Units below serial number 1719 that do not have Service Bulletin 3 incorporated have the older style Logic, Pre-amp and Power Supply Boards. These older boards are no longer available and if any of the three fail, all of the boards must be changed as the newer style boards are not compatible with the older boards. This modification can only be performed at the factory and since the cost of the service bulletin is so close to the price of an overhauled unit; it is typically not cost effective to add this service bulletin.