The TS-520S requires the DC-1 DC-DC converter for +13.8VDC operation. The DC-1 mounts on the back of the rig and the wires pass through an access plate on the rear panel. It's going to be tough finding one these days, and they were also quite expensive. If my memory serves me correctly, they were around $75 to $100 in 1978. The original TS-520, on the other hand, included the converter as standard equipment.
Also, if you are successful in finding a DC-1, please ensure your mounting for the TS-520S is solid. The TS-520S is heavy, and you certainly don't want it "flying around" in an emergency situation!
Good luck!
73,
Don Allen W9CW