Circuit feedback with mutual inductance on a toroid
Toroids have one serious problem. Due to eddy currents there are peaks and dips in the magnetic flux around the core. The way this is avoided is to make sure each winding wraps around the entire perimeter of the toroid. The red winding I see in the photo is bunched up in one spot. That spot could be a dead zone or a hot zone in terms of magnetic flux density. The chances of a more predictable output and/or transfer of energy is much greater if the red winding is spread out around the entire toroid. Having wound toroids of my own, I have run into this very problem myself. You can test it by using a single turn that you can move around the toroid, with the turn connected to an oscilloscope. As you move the sense wire around the toroid you will see peaks and dips in signal strength. Always have each winding spread out around the toroid as much as possible.