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Custom probe needed for MI Model 904 Gaussmeter
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enut11:
**Discussion moved from REPAIR section **

I have a Magnetic Instrumentation (MI) model 904 Gaussmeter that came without a probe and would like to get it working.

Have not been able to find any info or manual online.

I believe this type of instrument uses a Hall Effect chip. These chips normally have 3 pins: power, ground and output.

You can see from the photos that the probe socket has 4 wires so maybe the fourth is a shield?

Our local supplier (Jaycar) has a UGN3503U sensor that I will try. It needs a nominal 5v supply.

Ebay has the following linear chips on offer:
https://www.ebay.com.au/itm/10-PCS-AH49E-TO-92S-AH-49E-LINEAR-HALL-EFFECT-IC/122490059728
https://www.ebay.com.au/itm/4PCS-HAL401SF-A-Encapsulation-SOT89-3-Linear-Hall-Effect-Sensor-IC/372247259357

I have never used a Hall chip so any feedback from the Forum would be appreciated.
enut11:
More MI Gaussmeter 904 pix
enut11:
What I have been able to determine so far referring to the Gaussmeter input socket:

Front switch set to READ:
Black wire is grounded.
Brown wire is signal to the PMI OP02 opamp pin 2.
Orange wire is +15v (from the LM317 connected as constant current source).
Yellow wire is ground for CC source only.

Front switch set to CAL:
Black wire is open.
Brown wire is open.
Orange wire is grounded.
Yellow wire is ground for CC source.
A constant current 60-120mA, variable by front panel pot, is applied to a precision 0.38ohm resistor generating a CAL voltage fed to the  PMI OP02 opamp.

The second opamp LM384 is used for meter drive and North/South field indication.

From this I conclude that input connector:
Pin 1 (Brown) is Hall sensor output.
Pin 2 (Black) is Hall sensor ground.
Pin 3 (Orange) is Hall sensor supply.
Pin 4 (Yellow) only used for calibration.

I found it odd that the Hall sensor was powered by an unregulated supply.
enut11
enut11:
Part schematic around the input socket.

I measured the sensitivity of the input opamp via R37 and it is 100uV to 100mV over the full front panel switching.

It appears that power for the probe must come from the LM317 current source?

I think this would rule out conventional Hall Sensor ICs which need a regulated 5v supply and output a +/- signal centered on 2.5v

Note also the two power supply grounds are not connected, unless this is done via the (missing) probe plug.
enut11:
When in CAL mode the input opamp is connected across a precision 0.38ohm 0.1% resistor but only sees a voltage if input socket pins 3-4 are shorted.

The 36ohm pot adjusts the current source from about 60mA to 120mA. This presents a CAL voltage of about 25-50mV via R37 to the opamp.

When input socket Pin4 is not connected, an unregulated +15v is available at input socket Pin3.

So, what sort of magnetic sensor would be connected to the MI Gaussmeter 904?
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