Knitting machine Brother Kh910, card reading test 881 have error 3?

Knitting machine Brother Kh910, card reading is not work. I try to follow the Service Manual for modified Kh 910. I test the card reading by put no.881 and push M key, it show error no.3. I have ask the electronic guy to adjust VR501 and check pin CH501. However, i don't have a special card to put in the machine for the mark sensor checking. So, I try to adjust at the black mark on the original printed Mylar sheet, and then adjust the VR 501 to low the voltage to 0.6 volt. Then, i test no.881 again. It' s still error no.3. So, how to fix the card reading? do i need the special card? I bought this machine from ebay and ship to Thailand, Southeast Asia. Please help and recommend.