Thanks YouTube videos (mainly diodegonewild), I was convinced that modifying a ferrite transformer was easy. Just heat up the core to destroy the glue that holds the core together.
However, I have had many repair attempts where I had to separate the core of small power supplies to modify the windings. All of them have failed. And I always put the blame on something besides the transformer.
Not long ago, I began using a small flyback SMPS with its transformer taken away to test other flyback transformers. A simple idea. And that let me notice that taking apart the transformer killed it every time.
So, after that I performed an experiment. I sacrificed many flyback SMPS to unglue their cores through various methods (boiling, air, iron), and then test them again with to see which of them survive.
None of them survive.
So, simply heating the core until it can be separated, then putting it back together, without modifying anything else, makes the transformer useless. Even with the iron method, in which anything other than the core barely gets hot.
I did my test with small flyback SMPSs, in the range of a tiny 5W USB charger to a 90W CRT TV power supply.
I found a question on StackExchange which declares the same conclusion.
https://electronics.stackexchange.com/questions/497489/temperature-degradation-of-ferrite-coresI had originally planned to modify an ATX supply to suit my needs by modifying the transformer this way. That's cancelled now.
One of the comments in that StackExchange question says that bigger power transformers don't suffer from this problem, but I'm not willing to test that out.
My iron method consists of making the core make contact with the soldering iron, and over the span of ~20 min., slowly raise the temperature to ~160°C.
The core may be ready to take apart way before that. The signal I take is a certain smell, which I assume is the glue giving up.
After I discovered all of this, I have made many transformer mods without taking the core apart, even though painfully and slowly, and all of them have been successful.
Customizing windings without opening the core
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