I wanted to add something to this conversation that may be overlooked. I recently purchased a later model A6302 probe off e-bay. It seemed to be mostly working, but had AC overshoot that seemed too much. I finally figured out that the problem was with the core not mating perfectly, even though it would look like it was from casual observation. (The mu-metal housing looked to be perfectly mating, but the core itself was not).
It seems apparent that these probe cores are made by potting the ferrite in the mu-metal shield using a soft epoxy, and then the mating surfaces (mu-metal and ferrite) are ground/lapped flat/planar as one unit. I have much experience with epoxies and suspected that the epoxy had shrunk with time, pulling the core a little further back into the mu-metal housing, creating a gap between the ferrite surfaces. This is especially a problem with softer epoxies that have plasticizes in them.
Taking the probe apart I confirmed with a straight edge under the microscope that the core was indeed slightly below the surface of the mu-metal housing. I then starting lapping the cores, abrading the mu-metal until I could see that I was contacting the ferrite. I started with 600 grit sandpaper, working toward 1 micron diamond lapping film (used for polishing fiber optics) in multiple steps, all done on a flat glass surface.
After all this, the AC overshoot is nearly completely gone and the probe is performing to my expectations.
Some notes for "young players" on lapping, you have to have good technique!:
-You must be aware of how/where you're applying pressure as you're dragging the probe core across the sandpaper/lapping film, or you will put a tiny but meaningful convex curve in it. The ends or sides will get lapped a little more as the lapping drag force creates a tilt force and you have to compensate for this. I actually had to do some final lapping with half the core off the edge of the film to remove this small convexity I had put in the middle of it. Under the microscope I could see that I had a tiny tiny bit of a teeter totter thing going on, and it was affecting the performance of the probe.
-The ferrite is very brittle/frangible. Microscopic grains will come off, and then when you drag back over them they will take more grains out, leaving a rough surface in the ferrite. Consider this in your technique. I ended up lapping under a stream of water to wash the debris away.
-I was surprised by the probe sensitivity to this mechanical fitup. It's been talked about before, but I don't think it could be overstated. I was seeing effects that would be measured in microns.
-This process is not for the faint of heart and I don't recommend it for everyone!
I have attached a picture showing the small core pullback I'm talking about. Unfortunately this picture is half-way thought the process. I didn't think to take one at the beginning, so you're only seeing about half the gap here.
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