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Decided to unwind the HV winding of the flyback and see how many turns would need to be removed to find and remove the short. I was hoping it would be a short in the outer layer or two of the coil.
Got started by removing the black hard shell that coats the outer diameter of the HV coil. The material is quite different than most flybacks -- quite hard and durable. It was difficult to nibble at it with a diagonal wire cutter. First picture below shows the first nibbles out of the shell. Second picture shows the whole thing removed, which I also used a Dremel tool to assist. Once I get a few big nibbles out, then I was able to crack the whole thing off easily.
I left the Ringer and scope attached the entire time while I unwound the coil, so I could see immediately when the short was gone. After unwinding a few tens of layers, I could tell the unwinding was getting more difficult, and one spot in particular looked well cooked. Getting closer to the short!
Eventually got the short, and I also unwound one more layer of well cooked wire with the thought that a damaged winding layer would be likely to fail later. Still there are quite a few cooked layers present, but the short is gone. Alas, I had to remove quite a bit of the coil (perhaps something like 15%). Will it have enough HV back in the set? We'll find out.
The new ringdown is shown in the 3rd attached picture, and looks convincingly good. The last picture is the remaining HV coil, obviously smaller than when we started. I'll reattach the 3A3 anode lead and reinstall it in the set.
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