Most of the time the primary of a mic input transformer has a grounded center tap to reject common mode signals. That might be where some your hum is coming from. Remember seeing one cheaper mixer using a pair of 1k resistors across the primary with the center to ground to create sort of an artificial center tap. Also thanks for taking the time to make these videos.
Marka1986, hmmmm, I used to design custom equipment for radio stations, and I don't recall ever seeing or using an unbalancing transformer with a center tapped primary. Certainly there are circuit topologies that use center tapped transformers, but a simple winding provides common mode noise rejection without a center tap to ground. Even a quick check online shows many such circuits. As for the hum in my demonstration, none of the mic wiring was near a source of 60Hz magnetic field, so I doubt that the cable was picking up much of anything. I suspect poor foil routing in the Heathkit PCB, probably sharing a ground foil between the mic preamp and the adjacent power supply circuit. Heath dumped this product pretty quickly, probably only made one production run of them, and this might have been a contributing factor.
