Hypatia Model 306 Calibration Procedure Version 1.4

13 August 1991 © 1989-1991 Hypatia Inc.

Warning: Line voltages of as much as 132 Volts rms are present on exposed terminals within this instrument. Take great care to avoid contact with these potentially lethal points throughout this procedure. If you are unfamiliar with the precautions and techniques necessary in the course of servicing instruments with exposed line voltages, do not attempt to calibrate the instrument yourself. Instead, refer calibration to appropriately experienced technicians.

Warning: This instrument produces exposed high currents as a normal part of its function. High current shorts through rings, wrist bands or other conductive apparel can cause serious thermal burns which may result in permanent injury. Operators are advised to remove all conductive apparel, especially from the hands and wrists, prior to any operation or service of this instrument.

Required Instruments: One dc voltmeter with at least .5% accuracy and at least 200 µV resolution.

Common Reference: TP6 is the common reference for all the circuitry on the circuit board. The dc voltmeter's negative test lead should be connected to this test point for steps seven through sixteen of this procedure.

Test Points: There are six test points on the main circuit board. The test point nearest the front of the circuit board is TP1, the next test point toward the back of the circuit board TP2, and so on to TP6, the one nearest the rear of the circuit board.

Variable Resistors: There are five multiturn variable resistors on the main circuit board. The one nearest the front of the circuit board is VR1, the next toward the back of the circuit board VR2, and so on to VR5, the one nearest the rear of the circuit board. The last variable resistor, VR6, is located behind the red digital panel meter cover.

Shorting Bars: There are three square pin shorting bars located on the top (foil) side of the main circuit board. The location nearest the test points is SB1, the center location is SB2, and the location farthest from the test points is SB3.

Step by Step Procedure:

1. Set the Model 306 "current adjust" knob to its full counter clockwise position. Set the "power" switch to the off position and remove the power cord from the instrument.

2. Remove the six screws from each side of the instrument and remove the top cabinet half. Note that there is a lockwasher between the top and bottom cabinet half associated with both lower rear screws.

3. Secure the Model 306's test probe cables to the front panel connectors of the instrument, but leave the probe's measurement ends unconnected to each other or to any other conductor.

4. Connect an IEC power cord to the instrument and to a power source meeting the specifications printed above the IEC power connector.

5. Set the Model 306 "power" switch to the on position and allow fifteen minutes for thermal equilibrium to occur.

6. Being careful not to damage the black plastic housing, pry the red filter cover away from the front of the digital panel meter using a small flat blade screwdriver in the notch in the lower center or bottom center of the meter housing. A multiturn variable resistor, VR6, is located near the lower right corner of the front of the panel meter.

7. Connect the positive voltmeter lead to TP1 and adjust VR6 for a reading of 1.772 V ± 2.0 mV on the voltmeter. Replace the cover of the digital panel meter.

8. Remove the square pin shorting bar from location SB1 and set it aside. Move the shorting bar at location SB2 from the front and center of the three pins to the center and rear of these pins.

9. Connect the positive voltmeter lead to TP2 and adjust VR2 for a voltmeter reading of 0.0 mV.

10. Connect the positive voltmeter lead to TP3 and adjust VR3 for a voltmeter reading of 0.0 mV.

11. VR2 and VR3 are somewhat interactive. Repeat steps 9 and 10 in the same sequence until the voltmeter readings on TP2 and TP3 are 0.0 mV ± 1.0 mV and 0.0 mV ± 200 µV respectively without further adjustment.

12. Replace the shorting bars for locations SB1 and SB2 onto their original positions.

13. Remove the square pin shorting bar from location SB3 and set it aside. (It can be most easily accessed from the opposite side of the instrument.)

14. Connect the positive voltmeter lead to TP4 and adjust VR4 for a voltmeter reading of 0.0 mV.

15. Connect the positive voltmeter lead to TP5 and adjust VR5 for a voltmeter reading of 0.0 mV.

16. VR4 and VR5 are somewhat interactive. Repeat steps 14 and 15 in the same sequence until the voltmeter readings on TP4 and TP5 are 0.0 mV ± 1.0 mV and 0.0 mV ± 200 µV respectively without further adjustment.

17. Replace the shorting bar for location SB3 onto its original position and remove the voltmeter leads.

18. Place the Model 306 "meter mode" switch in the current position, if not already there, and verify a reading on the digital panel meter of 0.0 A ± 100 mA (probe ends remain unconnected to any conductor).

19. Connect the measurement ends of the Model 306's probes onto one another preferably through a load (directly is OK). Turn the "current adjust" knob clockwise until a reading of 30.0 A is obtained on the digital panel meter. Adjust VR1 until the 27.5 and 30.0 bar graph windows are simultaneously lighted with equal brightness. Turn the "current adjust" knob back to its full counter clockwise position.

20. Establish a connection between the probe's measurement ends and the 100 milliohm resistor on the back of the instrument. (A power connector may be required to serve as an adapter.) Set the "meter mode" switch to the milliohm position and turn the "current adjust" knob clockwise until a current of 17.5 Amps rms is reached. Verify a resistance reading of 101 milliohm ± 4 milliohm (100 milliohm ± 3% instrument tolerance plus ± 1% test resistor tolerance plus +~ 1 milliohm contact impedance). Accuracy checks at higher current levels require a quite rapid measurement technique to avoid damage to the resistor.

21. Set the "power" switch to the off position and remove the power cord from the instrument.

22. Replace the upper cabinet half using the twelve screws removed earlier. Before replacing the two lower rear screws, install a new number six lockwasher between the top and bottom cabinet half for each of these screws to insure proper grounding of the upper cabinet half. Secure all the screws tightly.

This Completes the Calibration Procedure.

© 1989-1991 Hypatia Inc.