  The fast dynamics of viscous calcium rubidium nitrate is investigated by
depolarized light scattering, neutron scattering and dielectric loss. Fast
$\beta$ relaxation evolves as in calcium potassium nitrate. The dynamic
susceptibilities can be described by the asymptotic scaling law of
mode-coupling theory with a shape parameter $\lambda = 0.79$; the temperature
dependence of the amplitudes extrapolates to $T_c\simeq 378$ K. However, the
frequencies of the minima of the three different spectroscopies never coincide,
in conflict with the factorization prediction, indicating that the true
asymptotic regime is unreachable.
