Attached is the schematic for the MK121C. Note there is nothing different about the schematic for the Medallion transformers - they are different internally than the earlier non-Medallion transformer but are connected the same. However, the "C" designation indicates the improved HF primary circuit, which is shown on the schematic. The schematic covers all the models that used the MK121 interface. For your 2-panel Model 1+1, you'll want the LF transformer tap set to the RED position. Other positions (for 3 and 4 panel systems) will offer less bass boost, which you may prefer. There's no harm in trying other positions.
Failure of the connection at the pin plugs and hook terminals is not uncommon after many years of handling. After resoldering, a reinforcement of heat shrink tubing will help ensure the integrity of the connection.
Likewise, corrosion on the exposed winding of the HF Balance resistor is not uncommon. It can be cleaned with some fine emery paper.
