  In standard ETC models the sideways and diagonal ETC interactions contribute
to $\delta R_b$ with opposite signs. The aim of this article is to study the
implications of the CDF value for m_t and the LEP value for R_b on $zt\bar{t}$
couplings where the LH sideways and diagonal ETC effects interfere
constructively. We find that for m_t=175 Gev, $\delta R_b =.0022$ and
m^2_s=m^2_d, F^t_v and F^t_a are modified by 19% and 7% respectively from their
SM values. The constrains implied by these deviations on diagonal ETC scenarios
and the feasibility of probing them at NLC through polarization and angular
distribution studies in $e^+e^- \to t\bar{t}$ are also considered.
