Hey guys, I really need your help with this one
I've recently bought a very nice F. W. Bell 615 gaussmeter (NASA surplus!!!) - no probe.
This is a Hall-effect type gaussmeter that uses an AC current in, and gets an AC voltage out.
I HAVE a high-end F. W. Bell hall probe, about the same vintage as the instrument, but with a different connector, and no model. pinout is known.
Connecting it to the 4 pins on the probe socket (I+, !-, V+, V-) yields some response to magnetic fields but WAY too little. something like 60 gauss, in a 20mm gap between two Nd magnets that should be about 4 kGauss!. Only the two lowest scales read anything at all.
The manual,
https://sites.fas.harvard.edu/~phys191r/Bench_Notes/bell615.pdf , sais to set current with a pot, so that the voltage drop over R11 is 100 mV. This cannot be done in my setup. At max 65 mV.
Now, the real probes has a 'cal' resistor, as seen in the schematic below, and the value of this is unknown. In normal operation, it's actually in parallel with R11, through R12.
Both R11 and R12 are 2 ohm. Shorting the sense resistor would in essence half the value of R11, which might let me set the current, but also ruin the 'cal' function.
So my question: Could the missing 'cal' resistor cause the effect I'm seeing here, is the probe mismatched to the instrument or broken, or is there another obvious fault?
Thanks for your time! I'm really stuck here.