On July 22, 2009, about 1400 Pacific daylight time, a Piper PA-30, N7504Y, experienced a landing gear collapse during the landing roll at Walla Walla, Washington. Walla Walla University was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The certificated commercial pilot, with a certified flight instructor (CFI) certificate, and one passenger were not injured. The airplane sustained substantial damage to the airframe. The local maintenance test flight departed Walla Walla about 1345. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported that he had remained in the airport traffic pattern, and completed one takeoff and landing. The pilot stated that, prior to the second landing, the landing gear was down, and all indications appeared normal. After reducing power for landing, he did not hear an audible alert warning horn. As the airplane touched down, the gear collapsed. He noticed smoke coming from the circuit breaker panel on the floor, and exited the airplane. The air traffic control tower personnel reported that they thought that the landing gear appeared to be down prior to touchdown. The operator reported that they had previously been having trouble with the landing gear circuit breaker popping during landing gear extension and retraction. Six flight hours prior to the accident, they had replaced the right main landing gear push/pull retraction/extension cable, the gear motor control contactor, and the gear motor circuit breaker. Ground and flight tests had been successfully completed without a recurrence. The operator stated that the airplane had about 8 to 10 landing gear cycles prior to the accident flight with no anomalies noted, which led them to believe that the situation had been resolved. Post accident examination revealed that the landing gear motor circuit breaker tripped. The landing gear warning horn circuit is powered by the gear motor circuit, and would not activate with the circuit breaker tripped.