But is it your circuit board in the photo?
Yes, it could be a loose contact, in fact I was lowering it with the insulated handle of the screwdriver and when it took the small kickback resting on the table, the relay started this on/off. Maybe that short circuit increased and the control board inhibits and enables the relay continuously now. I want to understand how the relay engagement sequence occurred at startup, slowly increasing the voltage with the variac is easy to understand and then I want to try to make the contact of the relay manually with a switch at the right time, just to exclude that the relay could have problems (in addition I unsolder and re-solder it, which is not a bad thing).
I also want to unsolder the two large filter electrolytics, and also measure their ESR, they are many years old (20?) and maybe they could be the cause of problems, who knows.
Then I will seriously start to unsolder the power part, also because I was wrong, everything is inhibited if I have the potentiometer at minimum (not maximum as I had stated), and as soon as I increase the power, the short circuit appears at the same time that the control board enables the signal on the pulse transformer and consequently the IGBTs start to work and in fact with the short I start to have an output voltage.