was on a charter flight from Kaminack Lake to Arviat with the pilot and 3 passengers on board. About 40 miles south of Kaminack, the engine began to run rough and when the pilot checked the magnetos, he found that the engine would not run on the left mag. The pilot completed the trip on one mag, and after landing, maintenance determined that the condenser on the left mag had failed. This condenser had been in service for 4.6 hours and the one it replaced had been in service for 172 hours. In a previous incident in May of this year involving the same a/c, a condenser failed after 84.4 hours of service, resulting in a precautionary landing. The operator says that the condensers were obtained from a Winnipeg overhaul shop, and were substandard. The overhaul shop has reportedly changed condenser suppliers, and the operator has reported the problem to Transport Canada.