Vintage research gaussmeter modifications/repair - help needed!

[ChristofferB]

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.

[ChristofferB]

Ok more info.

With a 1k variable resistor in place of the 'cal' probe resistor between pin B and F, the voltage drop over R11 lessens the lower resistance - around 23 mV at short.

I've tried shorting both B and F, and A and D, to obtain the max current draw (1 ohm, R11 and R12 in parallel.) 23 mV over R11. This makes sense as the resistance is halved, but No matter what I do, still cannot get 100 mV over R11.


With current adjust cranked to max:

With Hall probe: V over R11 =63 mV
with hall probe jack shorted R11 = 66 mV

Only output at very low ranges with very high field strenghts, ~ 5 kGauss gives something like 6 gauss on the 10G range.

This sounds like the current supply is faulty, and the hall signal amplification/detection is fine does it not?

Any suggestions on a fault in the current supply section (outputing via T2) that could explain low current?

I'm at a loss..
 

[ChristofferB]

Aha! Progress! As a hail mary, I replaced all the electrolytic caps. The instrument sprung back to life, scales reading predictable and so on.

With the probe (no cal res.) the scale reads within an order of magnitude on each scale, the 20mm gap now reading between 2 and 3 kGauss - much closer.

No magnet present reads 0.25 gauss, which is consistent with earths field.

But the current source STILL only reads 0.66V over 2 ohm. I don't buy that it's the probe not matching, since shorting the current out still only provides the same current.. Could the last transistor stage before the current xformer T2 be damaged?