The pilot was returning to the airport after a local maintenance flight. During the before landing cockpit checks the pilot did not detect an incomplete gear extension. After touchdown, at about 70 knots, both main landing gear legs collapsed. The nose gear remained extended during the event and the rear fuselage and left horizontal stabilizer contacted the concrete pavement. The operator's subsequent examination of the hydraulic power pack showed the electric pump motor was inoperative. The failure of the power pack before the end of the gear extension cycle would result in a fully extended nose gear and a partial extension of the main landing gear legs.