I have a cz20,that I can not get a threshold or gb to work right in auto tune. It will sill find targets but I am not getting the depth that I should. I am sure that if I sent it to fisher they would want to upgrade it to a cz21 to fix it .With the cost of the upgrade?Would I be better off to get another cz20 thats in good shape and try to get what I can out of mine,or do the upgrade?