On July 7, 2002, about 1145 Pacific daylight time, a Bell 47G-2, N2483B, experienced a partial loss of engine power during cruise flight and collided with power lines during a forced landing near Compton, California. The pilot/owner was operating the helicopter under the provisions of 14 CFR Part 91. The pilot and one passenger were not injured; the helicopter sustained substantial damage. Visual meteorological conditions prevailed, and a flight plan had not been filed. The personal local flight originated at the Compton/Woodley Airport about 1100. In a written statement, the pilot reported that about 45 minutes after departure, the helicopter was about 1,000 feet above ground level (agl), and moving in an easterly direction with an airspeed of 70 knots. He felt a "severe" vibration and noted a drop in the helicopter's rotor rpm. As the vibrations continued, he manipulated the flight controls in an effort to maintain a controlled descent. While searching for a suitable landing area, he verified that the engine rpm indicated zero. About 200 feet agl he noticed that power lines were in the helicopter's path, and turned the helicopter in a unsuccessful effort to avoid them and an accompanying pole. The helicopter came to rest in a dirt field on the corner of the 91 freeway and Alondra Blvd. During a telephone interview with a National Transportation Safety Board investigator, the brother of the pilot, a mechanic with inspection authorization (IA) who regularly worked on N2483B, examined the helicopter after the accident. He stated that a fan belt used in the integral system for cooling the engine had either broken or come loose. As a result, the belt wrapped around the pulley, which caused the transmission to drag. The pilot autorotated the helicopter into power lines and impacted terrain. The IA further noted that the engine ran fine after the accident, and the fuel system was free of particulates and contaminates. He said that he completed the annual inspection for the helicopter about 14 hours prior to the accident, at which time he believed that he examined the belts. He further stated the helicopter was stored outside, in a corrosive environment.