The 50 Hz miracle

After last part of yesterday's switch off shift, our beloved 50 Hz nightmare was again with us.
Basically, we switched stuff off and that made it worse.
After narrowing down the possible short list of actions which could have potentially caused it, today we carefully tried to undo them.
It looks like it was a success.
What allowed us to go back to the previous situation (actually, slightly better: light brown before, magenta after, blue today after the miracle)
was reconnecting the power supply cable to the NE Ring Heater power supply.
It is worth mentioning the power supply itself was (and still is) off.

Science marches on.