It is a mini pcb attached on top of the inverter board for my welder. If I'm not mistaken it is the logic control board. It basically consists of a transformer like coil that branches off to 4 sections of the board, each identical. (and each of these 4 sections goes to a group of mosfets on the inverter board) Each of the 4 sections has some capacitors, diodes, and resistors.
Is there any way to test a good zener to determine its value? I think you may be right about it being a 15 volt, just a hunch.