"...I have a LK-500 "B" model and it is running OK. It has no protection circuits in it that I can find other than a fuse. I cannot tell what is difference is between the "A" and "B" versions. I have the original manual and it seems to be a mish mash of both A and B information. Do you know?" ZA = no QSK (open frame relay) ZB = built-in QSK using vac relay 73, Roy K6XK