Weird OT issue ?

[catalin gramada]

Hi. Yes this was the circuit. My issue was related by accidental disconnection of nfb return which You remarked in the drawing. I didn't saw it the earth grounding path because is pretty unfamiliar for me on my bench. I did my hobby for years in a building without earthing connection at wall sockets and anyhow I used to use grounding paths soldered even for experiments. Now it was happen to use a strap wire which accidental was disconnected, is not happen to often to me, especially when work over hundred volts...but was happen and didn't know where to look. I have limited experience, I didn't learn from books, just test some circuits when have time and save what I consider relevant for futures builds. I found the source of problem by determination not by analysis. I checked DC resistance in circuit removing the load, the scope was conected over my load resistor, I did not saw it.
Got a lot of good advices over here, Thanks guys!