Hypatia Model 307 Calibration Procedure Version 1.6

4 June 1992 © 1989-1992 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. One digital counter with at least .1% accuracy and at least .1 Hz 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 9 through 18 of this procedure.

Test Points: There are nine 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. The remaining three test points are described by location in the procedure below.

Variable Resistors: There is a variable resistor at the top of an internal power converter labeled VR1. (This is separate from "VR1" located on the main circuit board). There are five multiturn variable resistors on the right side of 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 either in the front or in the rear of the digital panel meter.

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

Step by Step Procedure:

1. Set the Model 307 "main power" and "test current" switches to the off position. Set the "current adjust" control knob to its full counter clockwise position and remove the power cord from the instrument.

2. Remove the six screws from each side of the instrument cabinet 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 a Model 100 and any other model test probe 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 307 "main power" switch to the on position and allow fifteen minutes for thermal equilibrium to occur.

6. Connect the voltmeter leads to the test points marked "+" and "-" on the left rear of the main circuit board, and adjust the power converter's variable resistor VR1 for a voltmeter reading of 5.00V. (The left side is the timer display side.)

7. Connect the digital counter to the "-" test point from step six and the third test point on the left side, which is toward the front center of the instrument. Verify a reading of 10.0 Hz ± .1 Hz. No adjustment is available for this signal.

8. If a flush mounted panel meter is utilized, access VR6 from the rear of its internal housing. If a protruding panel meter is utilized, access VR6 as follows: Being careful not to damage the 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 of the meter housing. A multiturn variable resistor, VR6, is located near the lower right corner of the front of the panel meter.

9. 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 if the protruding type.)

10. 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.

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

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

13. VR2 and VR3 are somewhat interactive. Repeat steps 11 and 12 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.

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

15. 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.)

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

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

18. VR4 and VR5 are somewhat interactive. Repeat steps 16 and 17 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.

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

20. Place the Model 307 "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).

21. Connect the measurement ends of the Model 307's probes onto one another preferably through a load (directly is OK). Turn the "test current" switch on, then turn the "current adjust" knob slowly clockwise until 29.9 rocking to 30.0 A appears on the digital panel meter. Adjust VR1 until the 27.5 and 30.0 bar graph windows are simultaneously and equally lighted. Turn the "test current" switch off and the "current adjust" knob full counter clockwise.

22. 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, turn the "test current" switch on and 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 resistor damage. Turn the "test current" switch off and the "current adjust" knob full counter clockwise.

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

24. 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.

© 1989-1992 Hypatia Inc.