Current combinatorial auctions require bidders to specify the valuations of bundles at the start of the auction. We propose an alternative mechanism called RevalSlot, based on the common value model, which allows a bidder to participate in the auction even if she can only identify a range within which her valuations lie. This will increase bidder participation, and at the same time maximize revenue. As the auction progresses, bidders get information which helps them to converge to a value for each bundle. RevalSlot is a combination of two processes. One helps bidders to zero in on a value for each bundle, and the other is an ascending proxy auction. We present theoretical and experimental results which confirm the efficacy of our mechanism.