  We apply the coherent potential approximation (CPA) to a simple model for
disordered superconductors with d-wave pairing. We demonstrate that whilst the
effectiveness of an electronic Van Hove singularity to enhance the transition
temperature T$_c$ is reduced by disorder it is not eliminated. In fact we give
a qualitative account of changes in the T$_c$ vs. doping curve with increasing
disorder and compare our results with experiments on the
Y_{0.8}Ca_{0.2}Ba_2(Cu_{1-c}Zn_c)_{3}O_{7-\delta} alloys.
