The thermopower of the Ni_xB_100-x-ySi_y amorphous alloy system with a metalloid concentration ranging from 20 to 39 at.% has been measured from 77 up to 300 K. The thermopower, as well as the specific heat, clearly shows filling of the d band on increasing metalloid content. The thermopower is negative for x>or approximately=70 and increases rapidly as x decreases. For x<or=67 the thermopower is positive and decreases slightly with decrease in x. The high-temperature slope of the thermopower, the residual resistivity and the room-temperature coefficient of resistivity are simultaneously explained within the framework of the extended Ziman theory. In performing the corresponding calculations the authors have chosen the phase shifts deduced in such a way that they reflect the d-band filling on alloying (deduced from the specific heat measurements). The have also used Hall effect measurements which determine the number of conduction electrons. For x>or=78 the Ziman contribution to the thermopower is concealed by the large extra term which can be ascribed to magnetic fluctuations.