The flight was inbound to MTN when electrical power occurred and communication with ATC was lost. The pilot proceeded to MTN, entered the traffic pattern, to land on runway 14. According to the pilot, the engine lost power while he was on base leg and the airplane crashed less than a mile from the runway. A pax said the pilot turned left to avoid another just before the engine lost power. He said during the turn the airplane shaked. ATC personnel at MTN said they first noticed the airplane when it was on base leg, but another airplane had been cleared to land, whose pilot was then instructed to break off the approach. ATC then saw the accident airplane go through the final approach course, turn left, and descend to the ground. The examination of the airplane revealed the alternator belt failed, although it was not determined if the failure occurred pre or post impact. The alternator was damaged and was not tested. The alternator warning circuitry for the annunciator panel was found defective, which disabled the warning. Mechanical examination of the airframe and engine did not disclose evidence of mechanical malfunction.