это текст в кодировке win1251

 // читаем файл
StreamReader txtFile1 = new StreamReader(@"c:\temp\ttt.txt");
string ddd = txtFile1.ReadToEnd();
txtFile1.Close();
// Преобразуем
txtBoxRespons.Text = PrintHexBytes(ddd);

[...]

// функция слегка переделанная  с [url]https://msdn.microsoft.com/ru-ru/library/bb311038.aspx[/url]
public string PrintHexBytes(string input)
{
  StringBuilder hexOutpu = new StringBuilder();
  char[] values = input.ToCharArray();
  foreach (char letter in values)
  {
    int value = Convert.ToInt32(letter);
    hexOutpu.Append(String.Format("{0:X} ", value));
  }
  return hexOutpu.ToString();
}

char[] values = input.ToCharArray();

public string PrintHexBytes(string input)
{
  byte[] bytes = Encoding.Win1251.GetBytes(input); // так нельзя !
  StringBuilder hexOutput =new StringBuilder();
  for (int i = 0; i < bytes.Length; i++)
  {
    hexOutput.Append(String.Format("{0:X2} ", bytes[i]));
  }
  return hexOutput.ToString();
}

StreamReader txtFile1 = new StreamReader(@"c:\temp\ttt.txt", Encoding.GetEncoding(1251)); // тут надо было указывать!
string ddd = txtFile1.ReadToEnd();
txtFile1.Close();
txtBoxRespons.Text = PrintHexBytes(ddd);

[...]

public string PrintHexBytes(string input)
{
  byte[] bytes = Encoding.Convert(Encoding.UTF8, Encoding.GetEncoding("windows-1251"), Encoding.UTF8.GetBytes(input));

  StringBuilder hexOutput = new StringBuilder();
  for (int i = 0; i < bytes.Length; i++)
  {
    hexOutput.Append(String.Format("{0:X2} ", bytes[i]));
  }
  return hexOutput.ToString();
}

StreamReader txtFile1 = new StreamReader(@"c:\temp\ttt.txt");

BinaryReader binFile1 = new BinaryReader(txtFile1.BaseStream);
byte[] bytes1 = new byte[1000];
int count=binFile1.Read(bytes1, 0, 1000);
            
txtFile1.Close();
txtBoxRespons.Text = PrintHexBytes(bytes1, count);

[...]

public string PrintHexBytes(byte[] bytes,int count)
{
  StringBuilder hexOutput = new StringBuilder();
  for (int i = 0; i < count; i++)
  {
    hexOutput.Append(String.Format("{0:X2} ", bytes[i]));
  }
  return hexOutput.ToString();
}

public static string ByteArrayToString(byte[] ba)
{
  StringBuilder hex = new StringBuilder(ba.Length * 2);
  foreach (byte b in ba)
    hex.AppendFormat("{0:x2}", b);
  return hex.ToString();
}

StreamReader txtFile1 = new StreamReader(@"c:\temp\ttt.txt");

BinaryReader binFile1 = new BinaryReader(txtFile1.BaseStream);

StringBuilder hexOutput = new StringBuilder();
for (int i = 0; i < txtFile1.BaseStream.Length; i++)
{
    hexOutput.Append($"{binFile1.ReadByte():X2} ");
}
txtFile1.Close();
txtBoxRespons.Text = hexOutput.ToString();

StringBuilder hexOutput = new StringBuilder();

StringBuilder hexOutput = new StringBuilder((int)txtFile1.BaseStream.Length*2);

#region Неизменяемая часть
StreamReader txtFile1 = new StreamReader(@"c:\temp\ttt.txt");

BinaryReader binFile1 = new BinaryReader(txtFile1.BaseStream);
byte[] bytes1 = new byte[txtFile1.BaseStream.Length];
int count = binFile1.Read(bytes1, 0, (int)txtFile1.BaseStream.Length);

txtFile1.Close();
#endregion
txtBoxRespons.Text = PrintHexBytes(bytes1);

[...]

public string PrintHexBytes(byte[] bytes)
{
  return BitConverter.ToString(bytes).Replace("-", " ");
}

Неизменяемая часть
txtBoxRespons.Text = ByteArrayToHexViaLookup32(bytes1);

[...]

private static readonly uint[] _lookup32 = CreateLookup32();

private static uint[] CreateLookup32()
{
    var result = new uint[256];
    for (int i = 0; i < 256; i++)
    {
        string s = i.ToString("X2");
        result[i] = ((uint)s[0]) + ((uint)s[1] << 16);
    }
    return result;
}

private static string ByteArrayToHexViaLookup32(byte[] bytes)
{
    char[] result = new char[bytes.Length * 3];
    for (int i = 0; i < bytes.Length; i++)
    {
        uint val = _lookup32[bytes[i]];
        result[3 * i] = (char)val;
        result[3 * i + 1] = (char)(val >> 16);
        result[3 * i + 2] = (" ".ToCharArray())[0];
    }
    return new string(result);
}

Неизменяемая часть
txtBoxRespons.Text = ByteToHexBitFiddle(bytes1);

[...]
static string ByteToHexBitFiddle(byte[] bytes)
{
    char[] c = new char[bytes.Length * 3];
    int b;
    for (int i = 0; i < bytes.Length; i++)
    {
        b = bytes[i] >> 4;
        c[i * 3] = (char)(55 + b + (((b - 10) >> 31) & -7));
        b = bytes[i] & 0xF;
        c[i * 3 + 1] = (char)(55 + b + (((b - 10) >> 31) & -7));
        c[i * 3 + 2] = (" ".ToCharArray())[0];
    }
    return new string(c);
}

StreamReader txtFile1 = new StreamReader(@"c:\temp\ttt.txt");  
    
BinaryReader binFile1 = new BinaryReader(txtFile1.BaseStream);  
byte[] bytes1 = new byte[txtFile1.BaseStream.Length];  
int count = binFile1.Read(bytes1, 0, (int)txtFile1.BaseStream.Length);  
    
txtFile1.Close();  

StreamReader txtFile1 = new StreamReader(@"c:\temp\ttt.txt");  
    
BinaryReader binFile1 = new BinaryReader(txtFile1.BaseStream);  
byte[] bytes = binFile1.ReadBytes((int)txtFile1.BaseStream.Length);  
    
txtFile1.Close();