Heathkit Model IM-1210
Digital Multimeter
[3-digit LED display]
Calibration Procedure
· Disconnect all external cables, components, and wires from the output terminals.
· All direction adjustments are made facing the front of each potentiometer.
· The built-in calibration standards provide accuracy within the specifications for this meter. However, to improve the accuracy, this meter can be calibrated with precision AC and DC voltages and resistance standards.
Ancillary Equipment Needed
Frequency Counter
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( ) Disconnect the line cord from the AC power lines.
( ) Set the POWER switch to the OFF position.
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CIRCUIT BOARD INITIAL ADJUSTMENTS
Refer to Figure 2-1
Illustration Booklet page 4
( ) Adjust the CLOCK ADJ control to maximum counter-clockwise.
( ) Adjust the ZERO ADJ control to maximum counter-clockwise.
( ) Adjust the DC CAL control to maximum counter-clockwise.
( ) Adjust the 2M CAL control to the center of rotation.
( ) Adjust the 200K CAL control to the center of rotation.
( ) Adjust the 20K CAL control to the center of rotation.
( ) Adjust the 2K CAL control to the center of rotation.
( ) Adjust the 200 CAL control to the center of rotation.
( ) Adjust the AC CAL control to the center of rotation.
( ) Adjust the AC REF ADJ control to the center of rotation.
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FRONT PANEL INITIAL ADJUSTMENTS
Refer to Figure 3-1
Illustration Booklet page 5
( ) Connect the line cord to the AC power lines.
(Allow the unit to warm up for 30 minutes)
( ) Set the FUNCTION switch [SW1] to the DCV position.
( ) Set the RANGE switch [SW2] to the 2 position.
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Refer to Figure 2-1
Illustration Booklet page 5
· The pink shaded 8’s represent unlighted 7-segment displays.
( ) The display should show -.88.
( ) Adjust the ZERO ADJ control for a display of 8.00.
(It may be necessary to adjust the DC CAL control clockwise slightly.)
( ) Connect a test lead to the positive (red) input jack.
( ) Connect the same test lead to the negative (black) input jack.
( ) Adjust the ZERO ADJ control for a display of -.88.
( ) Adjust the ZERO ADJ control slowly until the display just changes to 8.00.
( ) Disconnect the test lead from the negative (black) input jack.
( ) Connect the same test lead to the DC CAL TP (test point) [front lead of R91, circuit board – left rear].
( ) Adjust the DC CAL control clockwise slowly.
· The count should increase, reach a maximum, and then decrease again.
· Note the maximum display reading.
( ) Adjust the DC CAL control to maximum counter-clockwise.
( ) Adjust the DC CAL control clockwise slowly until the display is 20 counts less than the noted maximum.
( ) Disconnect the test lead from the test point.
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CLOCK ADJUSTMENT
(Using a Frequency Counter)
( ) Set the FUNCTION switch to the OHMS position.
( ) Connect the Frequency Counter’s common input to the negative (black) input jack.
( ) Connect a test lead to the Frequency Counter’s “hot” input.
( ) Connect the same test lead to either jumper C [see Pictorial 1-11, page 18] or to pin 14 of IC3 (SN7490) [see Pictorial 2-3, page 21].
( ) Adjust the CLOCK ADJ control for a Frequency Counter reading of 60 Hz.
( ) Disconnect both test leads from the meter.
( ) Proceed to the DC VOLTS CALIBRATION section.
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CLOCK ADJUSTMENT
(Without a Frequency Counter)
( ) Set the FUNCTION switch to the OHMS position.
( ) Adjust the CLOCK ADJ control for an “infinity” display of between ‘8.85 and ‘8.95.
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DC VOLTS CALIBRATION
Refer to Figure 2-1
Illustration Booklet page 5
( ) Reference the calibration voltage recorded on the transformer’s insulator.
( ) Observe that the calibration voltage number has four digits, whereas the meter can display only three digits.
( ) Round the third digit up by one if the fourth digit is greater than 4.
· If the number is greater than 1.99, the first digit will not be displayed and the OVER lamp shall be lit.
( ) Set the FUNCTION switch to the DCV position.
( ) Connect a test lead to the positive (red) input jack.
( ) ( )1 Connect the same test lead to the negative (black) input jack.
( ) ( )2 Adjust the ZERO ADJ control slowly until the display just changes to 8.00.
( ) ( )3 Disconnect the test lead from the negative (black) input jack.
( ) ( )4 Connect the same test lead to the DC CAL TP (test point) [front lead of R91].
( ) ( )5 Adjust the DC CAL control to maximum counter-clockwise.
( ) ( )6 Adjust the DC CAL control clockwise until the reading matches the rounded calibration reference voltage.
( ) ( )7 Disconnect the test lead from the test point.
( ) Repeat the above 7 marked steps.
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AC LINE VOLTAGE COMPENSATION
Refer to Figure 2-1
Illustration Booklet page 5
♦ The meter must be calibrated at the same AC line frequency where it will be used. If it is not, the meter shall be inaccurate on the higher portions of its range.
♦ This procedure is a temporary compensation for any line voltage variation during the calibration process.
( ) Set the RANGE switch to the 20 position.
( ) Connect the same test lead to the +V TP (test point) [bracket of Q15].
· If the reading is between 7.8 and 8.6, no adjustment to the DC CAL voltage is required at this time.
· If the reading is less than 7.8 subtract .003 from the DC CAL voltage on the calibration label.
· If the reading is greater than 8.6 add .003 to the DC CAL voltage on the calibration label.
( )# Round the adjusted calibration voltage as described at the beginning of the DC VOLTS CALIBRATION section above.
( ) Temporarily save this adjusted DC CAL voltage for later use.
( ) Disconnect the test lead from both the test point and the positive (red) input jack.
( )* Perform the CLOCK ADJUSTMENT procedure as detailed in those sections above.
( ) Set the RANGE switch to the 2 position.
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TRANSISTOR SELECTION
♦ This procedure only needs to be performed at the initial calibration of the unit, or if either transistor Q1 or Q4 has been replaced. Otherwise, skip this section.
· Two transistor sockets should be installed to avoid potential circuit board damage.
( ) Disconnect all test leads from the input jacks.
( ) Set the FUNCTION switch to the DCV position.
( ) Adjust the ZERO ADJ control for a display of between -.88 and 8.00, but showing 8.00 most of the time.
( ) Disconnect the line chord from the AC power lines.
( ) Interchange the two transistors at Q1 and Q4 [see Pictorial 2-2, page 20].
( ) Connect the line cord to the AC power lines.
( ) If the display shows 8.00, go to the DC VOLTS CALIBRATION continued section.
( ) If the display shows -.88, perform the following 4 marked steps.
( )1 Disconnect the line chord from the AC power lines.
( )2 Interchange the two transistors at Q1 and Q4.
( )3 Connect the line cord to the AC power lines.
( )4 Adjust the ZERO ADJ control for a display of between -.88 and 8.00, but showing 8.00 most of the time.
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DC VOLTS CALIBRATION continuation
( ) Connect a test lead to the positive (red) input jack.
( ) Connect the same test lead to the negative (black) input jack.
( ) Adjust the ZERO ADJ control for a display of between 8.01 and 8.00, and then stable at 8.00.
( ) Disconnect the test lead from both input jacks.
( )* Perform the CLOCK ADJUSTMENT procedure as detailed in those sections above.
( ) Set the FUNCTION switch to the DCV position.
( ) Connect a test lead to the positive (red) input jack.
( ) Connect the same test lead to the DC CAL TP (test point) [front lead of R91].
( ) Adjust the DC CAL control to maximum counter-clockwise.
( ) Adjust the DC CAL control clockwise until the display matches the corrected reference calibration voltage.
( ) Disconnect the test lead from the test point.
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AC VOLTS CALIBRATION
Refer to Figure 2-1
Illustration Booklet page 5
( ) Set the RANGE switch to the 2 position.
( )1 Set the FUNCTION switch to the DCV position.
( )2 Connect the same test lead to the AC REF TP (test point) [circuit board – left rear].
( )3 Adjust the AC REF ADJ control for a display of 1.65 [circuit board – left rear].
(The display may vary of between 1.64 and 1.66 – a proper adjustment will be stable at 1.65.)
( )4 Disconnect the test lead from the test point.
( )5 Set the FUNCTION switch to the ACV position.
( )6 Connect the same test lead to AC REF TP (test point) again and allow the display to stabilize.
· DO NOT adjust the AC REF ADJ control [circuit board – left rear]!
( )7 Adjust the AC CAL control for a display of ‘8.00 [circuit board – right rear].
(The display may vary of between 1.99 and ‘8.01, – a proper adjustment will be stable at ‘8.00.)
( )8 Disconnect the test lead from the test point.
( ) Repeat the above 8 marked steps until the correct displays are obtained in both switch positions.
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OHMS CALIBRATION
Refer to Figure 2-1
Illustration Booklet page 5
( ) Set the FUNCTION switch to the OHMS position.
( )1 Set the RANGE switch to the 200 W position.
( ) Connect the same test lead to the 200 W resistor.
( ) Adjust the 200 ADJ control for a display of ‘800.
( ) Disconnect the test lead from the test point.
( )2 Set the RANGE switch to the 2 kW position.
( ) Connect the same test lead to the 2 kW resistor.
( ) Adjust the 2000 ADJ control for a display of ‘8.00.
( ) Disconnect the test lead from the test point.
( )3 Set the RANGE switch to the 20 kW position.
( ) Connect the same test lead to the 20 kW resistor.
( ) Adjust the 20K ADJ control for a display of ‘80.0.
( ) Disconnect the test lead from the test point.
( )4 Set the RANGE switch to the 200 kW position.
( ) Connect the same test lead to the 200 kW resistor.
( ) Adjust the 200K ADJ control for a display of ‘800.
( ) Disconnect the test lead from the test point.
( )5 Set the RANGE switch to the 2 MW position.
( ) Connect the same test lead to the 2 MW resistor.
( ) Adjust the 2M ADJ control for a display of ‘8.00.
( ) Disconnect the test lead from the test point.
( ) Repeat the five marked ohms adjustment subsections above until the proper readings are obtained.
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( ) Disconnect all test leads from the unit.
( ) Set the FUNCTION switch to the OFF position.
( ) Disconnect the line cord from the AC power lines.
( ) Calibration procedure is complete.