Heathkit IT-11 and IT-28 calibration procedure

The following procedure applies for both the Heathkit IT-11 and the IT-28 CAUTION: The Heath IT-11 and IT-28 cap testers and all similar cap testers have HIGH VOLTAGE on the chassis and at the test terminals. If you are not familiar with the equipment or the dangers, stop now and find a local mentor to help you. Until you are familiar with the hazards, I strongly suggest using a meter when testing for electrolytic leakage at high voltage. (Note that the load of the meter itself can be mistaken for a bit of leakage) For safety, always start with the right lever set to "DISCHARGE" before powering the instrument.

For leakage calibration, Heath specifies two procedures, the first without and the second with a milliammeter. According to the manual, the first is quite satisfactory, however, the second procedure will provide greater accuracy. Heath provided 2 extra resistors for calibration without a milliammeter. Those are 100 K ohm and 1.5 megohm.Preset the BRIDGE-LEAKAGE switch lever to DISCHARGE, the type lever (on the left side) to ELECTROLYTIC and voltage to "3" before proceeding with calibration.

The following is quoted from the manual:

Calibration with no milliammeter
1. Turn the instrument on, and allow it to warm up.

2. Connect a 100K ohm (brown-black-yellow) resistor across the TEST terminals.

3. Set the controls as follows:


VOLTAGE -- 300 volts
BRIDGE-LEAKAGE -- LEAKAGE

4. Adjust the eye tube to "just closed" with the top (AD) calibrate control

5. Move the BRIDGE-LEAKAGE switch to DISCHARGE and remove the 100 K ohm resistor.

6. Connect a 1.5 megohm (brown-green-green) resistor across the TEST terminals.

7. Set the controls as follows:

VOLTAGE -- 25 volts
Type -- MIN.'LYTIC
BRIDGE-LEAKAGE -- LEAKAGE

8. Adjust the eye tube to the "just closed" position with the bottom (AB) calibrate control.

9. Set the controls as follows:

VOLTAGE -- 3 volts
Type -- PAPER
BRIDGE-LEAKAGE -- LEAKAGE

10. Adjust the eye tube to "just closed" with the center (AC) calibrate control

11. Return the BRIDGE-LEAKAGE switch to DISCHARGE and remove the 1.5 megohm resistor.

Calibration with a milliammeter

1. Connect the meter in series with a 2 megohm control (not supplied) across the TEST terminals

2. Turn the instrument on, and allow it to warm up.

3. Set the controls as follows:

VOLTAGE -- 50 volts
Type -- ELECTROLYTIC
BRIDGE-LEAKAGE -- LEAKAGE

4. Adjust the 2 megohm control until the meter reads 2 mA.(milliamps)

5. Adjust the eye tube to "just closed" with the top (AD) calibrate control

6. Set the controls as follows:

Type -- MIN.'LYTIC
BRIDGE-LEAKAGE -- LEAKAGE

7. Reduce the VOLTAGE switch setting and rotate the 2 megohm control until the meter reads 15uA (microamps)

8. Adjust the eye tube to the "just closed" position with the bottom (AB) calibrate control.

9. Set the controls as follows:

Type -- PAPER
BRIDGE-LEAKAGE -- LEAKAGE

10. If necessary, reduce the VOLTAGE switch setting, and increase the 2 megohm control until the meter reads 2 uA (microamps)

11. Set the eye tube to "just closed" position with the center (AC) calibrate control

Notes on Heath's calibration procedure without a milliammeter. The use of a 100 K ohm resistor with the 300 volt setting actually results in a leakage calibration of 3 milliamps, not 2 milliamps. If I were to use that procedure and the 100 K ohm resistor, I would use the 200 volt setting. A 100 K ohm resistor at 200 volts will draw 2 mA.

Following the "mini.'lytic" procedure as described using the 1.5 megohm resistor sets the instrument at 16.7 uA, close enough to the 15 uA that Heath intended.

The same 1.5 megohm resistor at a 3 volt setting would indeed draw 2 uA as Heath intended. Note that all of the settings depend upon the accuracy of the voltages and resistors. With a good digital meter, those are easy to check.

Click to go back Part Two "Capacitor analyzers without meters" which includes the Heathkit IT-11 and IT-28