The purpose of the flight was to complete the training for an aircraft endorsement. Two circuits were flown after the initial take-off, during which all radio communication equipment progressively failed. No alternator failure light was observed. When the landing gear was finally selected down neither the instructor or the trainee could determine if the landing gear had correctly extended. They assumed it had extended however as the landing gear warning horn did not sound when the throttle was retarded. The aircraft subsequently landed with the landing gear retracted. No fault could be found with the landing gear or radio systems after the accident. The reason for the apparent loss of electrical power could not be established.