The pilot arrived at the airport and discovered that the airplane's battery was weak. He requested an electrical boost from a local mechanic to start the engine. After starting the engine, the pilot allowed the engine to run for several minutes before taxiing to the runway for takeoff. The pilot reported that the runup procedure was normal. He also turned off all electrical equipment, but left one communication radio on. After liftoff, the pilot discovered that the landing gear would not retract completely. The pilot elected to continue the flight and, while en route, he attempted manually to lower the main landing gear, but felt pressure on the emergency extension handle and decided to discontinue the extension operation. During the landing roll, at the destination airport, the right main landing gear collapsed and the airplane veered off the right side of the runway. As the right wing was lifted up, during the wreckage recovery phase, the pilot lowered the right main gear with the emergency extension handle. Examination of the electrical system disclosed that a diode in the alternator had slipped from its normally installed position.