  The dynamo equations are solved numerically with a helical forcing
corresponding to the Roberts flow. In the fully turbulent regime the flow
behaves as a Roberts flow on long time scales, plus turbulent fluctuations at
short time scales. The dynamo onset is controlled by the long time scales of
the flow, in agreement with the former Karlsruhe experimental results. The
dynamo mechanism is governed by a generalized $\alpha$-effect which includes
both usual $\alpha$-effect and turbulent diffusion, plus all higher order
effects. Beyond the onset we find that this generalized $\alpha$-effect scales
as $O(Rm^{-1})$ suggesting the take-over of small-scale dynamo action. This is
confirmed by simulations in which dynamo occurs even if the large-scale field
is artificially suppressed.
