Hallo! Interesting that both cracked ferrite, maybe it got too high a frequency? I have the full description but unfortunately the type of ferrite is not in it so I can rewind. This is the total in my book:I'm not an expert on this particular device, but it seems a bit unlikely to manufacture a "broken ring", and create a winding on it, when the desired magnetic properties could be acheived by controlled use of core material instead. The surface coating seems to have been torn apart. I'd be inclined to guess the core has overheated and broken apart and created a gap.