )1        Disconnect the test lead, coming from the IN binding post, to the OUT binding post.

 

(    )         Disconnect the Frequency Counter from the Audio Generator’s output terminals.

 

* * * * * * *

ATTENUATOR ADJUSTMENT

 [Circuit # 2]

 

(    )2        Connect the test lead from the red IN binding post to the output terminal of the Audio Generator.

 

(    )2        Connect a shorting jumper across the 4.7 kW resistor from the central solder junction of the three resistors to ground. (A shorting jumper should still be connected after using Circuit 1 above.)

 

(    )         Set the RANGE switch to the 3 volts position.

 

(    )         Adjust the output level of the Audio Generator to produce the same meter indication as noted above.

 

·         A voltage ratio of 10:1 is now established.

 

(    )2        Disconnect the shorting jumper across the 4.7 kW resistor.

 

(    )2        Disconnect the test lead, coming from the IN binding post, to the output terminal of the Audio Generator.

 

* * * * * * *

10% IM ADJUSTMENT

 [Circuit # 3]

 

(    )3        Connect the test lead from the IN binding post to the central solder junction of the three resistors.

 

(    )         Set the RANGE switch to the 10% IM position.

 

(    )         Set the FUNCTION switch to the % IM position.

 

(    )         Adjust the I. M. ANALYZER INPUT control for a full scale meter indication.

 

(    )         Set the FUNCTION switch to the SET LEVEL position.

 

(    )         Set the RANGE switch to the 3% IM position.

 

(    )         Adjust the IM CALIBRATE control Z [chassis – center] for a full scale meter indication.

 

(    )         Adjust the I. M. ANALYZER INPUT control to the maximum counter-clockwise OFF position,

 

(    )         Disconnect the resistor network from both instruments.

 

(    )         Disconnect all test leads from the front panel terminals.

 

(    )         Calibration procedure is complete.