  Effective theories with the matter content of the minimal supersymmetric
Standard Model below the string scale M_string predict a wrong value for the
weak--mixing angle sin^2\theta_W and strong coupling constant \alpha_S at the
scale M_Z. To resolve this problem one needs large threshold corrections. At
the same time one would like to avoid introducing new intermediate scales that
are small compared to M_string. Two requests which seem to be incompatible. We
show how both requirements can be satisfied in a class of (0,2) heterotic
superstring compactifications with a natural choice of the vevs of the moduli
fields entering the moduli dependent string threshold corrections.
