On April 26, 2001, about 1315 eastern daylight time, a Cessna 210N, N6585A, registered to and operated by Air Carriers, Inc., as fast check flight 269, experienced collapse of the main landing gear during the landing roll at the Gainesville Regional Airport, Gainesville, Florida. Visual meteorological conditions prevailed at the time and an instrument flight rules (IFR) flight plan was filed for the 14 CFR Part 135 non-scheduled domestic cargo flight. The airplane was substantially damaged and the commercial-rated pilot, the sole occupant, was not injured. The flight originated at 1155 from the Hernando County Airport, Brooksville, Florida. The pilot stated that the first two legs in the accident airplane were uneventful. The flight departed on the third leg and approximately 15 minutes into the flight, he noted a problem with the airplane's radios. He attempted to troubleshoot the radio problem and checked the ammeter which indicated a slight discharge. He turned on all the aircraft's light to see if the ammeter would change; it did not. He then secured all electrical equipment with the exception of the No. 1 communication transceiver and transponder. He checked the circuit breakers and reset the alternator switch; with negative results. On approach to the destination airport he remained high and began to circle awaiting light gun signals from the tower. He received a signal that the flight was cleared to land and lowered the landing gear electrically. He reported hearing the landing gear motor operate, felt a sensation that he equated to was the landing gear locking into position, and looked out and thought that the main landing gear were locked into position; he could not see the gear down and locked light due to sunlight. After touchdown, the main landing gear collapsed; the nose landing gear did not collapse. Examination of the airplane by an Federal Aviation Administration (FAA) airworthiness inspector revealed that the "A+" electrical cable which is connected to the alternator and to a terminal board at the firewall, separated from the wire connector at the terminal board at the firewall. Further examination determined that the cable was locally manufactured and appeared to be too short which did not allow for vibration. A copy of the FAA inspector statement is an attachment to this report.