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.