Try following function..
public string ConvertEBCDICtoASCII(string strEBCDICString) {
    int[] e2a = new int[256]{
        0, 1, 2, 3,156, 9,134,127,151,141,142, 11, 12, 13, 14, 15,
        16, 17, 18, 19,157,133, 8,135, 24, 25,146,143, 28, 29, 30, 31,
        128,129,130,131,132, 10, 23, 27,136,137,138,139,140, 5, 6, 7,
        144,145, 22,147,148,149,150, 4,152,153,154,155, 20, 21,158, 26,
        32,160,161,162,163,164,165,166,167,168, 91, 46, 60, 40, 43, 33,
        38,169,170,171,172,173,174,175,176,177, 93, 36, 42, 41, 59, 94,
        45, 47,178,179,180,181,182,183,184,185,124, 44, 37, 95, 62, 63,
        186,187,188,189,190,191,192,193,194, 96, 58, 35, 64, 39, 61, 34,
        195, 97, 98, 99,100,101,102,103,104,105,196,197,198,199,200,201,
        202,106,107,108,109,110,111,112,113,114,203,204,205,206,207,208,
        209,126,115,116,117,118,119,120,121,122,210,211,212,213,214,215,
        216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,
        123, 65, 66, 67, 68, 69, 70, 71, 72, 73,232,233,234,235,236,237,
        125, 74, 75, 76, 77, 78, 79, 80, 81, 82,238,239,240,241,242,243,
        92,159, 83, 84, 85, 86, 87, 88, 89, 90,244,245,246,247,248,249,
        48, 49, 50, 51, 52, 53, 54, 55, 56, 57,250,251,252,253,254,255};
    char chrItem = Convert.ToChar("0");
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < strEBCDICString.Length; i++) {
        try {
            chrItem = Convert.ToChar(strEBCDICString.Substring(i, 1));
            sb.Append(Convert.ToChar(e2a[(int)chrItem]));
        } catch (Exception ex) {
            Console.WriteLine(ex.Message);
            return string.Empty;
        }
    }
    string result = sb.ToString();
    sb = null;
    return result;
}