During a landing on RWY 14R, the aircraft veered off the left side of the runway, hit a runway sign, and was damaged. At the same time the DHC-6 was approaching to land, a Boeing 727 was also in the traffic pattern to land on RWY 14L. According to tower personnel, the Boeing landed about 1 mile after the DeHavilland landed. However, there was evidence that the Boeing closed and possibly passed the DeHavilland during the landing sequence. The wind was from 360 degrees at 10 knots, which would have drifted wake turbulence toward RWY 14R. The DeHavilland captain reported that the power levers were stiff and that the left prop sounded like it surged during the landing just after the copilot (who was flying the aircraft) selected ground beta. When the captain realized that the copilot could no longer control the aircraft, he took the controls but was unable to avoid colliding with the runway sign. There was minor damage to the landing gear strut fairings.