  A model of B20 helimagnet with strong (frustrating) defect bonds is
considered. Single defect provides dipolar field-like distortion of the helical
magnetic ordering. At finite defect bond concentrations, the spiral vector
acquires correction to its value in the pure system. Moreover, it is shown that
the strong defects of sufficient but small concentration are able to destroy
long-range order driving the system to the phase with short-range helical
ordering. It is argued that the developed approach allows describing main
experimental observations in Mn$_{1-x}$Fe$_x$Si and Mn$_{1-x}$Co$_x$Si
compounds.
