  For more than a decade from the late 1980s to the late 1990s, the intraday
variable (IDV) quasar 0917+624 was known to be strongly variable with amplitude
variations of 10-20% within 0.8 to 1.6 days. During that period it showed
faster variability in the polarized flux and the polarization angle than in
total intensity. However, since 2000 the IDV of this source has almost ceased,
attaining a variability index of only less than a few percent. If interstellar
scintillation was responsible for the IDV in the past, one possible explanation
for the sudden change of the variability mode could be an intrinsic change in
the morphology of the source, such as an increase in the size of the structural
components that are responsible for the scintillation. New jet components
appearing at the jet base could lead to a temporarily larger size of the
scintillating component when compared to the scattering size of the
interstellar medium, causing quenched scintillation. In order to test this
scenario, VLBI observations of 0917+624 from 1999 to 2005 have been analysed.
Here, we summarize first results and discuss them in the framework of the
quenched scintillation model.
