help with sears model 21357 incubator

hello all , I am new to this, we have had ducks, bought as peeps, for 4 yrs now, great eggs, we have 3 girls and 2 boys , the boys are pekin whites , the girls are 2 pekins and 1 rowen, so I saw this incubator on c/l and bought it, now its round metal like 20' round, I pluged it in and it went to 120 deg. so I turned the temp nob down ,way down and the light stayed on , there is a round light on top ,next to the temp knob, so I think the light should have gone out??? is my wafer thermometer bad??? local feed store lady told me it needs to open ,, and the wafer did when I plugged it in , I have no directions to this , so I am asking for some guidance and help please ,thank you tommy