MK-1 = Modifications to use a SB-10 SSB Adapter. MK-3 = Loading Control Modification. What about MK-2? Was it the "Improved Keying for the DX-100?" Were there others from Heathkit? 73, Gary H. Harmon, Jr. / K5JWK 6003 Archwood San Antonio, TX 78239-1504 210.657.1549h 210.884.6926c gharmon at idworld.net "Being a grandparent is a marvelous thing!"